Ultra-fine grain high-strength steel cord steel with excellent heat resistance and production method of ultra-fine grain high-strength steel cord steel

Abstract

The invention discloses ultra-fine grain high-strength steel for a steel cord with excellent heat resistance and a production method thereof, the steel for the steel cord comprises the following main chemical components in percentage by weight: 0.86%-0.94% of C, 0.25%-0.45% of Mn, 0.30%-0.40% of Cr, 0.32%-0.45% of Si, 0.025%-0.040% of V and 0.0005%-0.0009% of B, wherein X is more than or equal to 0.80 and less than or equal to 1.20 and is equal to Mn / Cr; 0.32 < = Y = 10 * V + 100 * B < = 0.45; the sorbite rate of the steel for the steel cord reaches 95% or above, pearlite clusters are small, sorbite lamellar spacing is small, meanwhile, a sorbite structure is distributed in a multi-orientation mode, and a steel wire made of the steel for the steel cord is good in torsion fracture, high in strength and toughness and excellent in heat resistance; the production process is stable and reliable, and is suitable for large-scale industrial production.

Description

Technical Field

[0001] The present invention belongs to the technical field of ultra-high strength steel cord steel, and in particular relates to an ultra-fine grain high strength steel cord steel with excellent heat resistance and a production method thereof. Background Art

[0002] In recent years, with the explosive growth of my country's automotive industry, the tire industry has experienced rapid development. Tire production and quality are indicators of a country's automotive industry development level. Radial tires offer superior safety, high speed, fuel efficiency, and driving performance, and are the future direction of tire development. Steel cord is widely used in radial tire production due to its high strength, excellent wear resistance, rapid heat dissipation, and excellent elasticity. As one of the primary raw materials for tire carcasses, it is known as the "crown" of wire products.

[0003] The concepts of low-carbon economy and energy conservation and emission reduction are gaining popularity. Green and environmentally friendly vehicles have become an inevitable trend in the development of the automotive industry. Consequently, automobile tires are moving towards lightweighting. This has led to increasing requirements for radial tires in terms of safety, comfort, and economy. The development of steel cord towards higher strength, higher thermal conductivity, and longer fatigue life has become a major trend in radial tires to adapt to the development of green tires. The mainstream steel cord in the market is gradually transitioning from high-strength steel cord (HT) to ultra-high-strength steel cord (ST) and ultra-high-strength steel cord (UT).

[0004] Currently, the main way to improve the strength level of steel cord is to increase the carbon content. However, as the carbon content increases, the thermal conductivity of the steel wire continues to decrease. The heat generated by friction during driving will reduce the fatigue resistance of the tire. Therefore, the development of steel cords with higher strength and higher thermal conductivity will become the future development trend of the steel cord industry.

[0005] Chinese patent CN113789465A discloses a grade 92 steel cord with a rough-draw breakage rate of less than 5 per 1,000 tons and a production method thereof. The method comprises: hot metal desulfurization → converter smelting → argon blowing → LF furnace treatment → RH vacuum treatment → billet continuous casting → billet heating → high-speed wire rolling → Stelmor controlled cooling → coiling; rapid cooling above 665°C ensures sufficient undercooling and prevents abnormal precipitation of cementite; slow cooling after 665°C prolongs pearlite growth time, resulting in larger pearlite lamellar spacing and a thicker, denser oxide scale. The larger pearlite lamellar spacing ensures strong plasticity in the wire rod, while the thicker, denser oxide scale improves the integrity of the scale during rough-drawing, preventing scratches on the wire rod surface caused by fine oxide scale adhering to the rough-drawing wheels, thereby ensuring the surface quality of the wire rod. This patent reduces the rough-draw breakage rate from the perspective of steel cord manufacturing, but fails to meet the requirement for reduced heat resistance due to high strength.

