JP2012021217A - Steel wire material excellent in mechanical descaling property and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel wire material which is suppressed in scale detachability during production and improved in mechanical descaling properties, and to provide a useful method for producing such a steel wire material.SOLUTION: The steel wire material has a predetermined chemical composition and includes a scale layer formed on the surface. The scale layer has a FeO layer having an average thickness of 5 μm or less and an oxide layer containing voids which is formed under the FeO layer. When the oxide layer is microscopically observed in a cross section perpendicular to the axis of the steel wire material, the part, where the percentage of the voids on a line segment along the circumference of a circle concentric with the surface of the steel wire material is 10% or more and less than 50%, has an average thickness of 0.50 μm or more.

Description

  The present invention relates to a steel wire useful as a material such as saw wire used for cutting semiconductor tire silicon (steel cord, bead wire), hose wire, and semiconductor wire, and particularly after hot rolling (drawing). The present invention relates to a steel wire excellent in mechanical descaling property before processing) and a useful method for producing such a steel wire.

  The steel wire used as a material for the tire cord, hose wire, and saw wire includes a wire drawing step (primary wire drawing, secondary wire drawing) in the secondary processing, and before the wire drawing processing (hot rolling) Later), it is necessary to remove the oxide film (scale) on the surface in order to ensure good wire drawing workability. As a method for removing the scale, pickling (a method for removing by dipping in an acid) and mechanical descaling (a method for removing mechanically by applying an elastic strain: hereinafter, abbreviated as “MD”). )It has been known.

  The scale removal before the wire drawing is mainly performed by pickling. However, pickling has problems such as working environment and disposal of acid after use. For this reason, replacing pickling, MD which removes a scale mechanically has come to be performed widely. However, if the MD is insufficient and the scale remains attached to the surface, in the subsequent wire drawing process, the surface flaws will occur on the product, the processing die will be worn out and the life will be reduced, etc. Cause it.

  For these reasons, steel wires excellent in scale peelability during MD have been demanded, and various techniques have been proposed so far.

  For example, in Patent Document 1, as a method of manufacturing a steel wire rod, the mechanical descaling property (MD property) is improved by oxidizing in a wet atmosphere after hot rolling and ensuring the thickness of FeO in the scale. In addition, a method for suppressing generation of a tertiary scale that occurs due to peeling of the scale during manufacturing, particularly when the coils are bound is disclosed. With this technology, the MD property has been remarkably improved. However, even in this technology, the MD property may be insufficient due to the separation of the scale during production and the formation of a tertiary scale on the new surface. is there.

JP 2008-49391 A

  The present invention has been made under such circumstances, and an object of the present invention is to provide a steel wire rod that suppresses scale peeling during manufacturing and improves mechanical descaling property before wire drawing, and so on. It is to provide a useful method for producing a simple steel wire rod.

  The steel wire rod of the present invention that has achieved the above-mentioned object has C: 0.50 to 2.0% (meaning of mass%, the same applies hereinafter), Si: 0.10 to 0.50%, and Mn: 0.10 Each of which is composed of FeO layer having an average thickness of 5 μm or less. And an oxide layer containing voids formed below the FeO layer, and when the oxide layer is observed under a microscope in a cross section perpendicular to the axis of the steel wire, the surface of the steel wire is concentric The ratio of the void occupying the line segment along the line is 10% or more and less than 50%, and the average thickness is 0.50 μm or more.

  The steel wire rod of the present invention is particularly useful when applied to tire cords, hose wires or saw wires.

  In producing the steel wire as described above, after finish rolling, at a temperature higher than 720 ° C., the contact time with the humid atmosphere is 0 to 5 seconds while cooling to 720 ° C., and the dew point is 30 to 80 ° C. What is necessary is just to make it contact at 530-720 degreeC for 1 second or more in a humid atmosphere. The wet atmosphere is basically an atmosphere containing a water vapor atmosphere or mist water, but may be a mixed atmosphere of the two.

  Moreover, after finishing rolling, it can manufacture also by cooling to 720 degrees C or less in an air atmosphere, and making it contact at 530-720 degreeC for 1 second or more in a humid atmosphere with a dew point of 30-80 degrees C continuously.

  According to the present invention, after finish rolling, by appropriately controlling the atmospheric conditions in which the steel wire is in contact, the scale structure on the surface of the steel wire is improved, thereby realizing a steel wire excellent in MD properties, Such a steel wire is extremely useful as a material for a tire cord, a hose wire, or a saw wire.

It is sectional drawing which showed typically the structure of the scale in the steel wire rod surface of this invention.

  The present inventors have studied from various angles with the aim of realizing a steel wire that exhibits excellent mechanical descaling properties. As a result, after hot rolling, the steel wire is cooled to 720 ° C. or lower in an air atmosphere, and then contacted with a humid atmosphere at a dew point of 30 to 80 ° C. for 1 second or longer in a temperature range of 530 to 720 ° C. ( In the following, it has been found that the scale structure on the surface of the steel wire is improved to have excellent mechanical descaling properties.

