WO2023056658A1 - Cord steel wire rod suitable for deep drawing and manufacturing method therefor - Google Patents

Abstract

The present invention relates to a cord steel wire rod suitable for deep drawing and a manufacturing method therefor. The wire rod has a metallographic structure of sorbite+pearlite+ferrite, wherein the proportion of the sorbite is controlled within 60-70%, the proportion of the pearlite is controlled within 30-40%, and the ferrite exists as a small amount of tissue and has a proportion of less than or equal to 10%. The thickness of a pearlite lamella is 0.10-0.35 mm, the size of a pearlite colony is 10-20 um, and the feature of the pearlite corresponds to a smaller dislocation density. The level of central carbon segregation is evaluated according to YB/T4413 "metallographic evaluation method for central carbon segregation of high-carbon steel wire rod" and lower than or equal to level 1, and the level of network cementite in the tissue is lower than or equal to level 1. The tensile strength σ is (103762 * Ceq to 114606 * Ceq) MPa, and the reduction of area is 38% or above, such that the cord steel wire rod is more suitable for deep drawing.

Description

A cord steel wire rod suitable for deep drawing and its manufacturing method technical field

The invention belongs to the technical field of metallurgy, and in particular relates to a cord steel wire rod and a manufacturing method thereof.

Background technique

The rapid development of automobile lightweight and the rapid rise of new energy vehicles have gradually increased the strength requirements of steel cords for automobile tire frames. The strength grade of the steel cord has gradually developed from the initial ordinary strength (NT) grade to the current super high strength (ST) grade super high strength and extremely high strength (UT) grade ultra high strength grade.

The higher the strength of the steel wire, the thinner the diameter after drawing and the more complex the structure. Therefore, for the wire rod for cord steel, the inclusions in the steel should be reasonably controlled, mainly MnS and deformable 40% SiO ₂ +15% Al Deformable inclusions such as ₂ O ₃ +20% CaO composite inclusions will help to improve the drawability of the wire rod. In addition, the proportion of sorbite structure in the wire rod structure will also affect the drawing performance of the wire rod. Generally, the wire rod undergoes isothermal transformation in the temperature range of 600-650 ℃ after cooling, and the transformed structure is sorbite. For high carbon For steel wire rods, the proportion of sorbite is generally above 85%. If the sorbite is too high and the pearlite sheets are thin, the tensile strength of the wire rod will be too high, the dislocation density will be high, and the drawing hardening will be increased. It is good for deep drawing; the proportion of sorbite is too low, the strength of the wire rod is low, the plasticity is poor, and the deformation ability of the wire rod is poor, which is also not suitable for deep drawing. Therefore, only by controlling the proportion of sorbite in the wire rod and the tensile strength within an appropriate range can the requirements of deep drawing of wire rod and low broken wire rate be met, and further promote the development of steel cord in the direction of high strengthening and refinement .

By solving the above technical problems, the popularization of production of UT grade steel cords above 4000MPa and wire rods for cutting steel wires can be realized.

Contents of the invention

The object of the present invention is to provide a cord steel wire rod suitable for deep drawing, which is intended to reduce the proportion of sorbite in the wire rod structure. The metallographic structure of the wire rod is sorbite+pearlite+ferrite, wherein the proportion of sorbite is controlled at 60-70%, the proportion of pearlite is 30-40%, and the ferrite exists in a small amount. Ratio ≤ 10%. The proportion of each structure in the wire rod refers to the area ratio of each structure in the metallographic structure map (such as SEM enlarged image) of the wire rod.

Further, the present application intends to obtain a pearlite structure with thicker sheets, the thickness of the pearlite sheet is 0.10-0.35mm, and the size of the pearlite group is 10-20um, so as to reduce the dislocation density of the metallographic structure to avoid tensile strength. Too large will inhibit drawing hardening.

Further, the present application intends to control the central carbon segregation level of the wire rod. According to YB/T 4413 "High Carbon Steel Wire Rod Central Carbon Segregation Metallographic Evaluation Method", the central carbon segregation level of the wire rod of the present invention is ≤1 Grade, so that the precipitation of reticulated cementite during the cooling process of the wire rod can be effectively suppressed. The grade of reticulated cementite in the structure is ≤ grade 1, which is grade 0 in this application.