[0006] Chinese patent CN113088818A discloses an ultra-high strength steel cord, a wire rod for ultra-high strength steel cord and a production method thereof. The chemical composition of the ultra-high strength steel cord wire rod includes, by mass percentage, C 0.90-0.94%, Si 0.17-0.23%, Mn 0.35-0.48%, Cr 0.17-0.23%, Ti≤0.001%, Al≤0.001 8%, Sn≤0.005%, As≤0.005%, S≤0.005%, O≤0.0025%, N≤0.0020%, and the rest are Fe and other inevitable impurities; among which, the contents of Mn and Si satisfy: Mn / Si=1.8~2.2; the diameter of the wire rod is 5.0~5.5mm, the tensile strength is 1250~1350MPa, the cross-sectional shrinkage rate is ≥40%, the size of the maximum inclusion is ≤25μm, the area proportion of sorbite and pearlite in the metallographic structure is ≥99%, and a single wire with a diameter of 0.30mm can be drawn out, and the tensile strength of the single wire is 3750~3900MPa, which can then be twisted into ultra-high strength steel cord. The production method includes pretreatment, converter smelting, refining, continuous casting, cogging, high-speed wire rolling, and temperature-controlled cooling. Combined with control of chemical composition and ratio, it can reduce center segregation, control abnormal carbide network structures, improve the product's tensile strength, and reduce wire breakage. While this patent achieves increased steel cord strength through composition optimization, it cannot address the reduced heat resistance associated with higher strength. Summary of the Invention

[0007] In order to solve the above technical problems, the present invention provides an ultrafine-grained high-strength steel for steel cord with excellent heat resistance and a production method thereof. The sorbitization rate of the steel cord steel reaches more than 95%, the pearlite clusters are small, the spacing between the sorbite lamellae is small, and the sorbite structure presents a multi-oriented distribution. The steel wire made therefrom has a good torsional fracture, high strength, high toughness, and excellent heat resistance; its production process is stable and reliable, and is suitable for large-scale industrial production.

[0008] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0009] The present invention provides an ultrafine-grained high-strength steel for steel cord with excellent heat resistance. The chemical composition and weight percentage of the steel cord steel are as follows: C 0.86-0.94%, Mn 0.25-0.45%, Cr 0.30-0.40%, Si 0.32-0.45%, V 0.025-0.040%, B 0.0005-0.0009%, Alt≤0.001%, Ti≤0.0005%, P≤0.010%, S≤0.010%, O≤0.0020%, N≤0.0065%, and the remainder is Fe and unavoidable impurity elements.

[0010] Where, 0.80≤X=Mn / Cr≤1.20; 0.32≤Y=10*V+100*B≤0.45. In the calculation formulas for X and Y, the value of each component is its content in the steel × 100.

[0011] The hot-rolled microstructure of the steel cord steel is sorbite, with a sorbitization rate of ≥95%, a sorbite interlamellar spacing of ≤130 nm, and ≥8 sorbite orientation positions. The steel cord steel provided by the present invention has fine pearlite clusters, fine sorbite interlamellar spacing, and a multi-oriented sorbite structure. This reduces orientation inconsistency and fragmentation during drawing compared to conventional structures, resulting in high strength.

[0012] The hot-rolled tensile strength R of the steel cord steel m ≥1270MPa, same circle difference ≤65MPa, section shrinkage Z ≥42%; thermal conductivity ≥60.5Wm at 40℃ -1 .K -1 , 80℃ thermal conductivity ≥54.0Wm -1 .K -1 , 120℃ thermal conductivity ≥44.3Wm -1 .K -1 .

[0013] The non-metallic inclusions of hot-rolled steel cord steel are Class B and Class D ≤ 0.5 level, Class A and Class C ≤ 1.5 level, the maximum size of inclusions (longitudinal width, transverse size) ≤ 25 μm, the maximum width of titanium inclusions ≤ 5 μm, and the number of inclusions is < 1000 / cm 2 , there are no pure Al2O3 inclusions.

[0014] The steel wire made of the steel used for the steel cord has a tensile strength of ≥4000 MPa, can be twisted 360° ≥22 times, has a good torsional fracture, has high strength and high toughness, and also has excellent heat resistance.