The reason why the MD property of the steel wire material becomes excellent by performing the oxidation treatment as described above could be considered as follows. That is, when exposed to a moist atmosphere at a temperature of 530 to 720 ° C., the action of the moist atmosphere promotes the formation of oxides by oxygen diffusion in the steel wire, and the dense oxide scale normally formed in the air atmosphere [ It is considered that an oxide layer (scale layer) containing a large amount of voids (voids) is formed below the magnetite (Fe ₃ O ₄ ) or wustite (FeO)]. Such a scale structure is schematically shown in FIG. 1 (cross-sectional view).

As the oxidation treatment temperature is further increased beyond 720 ° C., the thickness of the FeO layer is increased, and the scale layer is formed into a dense scale property that is formed in an air atmosphere. The physical layer (hereinafter sometimes referred to as “void-containing oxide layer”) is reduced or eliminated. As described above, the change in the scale layer structure due to the difference in the processing temperature is that the chemical composition of the steel wire targeted by the present invention has an A ₁ transformation point of about 720 ° C. It is considered that diffusion of iron atoms occurs actively when the temperature exceeds. Therefore, an oxide layer containing many voids (void-containing oxide layer) can be formed for the first time by controlling the oxidation treatment temperature to 530 to 720 ° C., and a steel wire excellent in MD property can be realized.

  In addition, during the manufacturing process, especially when the secondary scale is peeled off at a heating stage of about 200 to 500 ° C., a new thin scale (tertiary scale: so-called blue scale) is formed on the new surface, and the MD property of the steel wire is reduced. May be exacerbated. However, the void-containing oxide layer formed according to the present invention also exhibits the effect of reducing the difference in thermal expansion / contraction between the scale layer and the steel wire rod (ground iron), so that the adhesion of the secondary scale during cooling is improved. It also maintains the effect of suppressing peeling.

By the above-described method, the structure of the scale layer on the surface of the steel wire is controlled to the form as shown in FIG. 1, and the MD property is improved. In order to exert such an effect, the average thickness of the FeO layer is set to 5 μm or less (preferably 4 μm or less), and the void-containing oxide layer is observed with a microscope in a cross section perpendicular to the axis of the steel wire rod. When the ratio of voids occupying the segment along the circumference concentric with the surface of the steel wire rod (referred to as “void ratio”) is 10% or more, the average thickness of the portion less than 50% is 0.50 μm or more. (Preferably 0.7 μm or more), and even if any requirement is lacking, the object of the present invention cannot be achieved. In addition, the upper limit of the void ratio was set to less than 50% in order to prevent scale peeling at the time of manufacture (particularly during coil binding). Further, the thickness of the Fe ₃ O ₄ layer shown in FIG. 1 also varies depending on the manufacturing conditions, but does not affect the MD property unlike the FeO layer.

  In order to form the void-containing oxide layer as described above, it is also important to control the dew point of the humid atmosphere to 30 to 80 ° C (preferably 40 to 80 ° C). If the dew point is less than 30 ° C., the effect of the oxidation treatment is small, and the void-containing oxide layer as described above cannot be formed. On the other hand, when the dew point exceeds 80 ° C., not only the scale generation becomes excessive and the scale loss increases, but also scale peeling occurs (peeling after rolling), which causes the formation of a tertiary scale.

  The dew point in the wet atmosphere can be measured by collecting the atmospheric gas near the surface of the steel wire and measuring it with a dew point meter. To form a wet atmosphere controlled to the dew point as described above, steam or mist water is used. What is necessary is just to make it vaporize on the surface of a hot steel wire rod by spraying. At this time, in order to form a wet atmosphere having a dew point of 30 to 80 ° C. with mist water, it is preferable to sufficiently evaporate the mist by setting the particle diameter of the mist water as fine as 100 μm or less. The oxidation treatment time must be at least 1 second (preferably 10 seconds or more), but is preferably 100 seconds or less (more preferably 90 seconds or less) from the viewpoint of reducing the scale loss of the steel material. ).

  Although the basic conditions of the oxidation treatment in the present invention are as described above, the contact treatment (oxidation treatment) in a wet atmosphere was started at a temperature exceeding 720 ° C., and then heated at 530 to 720 ° C. for 1 second or longer. Even in this case, a void-containing oxide layer is formed. However, when heating for 5 seconds or more in a humid atmosphere with a dew point of 30 ° C. or higher at a temperature higher than 720 ° C., the FeO layer becomes too thick (thicker than 5 μm). Since the scale is peeled off at the time of coil binding and a tertiary scale is easily formed, the MD property is deteriorated.

  In manufacturing the steel wire of the present invention, conditions other than the oxidation treatment described above may follow a conventional method. A steel material having a predetermined chemical composition (described later) is melted to form a steel billet, and the steel billet is heated to about 1250 ° C. or less in a heating furnace, and subjected to a rough rolling process and a finishing rolling process, and a diameter of 10 mm or less ( For example, the steel wire is made by hot rolling up to about 5 mm. At this time, if rolling is performed while the high-temperature oxide film (primary scale) formed in the heating furnace adheres to the surface, a part of the scale is embedded in the surface of the steel wire rod, resulting in product defects, a rolling roll In general, scale removal (descaling) is performed before each of the rough rolling process and the finishing rolling process. The descaling performed before these hot rolling steps can be usually performed by spraying high-pressure water (the spray pressure at this time is called “descaling pressure”).