Furthermore, the thickness of the _oxide skin on the surface of _the wire rod is 13-18um, which is _5um thicker than the average thickness of the existing oxide layer _. It is easy to achieve the ideal (mechanical) descaling effect, which helps to significantly reduce the wire rod breaking rate during drawing.

The tensile strength σ of the wire rod of the present invention is in (103762*Ceq～114606*Ceq) MPa, the carbon equivalent Ceq=C+Mn/6+Cr/5 in the formula, and the element symbol represents the weight percentage of this element in the steel in the formula content, moderate strength, more suitable for deep drawing, significantly reducing the broken wire rate. The wire rod tensile strength range σ=(120387*Ceq～131231*Ceq) MPa produced by the prior art, the tensile strength is mainly determined by two factors: one is the chemical composition, i.e. carbon equivalent; the other is the cooling strength during cooling , The phase transition temperature of the wire rod, the cooling intensity is small, the phase transition temperature is high, and the wire rod strength is low; the cooling intensity is large, the phase transition temperature is low, and the wire rod strength is high. This application mainly prevents the tensile strength from being too high by controlling the cooling intensity.

The chemical composition of the wire rod is calculated by mass percentage: C: 0.70-0.99%, Si: 0.15-0.30%, Mn: 0.15-0.60%, Cr: 0.01-0.50%, and the balance is Fe and unavoidable impurity elements. The strength involves 72, 82, 86 and 92 cord steels.

The action principle of the chemical elements in the application wire rod is as follows:

C is the main strengthening element in high-carbon hard wire steel. It improves the strength of steel through solid solution strengthening and precipitation strengthening. With the increase of carbon content, the strength of wire rod increases, and the strength of drawn steel wire also increases. However, with the increase of carbon content in hypereutectoid steel, the probability of precipitation of reticular cementite in the core of wire rod also increases. Therefore, the range of C content in the present invention is set at 0.78-0.99%.

Si is the main deoxidizing element in cord steel, and harmless SiO2 inclusions are generated through Si deoxidation. But Si is also an element that strengthens ferrite at the same time. Too high Si content in the cord steel will cause the ferrite phase plasticity in the pearlite to decrease, and the steel wire drawing ductility will deteriorate. Therefore, the Si content of the present invention is controlled at 0.15- 0.30%.

In cord steel, Mn can be deoxidized to form deformable MnS inclusions, and at the same time it is the main strength-enhancing element in steel. However, Mn is also an element that is easy to segregate. Excessively high Mn will aggravate the segregation of steel, and at the same time, it will improve the hardenability of steel, increase the degree of supercooling when austenite is cooled, make the pearlite sheet thinner, increase the strength, and improve plasticity. Difference. Therefore, the content of Mn in the present invention is controlled at 0.15-0.60%.

Cr in cord steel can promote the downward movement of the C curve, delay the time of sorbite phase transition, reduce the phase transition temperature, refine the interlamellar spacing of pearlite, significantly improve the plasticity index of steel, and significantly improve the drawing performance , to achieve large drawing deformation, reduce intermediate heat treatment, and improve the final strength of the drawn steel wire. However, if the Cr content in the cord steel is too high, it is easy to form supercooled structure bainite and martensite in the cooling process, and the plastic deformation is poor, which affects the drawing performance of the steel. In the present invention, Cr is preferably controlled below 0.35%.

In addition, the application also provides the manufacturing method of the cord steel wire rod, the specific production process: including KR molten iron pretreatment, converter smelting, LF refining, billet continuous casting, billet heating and rolling, and wire rod cooling in sequence.

in:

Smelting molten steel conforming to the composition design, using continuous casting technology to continuously cast molten steel into billets, controlling the superheat of the tundish at 15-30°C, equipped with electromagnetic stirring equipment, adopting dynamic light reduction equipment at the end of solidification, and pulling and straightening each zone The pressure roller adopts the displacement reduction mode, and the specific displacement reductions of the 1#-6# rollers are 2mm, 2mm, 3mm, 4mm, 4mm, and 4mm respectively. Compared with conventional continuous casting slab carbon segregation control which mainly adopts light reduction technology, the dynamic reduction of this application adopts the combined reduction technology of "light reduction + heavy reduction". Use under heavy pressure. Continuous casting slab carbon segregation index ≤ 1.05, wherein continuous casting slab carbon segregation index = continuous casting slab core C%/smelting C%.