[0015] The present invention also provides a method for producing the ultrafine-grained high-strength steel for steel cord with excellent heat resistance, which comprises the following steps: smelting→continuous casting→heating→high-wire controlled rolling→controlled cooling.

[0016] The smelting steps specifically include: molten iron pretreatment → converter smelting → LF refining. In order to achieve plasticization of inclusions, RH vacuum degassing treatment is not performed.

[0017] In the molten iron pretreatment step, to control Ti inclusions, low-Ti molten iron is used for production, and desulfurization is performed during pretreatment, with the desulfurization target value S≤0.005%. In order to control Al2O3 inclusions, the previous furnace of ladle production must not produce steel with Al≥0.010%.

[0018] In the converter smelting step, P is effectively removed and "resulfurization" is suppressed, and the end point C is controlled at ≤0.10%, P≤0.0080%, and S≤0.08%. Slag is blocked for tapping, and refined slag and lime are added when about 1 / 5 of the molten steel is tapped. When about 1 / 3 of the molten steel is tapped, deoxidizer and alloy are added in the following order: silicon balls → slag → silicon manganese → high carbon ferrochrome → recarburizer. Low Alt alloy is used to strictly prevent the alloy from bringing in Al.

[0019] In the LF furnace refining step, argon is blown from the bottom of the ladle throughout the entire process, and the argon flow rate is based on the requirement that the molten steel does not splash out of the ladle; in order to avoid the formation of large brittle Al2O3-type hard inclusions in the steel, a Si deoxidation process is adopted, and a silicon ball deoxidizer is selected as a deoxidizer. In order to modify the inclusions and make them deformable inclusions to prevent brittle fracture of the steel wire during drawing, lime and wollastonite are added to form slag, with basicity (CaO / SiO2) ≤1.0, Al2O3 ≤5%, Als in the steel ≤5ppm, and a large Ar gas flow rate is used for stirring as much as possible without exposing the slag surface; sufficient silicon carbide is added to the slag; the FeO content of the slag is controlled between 2 and 3%; argon is blown from the bottom of the ladle throughout the entire process, and the argon flow rate is based on the requirement that the molten steel does not splash out of the ladle; the use of Al-containing alloys is strictly controlled, the white slag time is ≥20 minutes, and alloys are added before and during refining to adjust the Si, Mn, Cr, V, and B contents based on the component analysis results before entering the LF furnace.

[0020] In the continuous casting step, in order to obtain low-segregation ingots, electric stirring of the crystallizer and electric stirring at the end + light pressure are used, the crystallizer electromagnetic stirring frequency is 1.5-2.5 Hz, the current is 180-220 A, the end electromagnetic stirring frequency is 2.5-3.5 Hz, the current is 240-260 A, the pressure reduction is 12-18 mm, the superheat is controlled at 20-40°C, and the pulling speed is 1.9-2.6 mm / min.

[0021] In the continuous casting step, the carbon segregation index of the continuous casting billet is ≤1.13.

[0022] In the heating step, the soaking zone temperature is 1200-1250°C, and the soaking zone holding time is ≥80 minutes. This can further improve the segregation of the slab and avoid the occurrence of abnormal structures such as central martensite and network cementite during wire rod rolling. The slab heating uses high-temperature diffusion to improve segregation.

[0023] In the high-speed wire controlled rolling step, the starting rolling temperature is 950-1000°C, the finishing rolling temperature is 810-840°C, the sizing temperature is 870-910°C, and the spinning temperature is 890-930°C.

[0024] In the controlled cooling step, a three-stage cooling method is adopted, wherein the first stage has a cooling rate of 7 to 11°C / s, cooling to 600 to 650°C; the second stage has a cooling rate of 7 to 9°C / s, cooling to 500 to 550°C; and the third stage has a cooling rate of ≤3°C / s, cooling to 300 to 400°C.

[0025] The present invention also provides a steel cord prepared by using the steel for ultrafine-grained high-strength steel cord with excellent heat resistance according to the present invention.

[0026] The preparation method of the steel cord is: peeling or pickling → rough drawing Specifications → 1100℃ heating → pickling → water bath → electroplating → 40℃ wet drawing Specifications → Twisted strands.

[0027] Furthermore, the rough drawing is performed 13 times.

[0028] The functions and controls of the various components in the steel cord steel provided by the present invention are as follows:

[0029] C: The carbon element is essential for achieving high strength and hardness. To achieve the high strength required for high-strength steel cord, the carbon content must be above 0.86%. However, excessive carbon content increases carbon segregation, which can lead to the formation of abnormal structures such as central martensite and network cementite, impairing drawing performance. Furthermore, as the carbon content increases, thermal conductivity decreases, the critical drawing strain of the steel wire gradually decreases, and the material's drawing performance also gradually declines. Therefore, the carbon content should be controlled between 0.86% and 0.94%.

[0030] Si: As a solid solution hardening element, Si can significantly increase the strength of high carbon steel. Increasing the Si content in high carbon steel wire rods helps reduce the segregation of carbon atoms in the ferrite lamellae, reducing the formation of central martensite and network cementite. Si can also significantly delay the exothermic peak of the steel wire to a higher temperature range, thereby improving the thermal stability of the steel wire. Therefore, the Si content should be controlled between 0.32% and 0.45%.

[0031] Mn: Mn is an effective element for deoxidation and desulfurization, and can also improve the hardenability and strength of steel. However, excessive Mn content can easily cause segregation, resulting in the formation of harmful structures such as central martensite and network cementite, which deteriorates the toughness of the steel. Therefore, the Mn content is controlled between 0.25% and 0.45%.

[0032] Cr: It is a strong carbide-forming element and exists in the cementite lamellae to form alloy cementite, which can effectively inhibit the precipitation of the proeutectoid phase, increase the sorbitization rate, and achieve high strength and high toughness of the material; at the same time, the addition of Cr shifts the continuous cooling transformation curve of the steel to the right, thereby refining the interlamellar spacing. At the same time, Cr can also reduce the activity of C, which can reduce the decarburization tendency of the steel surface during heating, rolling and heat treatment, and is used to obtain high fatigue resistance. In addition, increasing the Cr content can inhibit the dissolution of cementite during drawing, reduce the amount of cementite dissolved, and thus improve the torsional stratification of the steel wire, so the Cr content is controlled at 0.30% to 0.40%.

[0033] V: The V element can significantly refine the grain size, which not only increases the strength and toughness of steel but also improves its low-temperature performance. Furthermore, V is a strong carbide-forming element. Its precipitation at the austenite grain boundaries during the initial phase transformation can reduce the grain boundary carbon content, effectively inhibiting the formation of network cementite. Furthermore, V (C, N) precipitated in ferrite can act as precipitation strengthening. Furthermore, due to the finer grains, corrosion resistance can also be improved. Excessively high V content results in higher costs. The V content can be controlled within the range of 0.025-0.040%.

[0034] Boron (B) significantly expands the low-melting-point region of SiO2-MnO-Al2O3 and SiO2-CaO-Al2O3 inclusions in molten steel, thereby improving their deformation capacity and achieving inclusion modification. However, excessive B content in steel can easily deteriorate hot workability, so it must be strictly controlled below 0.001%. Therefore, the B content can be controlled within a range of 0.0005% to 0.0009%.

[0035] Alt: As a strong deoxidizing element, Alt easily forms immutable Al2O3-type brittle inclusions with oxygen. Higher Alt content leads to larger and more brittle inclusions. Larger brittle inclusions, in particular, can cause drawing fractures, impacting serviceability, and significantly reducing fatigue life. Steel cord drawn wire is very fine, so to control inclusion morphology and size, the Alt content is strictly controlled to ≤0.001%.