Select the appropriate heating temperature for billet rolling. Specifically, the temperature in the high temperature section of the heating furnace before rolling is above 1180°C, the total heating time is above 120 minutes, and the high temperature time is above 60 minutes to ensure sufficient temperature and time for the billet to diffuse. Carbon segregation in the core is further suppressed by high temperature diffusion.

The final rolling temperature of the wire rod is controlled at 800-900°C, the rolling speed is set at 95-120m/s, and the spinning temperature is 850-950°C.

The wire rod coil after spinning is controlled and cooled on the air-cooled roller table, and the speed range of the roller table is 0.95-1.05m/s, increasing from 0.95m/s. Turn on the 1-3# fan to speed up the initial cooling rate of the wire rod, promote the rapid cooling of the wire rod from the spinning temperature to below 700°C, promote the refinement of the size of pearlite clusters, and inhibit the precipitation of network carbon. A small amount of ferrite will precipitate at this stage ;In the fast cooling stage, the cooling speed is preferably controlled at 15-20°C/s, and the opening degree of the fan air volume is adjusted according to the ambient temperature. The specific ambient temperature is above 20°C, and the opening degrees of 1-3# fans are 90%, 90%, and 70% respectively . The specific ambient temperature is below 20°C, and the opening degrees of 1-3# fans are 90%, 90%, and 50% respectively. The subsequent fans (the remaining fans after 3#) are all turned off. The main reason for the shutdown is to fully and slowly cool the wire rod into the phase transformation zone, transforming from austenite to sorbite and pearlite, increasing the sorbite phase transformation temperature, and realizing The phase transformation of sorbite is around 650°C, and at the same time, the phase transformation time is extended to realize isothermal transformation. The release of phase change latent heat during the phase change process will cause the temperature of the wire rod to rise again. With the completion of the phase change, the temperature of the wire rod will drop again. After the temperature of the wire rod drops below 570°C, the phase change will basically end. Set the heat preservation cover for slow cooling to promote the thickening of the oxide layer on the surface of the wire rod, and the FeO in the oxide layer is further transformed into Fe ₃ O ₄ , reducing the ratio of FeO/Fe ₃ O ₄ .

For the insulation cover setting on the air-cooled roller table: 1-11# insulation cover corresponds to the fast cooling and phase change intervals, the wire rod does not set the insulation cover on the air-cooled roller table corresponding to these two cooling intervals, and the insulation cover after 12# is all Close, ensure that the wire rod is fully insulated at about 570°C, fully cooled slowly, increase the thickness of the oxide scale, adjust the composition of the oxide layer to make it more conducive to descaling of the wire rod, and reduce the rate of wire breakage during drawing.

Compared with the prior art, the present invention has the advantages of:

(1) The wire rod produced by the present invention has thick pearlite sheets and small pearlite cluster size, which can adapt to the deep drawing of wire rods, reduce the tendency of cracks to occur in steel wires under the conditions of drawing with a large area reduction ratio, and effectively reduce fractures. The wire rate and the broken wire rate are 30% lower than the normal process.

(2) C in the wire rod: 0.70-0.99%, belongs to high-carbon wire rod steel, and the sorbite ratio of high-carbon wire rod steel is generally more than 85%. The cold process controls the proportion of sorbite in the wire rod to 60-70%, and the proportion of coarse flaky pearlite to 30-40%. The tensile strength of the wire rod can be effectively reduced by coarsening the sheet layer. On the one hand, it can reduce the wear of the abrasive tool and the surface heating of the steel wire during the rough drawing stage, reducing the risk of surface defects, thereby reducing the rate of wire breakage during drawing. On the other hand, as far as deep drawing is concerned, the pearlite sheet is too thin, and the true strain of the steel wire becomes smaller when the area shrinks during the drawing process, that is, the steel wire can maintain stable plasticity and the acceptable amount of drawing area reduction is reduced, that is The deep drawability of steel wire is reduced.