[0036] Ti: Ti and N, impurities present in molten steel during solidification, accumulate in the interdendritic space due to solidification segregation, resulting in the precipitation of TiN inclusions. TiN inclusions are brittle, non-deformable inclusions with very high hardness, often appearing as triangles, rhombuses, or squares. They pose a serious threat to the fatigue and processing properties of steel materials and are the most direct cause of wire rod breakage during drawing or stranding. Unlike oxide inclusions, TiN inclusions cannot be plasticized through process technology, so their precipitation can only be controlled by strictly controlling their content. Therefore, the Ti content should be ≤ 0.0005%.

[0037] S and P: Impurity elements such as S and P segregate at grain boundaries, significantly reducing delayed fracture resistance. P can form microsegregations during solidification and subsequently segregate at grain boundaries during austenitization, significantly increasing the steel's brittleness and delayed fracture sensitivity. S forms MnS inclusions and segregates at grain boundaries, further increasing the steel's delayed fracture sensitivity. Therefore, the P and S contents should be controlled within the range of P ≤ 0.010% and S ≤ 0.010%.

[0038] Oxygen and nitrogen: Oxygen forms various oxide inclusions in steel. Under stress, these inclusions easily cause stress concentration, leading to the initiation of microcracks and deteriorating the steel's mechanical properties, especially toughness and fatigue resistance. Therefore, measures must be taken in metallurgical production to minimize its content, controlling it to 0.0020% or less. Nitrogen precipitates as Fe₄N in steel, which diffuses slowly, causing aging. Nitrogen also reduces the steel's cold working properties, so the nitrogen content should be controlled to 0.0065% or less.

[0039] In order to improve the heat resistance of steel and thus improve fatigue performance, the present invention appropriately reduces the Mn element content and simultaneously increases the Cr element content, and satisfies 0.80≤X=Mn / Cr≤1.20. On the one hand, by adjusting the Mn content and the Cr content, the material still has sufficient hardenability, which can not only ensure sufficient strength but also prevent the occurrence of abnormal quenching structure; on the other hand, the thermal conductivity can be improved to ensure excellent fatigue performance even at high temperatures. In order to obtain high strength and improve toughness at the same time, V and B microalloying elements are added, and 0.32≤Y=10*V+100*B≤0.45 is satisfied. On the one hand, the fine and dispersed V carbides play a pinning role on grain boundary migration, hindering the growth of grains, which can effectively refine the size of pearlite clusters. At the same time, it can increase the nucleation driving force of cementite, so that cementite particles are formed at different positions of grain boundaries, reducing the continuity of network cementite and eliminating network cementite. On the other hand, the addition of B element expands the low melting point zone of SiO2-MnO-Al2O3 and SiO2-CaO-Al2O3 inclusions in the molten steel, thereby improving the deformation ability of inclusions and realizing inclusion modification.

[0040] The production method of steel for steel cord provided by the present invention adopts a "one-fire material production" process, and the production method is simple. By controlling the smelting process, a high-quality, low-inclusion ingot with a carbon segregation index of ≤1.13 is obtained. In order to further improve the segregation of the ingot and avoid abnormal structures such as central martensite and network cementite during wire rod rolling, high-temperature diffusion is used for heating the ingot to improve segregation, the target temperature of the soaking section is 1200-1250°C, and the holding time of the soaking section is ≥80min.

[0041] To achieve ultra-high strength, the rolling process adopts a low finishing temperature + high spinning temperature method. The starting rolling temperature of the high-speed wire rolling is 950-1000°C. The present invention adopts an appropriate low-temperature rolling method, and the finishing temperature is controlled at 810-840°C, which is conducive to grain refinement, thereby improving the tensile strength of the wire rod. The sizing temperature and spinning temperature are relatively high, 870-910°C for sizing and 890-930°C for spinning, providing conditions for subsequent controlled cooling, ensuring an increased cooling rate at a relatively high spinning temperature, obtaining a large degree of undercooling, and thereby reducing the spacing between the sorbite lamellae and improving the tensile strength of the wire rod.

[0042] To achieve a high sorbitization rate, a three-stage cooling method is employed. In the first stage, the fan is turned on at 100% and the cooling rate is 7-11°C / s, cooling to 600-650°C. This rapid cooling produces a microstructure with fine lamellar spacing and a high sorbitization rate. The second stage undergoes isothermal transformation at a cooling rate of 7-9°C / s and the fan is turned on at 50-80%, cooling to 500-550°C. The third stage involves a slower cooling rate to avoid the formation of central martensite, with a cooling rate of ≤3°C / s, cooling to 300-400°C. This three-stage cooling method effectively controls the transformation rate from the austenite region to the transformation point and throughout the entire transformation range, resulting in a smoother transformation process and a longer transformation time. This helps increase the sorbite content in the wire rod and reduce the formation of abnormal structures. It also facilitates controlling the final cooling temperature and rate, reducing or eliminating the formation of abnormal structures.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] The steel cord steel provided by the present invention has a sorbitization rate of more than 95%, a grain size of ≥9.0, a sorbitan interlamellar spacing of ≤130nm, and no central martensite or network cementite. m ≥1270MPa, same-turn difference ≤65MPa, section shrinkage Z ≥42%; steel wire tensile strength ≥4000MPa, 360° torsion ≥22 times, good torsion fracture, high strength, high toughness, and excellent heat resistance, thermal conductivity ≥60.5Wm at 40℃ -1 .K -1 , 80℃ thermal conductivity ≥54.0Wm -1 .K -1 , 120℃ thermal conductivity ≥44.3Wm -1 .K -1 Non-metallic inclusions of type B and type D are ≤0.5, type A and type C are ≤1.5, the maximum size of inclusions (vertical width, horizontal size) is ≤25μm, the maximum width of titanium inclusions is ≤5μm, and the number of inclusions is <1000 / cm 2 , without pure Al2O3 inclusions. Its production process is stable and reliable, suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 1 is the metallographic structure diagram of the steel cord steel in Example 1;

[0046] Figure 2 This is the metallographic structure diagram of the steel used for the steel cord in Comparative Example 3. DETAILED DESCRIPTION

[0047] The present invention provides an ultrafine-grained high-strength steel cord steel with excellent heat resistance, the chemical composition and weight percentage of which are as follows: C 0.86%-0.94%, Si 0.32%-0.45%, Mn 0.25%-0.45%, Cr 0.30%-0.40%, V 0.025%-0.040%, B 0.0005%-0.0009%, Alt≤0.001%, Ti≤0.0005%, P≤0.010%, S≤0.010%, O≤0.0020%, N≤0.0065%, and the remainder is Fe and unavoidable impurity elements;

[0048] Among them, 0.80≤X=Mn / Cr≤1.20; 0.32≤Y=10*V+100*B≤0.45.

[0049] The method for producing ultrafine-grained high-strength steel for steel cord with excellent heat resistance comprises the following steps: molten iron pretreatment → converter smelting → LF refining → continuous casting → heating → high-strength wire controlled rolling → controlled cooling.

[0050] To control Ti inclusions during hot metal pretreatment, low-Ti hot metal is used for production, and pretreatment desulfurization is performed, with the desulfurization target value S≤0.005%. Furthermore, to control Al2O3 inclusions, steel containing Al≥0.010% must not be produced in the previous ladle heat.

[0051] During the converter smelting step, P is effectively removed and "resulfurization" is suppressed, with the end point C controlled at ≤0.10%, P≤0.0080%, and S≤0.08%. When tapping, slag is blocked and refined slag and lime are added when about 1 / 5 of the molten steel is tapped. When about 1 / 3 of the steel is tapped, deoxidizer and alloy are added in the following order: silicon balls → slag → silicon manganese → high carbon ferrochrome → recarburizer. Low Alt alloy is used to strictly prevent Al from being brought into the alloy.