(3) The wire rod produced by the present invention has an oxide layer thickness of 13-18um, which is 5um thicker than the normal process, and the oxide layer is composed of FeO/Fe ₃ O ₄ =(2～2.5)/1, which is more conducive to the mechanical descaling of the wire rod , reduce the drawing loss on the surface of the wire rod when descaling is not clean, and affect the wire breaking rate of the steel wire.

Description of drawings

Fig. 1 is the microstructure figure of the wire rod of embodiment 1 of the present invention;

Fig. 2 is a schematic diagram of the oxide layer of the wire rod in Example 2 of the present invention.

Detailed ways

The present invention will be described in further detail below in conjunction with specific examples.

Taking smelting 100 tons of molten steel to manufacture cord steel wire rods as an example, the process of molten iron pretreatment + converter smelting + LF refining + continuous casting into billets was used to produce billets of each embodiment with the following components, and the carbon segregation index of the continuous casting billets Satisfy ≤1.05.

The continuous casting billet is heated in the furnace to ensure that the soaking temperature is above 1180°C, and the high temperature above 1180°C is maintained for more than 4 hours to ensure that the billet is fully heated and diffused to further reduce carbon segregation. In the austenite phase zone, the continuous casting billet is continuously rolled into wire rod. The final rolling temperature of the wire rod is controlled at 800-900°C, the rolling speed is set at 95-120m/s, and the spinning temperature is controlled at 900-950°C. Increase the spinning temperature to increase the stability of austenite, and the cooling of the wire rod coil after spinning adopts fast cooling + slow cooling + heat preservation. The speed of the roller table where the coil of the wire rod is located is set at 0.95m/s, and the speed of the roller table is gradually increased to 1.05m/s to widen the distance between the coils. Turn on the 1-3# fan to achieve fast cooling, accelerate pearlite nucleation, and inhibit pearlite growth. The fan opening degree is 90%, 90%, and 30%. After 3#, the fan is turned off for slow cooling, and the wire rod enters the phase transition zone. , the phase transition of the wire rod is under the condition of ambient temperature and its own latent heat release, and the phase transition temperature is increased to about 650°C, and the time of the sorbite phase transition is prolonged. When the temperature of the wire rod drops and drops below 570°C, the phase transition is considered Completed, at this _time correspondingly close _the 12# rear insulation cover to keep warm the wire rod after the phase change, and promote the transformation of the FeO layer on the surface of the wire rod into Fe ₃ O ₄ Adjust to (2～2.5)/1, the descaling effect of the wire rod is better, and the surface defect and the broken wire rate during drawing are reduced.

Examples 1-5 refer to Table 1 for the elemental composition of the wire rod and the segregation index of the slab.

Table 1

the	C	Si	mn	Cr	Ceq	Slab Segregation Index
Example 1	0.72	0.18	0.51	0.04	0.813	1.04
Example 2	0.82	0.18	0.51	0.03	0.913	1.05
Example 3	0.82	0.18	0.35	0.34	0.946	1.03
Example 4	0.86	0.18	0.51	0.02	0.953	1.03
Example 5	0.92	0.18	0.35	0.18	1.01	1.02

Refer to Table 2 for the specific process parameters of Examples 1-5 and two comparative examples.

Table 2

The detection performance of the final wire rods of Examples 1-5 and two comparative examples are shown in Table 3.

table 3

Through the comparison of the above examples and comparative examples, it can be proved that: using the controlled cooling method of the present application, especially the slow cooling in the controlled cooling process for phase transformation and the subsequent slow cooling with a cover, can significantly change the content of the sorbite structure, The distance between pearlite sheets can be controlled to control the tensile strength of the wire rod and improve the drawing performance.