[0052] During the LF furnace refining step, argon is blown from the bottom of the ladle throughout the entire process, and the argon flow rate is based on the requirement that the molten steel does not splash out of the ladle; in order to avoid the formation of large brittle Al2O3-type hard inclusions in the steel, a Si deoxidation process is adopted, and a silicon ball deoxidizer is selected as the deoxidizer. In order to modify the inclusions and make them deformable inclusions to prevent brittle fracture during wire drawing, lime and wollastonite are added to form slag, with basicity (CaO / SiO2) ≤ 1.0, Al2O3 ≤ 5%, and Als in the steel ≤ 5ppm. A large Ar gas flow rate is used for stirring as much as possible without exposing the slag surface; sufficient silicon carbide is added to the slag; the FeO content in the slag is controlled between 2 and 3%; argon is blown from the bottom of the ladle throughout the entire process, and the argon flow rate is based on the requirement that the molten steel does not splash out of the ladle; the use of Al-containing alloys is strictly controlled, and the white slag time is ≥ 20 minutes. Alloys are added before and during refining to adjust the Si, Mn, Cr, V, and B contents based on the composition analysis results before entering the LF furnace.

[0053] During the continuous casting step, electric stirring of the crystallizer and electric stirring at the end + light pressure are used. The crystallizer electromagnetic stirring frequency is 2.0Hz, the current is 200A, the end electromagnetic stirring frequency is 3.0Hz, the current is 250A, the pressure reduction is 15mm, the superheat is controlled at 20-40℃, the pulling speed is 1.9-2.6mm / min, and low-segregation defect-free ingots are obtained. The carbon segregation index of the ingot is ≤1.13.

[0054] In the heating step, the temperature of the soaking section is 1200-1250°C, and the holding time of the soaking section is ≥80 minutes.

[0055] In the high-speed wire controlled rolling step, the starting rolling temperature is 950-1000°C, the finishing rolling temperature is 810-840°C, the sizing temperature is 870-910°C, and the spinning temperature is 890-930°C.

[0056] In the controlled cooling step, a three-stage cooling method is adopted, wherein the first stage has a cooling rate of 7 to 11°C / s, cooling to 600 to 650°C; the second stage has a cooling rate of 7 to 9°C / s, cooling to 500 to 550°C; and the third stage has a cooling rate of ≤3°C / s, cooling to 300 to 400°C.

[0057] The present invention is described in detail below with reference to the embodiments.

[0058] The chemical composition and weight percentage of the steel for steel cord in each embodiment and comparative example are shown in Table 1, with the remainder being iron and unavoidable impurities.

[0059] Table 1 Chemical composition (wt%)

[0060]

[0061] The production method of steel cord steel in each embodiment and comparative example is as follows: molten iron pretreatment → converter smelting → LF refining → continuous casting → heating → high-speed wire controlled rolling → controlled cooling to obtain a specification of of wire rod.

[0062] Some of the main process parameters are shown in Table 2. The chemical composition of Comparative Example 4 is the same as that of Example 4.

[0063] Table 2

[0064]

[0065]

[0066] The microstructure of the hot-rolled wire rod produced by the above process is shown in Table 3, the inclusion rating is shown in Table 4, and the performance is shown in Table 5.

[0067] Table 3

[0068]

[0069] Table 4 Rating of non-metallic inclusions in hot-rolled wire rod

[0070]

[0071]

[0072] Table 5 Mechanical properties of hot rolled wire rod

[0073]

[0074] The hot-rolled wire rods in the above embodiments and comparative examples were prepared into steel wires according to the following steps and performance tests were performed: peeling or pickling → rough drawing to φ1.0-2.0 mm specification → heating at 1100°C → pickling → water bath → electroplating → wet drawing at 40°C to φ0.1-0.2 mm specification.

[0075] Table 6

[0076]

[0077]

[0078] From the above, it can be seen that the solution provided by the present invention can produce a tensile strength R m ≥1270MPa, same-turn difference ≤65MPa, cross-sectional shrinkage Z ≥42% of hot-rolled wire rod, thermal conductivity ≥60.5Wm at 40℃ -1 .K -1 , 80℃ thermal conductivity ≥54.0Wm -1 .K -1 , 120℃ thermal conductivity ≥44.3Wm -1 .K -1After the wire rod is made into steel wire, the tensile strength of the steel wire is ≥4000MPa, 360° torsion ≥22 times, the torsion fracture is good, it has high strength, high toughness, and excellent heat resistance.