Although the preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations of the present invention will occur to those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims (13)

1. A cord steel wire rod suitable for deep drawing, characterized in that: the metallographic structure of the wire rod is sorbite + pearlite + ferrite, wherein the proportion of sorbite is controlled at 60-70%, The proportion of pearlite is 30-40%, and the proportion of ferrite is less than or equal to 10%.
2. The cord steel wire rod suitable for deep drawing according to claim 1, characterized in that: the thickness of the pearlite sheet is 0.10-0.35mm, the size of the pearlite cluster is 10-20um, and the pearlite characteristics correspond to smaller dislocation density.
3. The cord steel wire rod suitable for deep drawing according to claim 1, characterized in that: the central carbon segregation level of the wire rod is evaluated according to YB/T 4413 "Metallographic Evaluation Method for Central Carbon Segregation of High Carbon Steel Wire Rod" , ≤ grade 1, the grade of reticular cementite in the organization is ≤ grade 1.
4. The cord steel wire rod suitable for deep drawing according to claim 1, characterized in that: the tensile strength σ of the wire rod is (103762*Ceq～114606*Ceq) MPa, where the carbon equivalent Ceq=C+ Mn/6+Cr/5, the element symbol in the formula represents the weight percentage of the element in the wire rod.
5. The cord steel wire rod suitable for deep drawing according to claim 1, characterized in that: the thickness of the oxide layer on the surface of the wire rod is 13-18um, FeO/Fe ₃ O ₄ =2-2.5:1 in the oxide layer, Has a better descaling effect.
6. The cord steel wire rod suitable for deep drawing according to claim 1, characterized in that: the area reduction of the wire rod is ≥ 38%.
7. The cord steel wire rod suitable for deep drawing according to claim 1, wherein the chemical composition of the wire rod is: C: 0.70-0.99%, Si: 0.15-0.30%, Mn: 0.15-0.60%, Cr: 0.01-0.50%, and the balance is Fe and unavoidable impurity elements.
8. The cord steel wire rod suitable for deep drawing according to claim 7, wherein the strength of the wire rod involves cord steels of grade 72, grade 82, grade 86 and grade 92.
9. A method for manufacturing a cord steel wire rod suitable for deep drawing, characterized in that: the molten steel after smelting is cast into a billet, the carbon segregation index of the billet is ≤1.05, and the carbon segregation index= billet core Part C%/Smelting C%;

   Heating the billet, the temperature in the high temperature section should be above 1180°C, and the holding time in the high temperature section should be more than 60 minutes, then rolling the billet into continuous wire rod, the final rolling temperature is 800-900°C, and the spinning temperature is 850-950°C;

   The wire rod coil after spinning is controlled and cooled on the air-cooled roller table: the fan is turned on to quickly cool the wire rod from the spinning temperature to below 700°C at a cooling rate of 15-20°C/s, and a small amount of ferrite will be produced at this stage , and promote the number of pearlite nucleation; then the wire rod enters the phase transformation zone, the fan is turned off, and the wire rod transforms from austenite to sorbite and pearlite at room temperature; when the temperature of the wire rod drops below 570°C, it is The wire coil is equipped with a heat preservation cover, so that the wire rod that has completed the transformation of the sorbite is fully and slowly cooled in the heat preservation cover, and the thickening of the oxide layer is promoted, and the FeO in the oxide layer is further transformed into Fe ₃ O ₄ .
10. The method according to claim 9, characterized in that: the smelting of molten steel involves KR molten iron pretreatment, converter smelting, and LF refining, and the LF refining is directly cast into a billet without vacuum degassing.
11. The method according to claim 9, characterized in that: the molten steel is cast into a billet by continuous casting process, the superheat of the tundish is controlled at 15-30°C, equipped with electromagnetic stirring equipment, and the solidification end adopts dynamic soft reduction equipment, In addition, light reduction is used in the early stage of solidification of the slab, and heavy reduction is used in the later stage of solidification.
12. The method according to claim 11, characterized in that: the dynamic light reduction device means that the pressure rollers in each area of the tension leveler adopt the displacement reduction mode, wherein the displacement reductions of the 1#-6# pressure rollers are respectively 2mm, 2mm, 3mm, 4mm, 4mm, 4mm.
13. The method according to claim 9, characterized in that: the speed of the air-cooled roller table is 0.95-1.05m/s, increasing from 0.95m/s to gradually increase the distance between coils of the wire rod.

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