[0079] In Comparative Example 1, although the contents of the chemical components meet the requirements of the present invention, the X value does not meet the requirements of the present invention, resulting in insufficient thermal conductivity.

[0080] In Comparative Example 2, although the contents of the chemical components meet the requirements of the present invention, the Y value does not meet the requirements of the present invention, resulting in insufficient toughness.

[0081] In Comparative Example 3, V and B were not added, resulting in insufficient strength.

[0082] In Comparative Example 4, the finishing rolling temperature is too high, and the cooling rate in the cooling step is not properly controlled, resulting in insufficient strength and toughness of the wire rod.

[0083] The above-mentioned reference embodiments provide a detailed description of an ultrafine-grained high-strength steel for steel cord with excellent heat resistance and a production method thereof. This is illustrative rather than restrictive, and several embodiments may be listed within the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. An ultrafine-grained high-strength steel cord steel with excellent heat resistance, characterized in that: The chemical composition and weight percentage of the steel cord steel are as follows: C 0.86-0.94%, Mn 0.25-0.45%, Cr 0.30-0.40%, Si 0.32-0.45%, V 0.025-0.040%, B 0.0005-0.0009%, Alt≤0.001%, Ti≤0.0005%, P≤0.010%, S≤0.010%, O≤0.0020%, N≤0.0065%, and the rest are Fe and unavoidable impurity elements; Among them, 0.80≤X=Mn / Cr≤1.20; 0.32≤Y=10*V+100*B≤0.

45.

2. The ultrafine-grained high-strength steel for steel cord with excellent heat resistance according to claim 1, characterized in that: The hot-rolled microstructure of the steel cord steel is sorbite, the sorbitization rate is ≥95%, the spacing between sorbite lamellae is ≤130nm, and the number of sorbite microstructure orientation positions is ≥8.

3. The ultrafine-grained high-strength steel for steel cord with excellent heat resistance according to claim 1, characterized in that: The hot-rolled tensile strength R of the steel cord steel m ≥1270MPa, same circle difference ≤65MPa, section shrinkage Z ≥42%; thermal conductivity ≥60.5Wm at 40℃ -1 .K -1 , 80℃ thermal conductivity ≥54.0Wm -1 .K -1 , 120℃ thermal conductivity ≥44.3Wm -1 .K -1 .

4. The ultrafine-grained high-strength steel for steel cord with excellent heat resistance according to claim 1, characterized in that: The steel cord is made of steel with a tensile strength of ≥4000 MPa and a 360° torsion of ≥22 times.

5. The method for producing ultrafine-grained high-strength steel for steel cord with excellent heat resistance according to any one of claims 1 to 4, characterized in that: The production method comprises the following steps: smelting→continuous casting→heating→high-wire controlled rolling→controlled cooling.

6. The production method according to claim 5, characterized in that In the continuous casting step, the carbon segregation index of the continuous casting billet is ≤1.

13.

7. The production method according to claim 5, characterized in that In the heating step, the temperature of the soaking section is 1200-1250° C., and the soaking section holding time is ≥80 min.

8. The production method according to claim 5, characterized in that In the high-speed wire controlled rolling step, the starting rolling temperature is 950-1000°C, the finishing rolling temperature is 810-840°C, the sizing temperature is 870-910°C, and the spinning temperature is 890-930°C.

9. The production method according to claim 5, characterized in that In the controlled cooling step, a three-stage cooling method is adopted, wherein the first stage has a cooling rate of 7 to 11°C / s, cooling to 600 to 650°C; the second stage has a cooling rate of 7 to 9°C / s, cooling to 500 to 550°C; and the third stage has a cooling rate of ≤3°C / s, cooling to 300 to 400°C.

10. A steel cord prepared from the steel for ultrafine-grained high-strength steel cord with excellent heat resistance according to any one of claims 1 to 4.

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