  Then, after finish rolling, at a temperature higher than 720 ° C., the contact time with the wet atmosphere is 0 to 5 seconds (preferably in the air atmosphere), and the cooling is performed to 720 ° C., within a temperature range of 530 to 720 ° C. Dew point: Oxidation is performed for 1 second or longer in a humid atmosphere of 30 to 80 ° C. As a result, an FeO layer having a predetermined thickness and an oxide layer containing a predetermined void are formed.

  In addition, although the scale (secondary scale) formed after descaling before finish rolling is shipped while adhering to the surface of the steel wire material, this secondary scale has a diameter in the secondary wire drawing process. : Removed by pickling or descaling before drawing to about 0.8 to 1.2 mm.

  The steel wire according to the present invention is basically made for the purpose of improving the mechanical descaling property (MD property), but depending on the use condition of the product, it is descaled by pickling. It can also be.

  The steel wire rod according to the present invention has an improved MD property by controlling the structure of the scale layer to an appropriate form, and the chemical composition of the steel wire rod only needs to satisfy the characteristics required as a steel wire rod. . From such a viewpoint, the basic components in the steel wire material targeted by the present invention are C: 0.50 to 2.0%, Si: 0.10 to 0.50%, and Mn: 0.10 to 3.0%. Each contained. The reason why the chemical composition of the steel wire material is determined in this way in the present invention is as follows.

[C: 0.50 to 2.0%]
C is an element necessary for ensuring the strength of the steel wire rod. In order to ensure the strength required for the tire cord after wire drawing, the C amount needs to be 0.50% or more. On the other hand, if the amount of C is excessive, cold workability deteriorates, so the upper limit of the amount of C was set to 2.0%. The amount of C is preferably 0.70% or more (more preferably 0.80% or more), preferably 1.2% or less (more preferably 0.84% or less).

[Si: 0.10 to 0.50%]
Si is an important element for securing the strength of the steel wire rod, and its lower limit is set to 0.10% as the minimum necessary Si content. However, if the Si content is excessive, the wire rod becomes too strong and impairs ductility, so the upper limit was set to 0.50%. The Si content is preferably 0.12% or more (more preferably 0.15% or more), preferably 0.40% or less (more preferably 0.30% or less).

[Mn: 0.10 to 3.0%]
Mn is an important element for securing the strength of the steel wire rod, and its lower limit is 0.10% as the minimum required Mn content. However, if the Mn content is excessive, the wire rod becomes too strong and impairs ductility, so the upper limit was set to 3.0%. The Mn content is preferably 0.30% or more (more preferably 0.40% or more), preferably 1.0% or less (more preferably 0.60% or less).

  The basic component composition of the steel wire of the present invention is as described above, and the balance is substantially iron. However, it is naturally allowed that inevitable impurities (for example, P, S, Cr, Cu, Ni, Al, O, N, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. are included in the wire. . However, these inevitable impurities are preferably controlled as follows.

[P: 0.05% or less (excluding 0%)]
P is segregated at the grain boundaries and impairs the toughness of the steel wire, so 0.05% or less is preferable. The P content is more preferably 0.02% or less, still more preferably 0.01% or less.

[S: 0.05% or less (excluding 0%)]
S forms MnS, which is a sulfide-based inclusion, and segregates during hot rolling of the steel wire, causing the steel wire to become brittle, so 0.05% or less is preferable. The S content is more preferably 0.015% or less, still more preferably 0.010% or less.

[Cr: 0.1% or less (excluding 0%)]
If a large amount of Cr is contained, the ductility is impaired. Therefore, in a steel wire that requires wire drawing, the content is preferably 0.1% or less. More preferably, it is 0.050% or less, More preferably, it is 0.03% or less.

[Cu: 0.1% or less (excluding 0%)]
Cu becomes a liquid layer at about 1080 ° C., and enters the austenite grain boundary to impair the drawability. Therefore, the Cu content is preferably 0.1% or less. More preferably, it is 0.040% or less, More preferably, it is 0.02% or less.

[Ni: 0.1% or less (excluding 0%)]
Ni is preferably concentrated to 0.1% or less in order to thicken unevenly on the surface of the steel wire, increase the unevenness of the scale surface, and deteriorate the scale peelability. More preferably, it is 0.040% or less, More preferably, it is 0.02% or less.

[Al: 0.03% or less (excluding 0%)]
When the Al content is excessive, a coarse oxide is generated and the wire drawing property is deteriorated. Therefore, the Al content is preferably 0.03% or less. More preferably, it is 0.01% or less, More preferably, it is 0.004% or less.

[N: 0.02% or less (excluding 0%)]
If the N content is excessive, the ductility deteriorates, so 0.02% or less is preferable. More preferably, it is 0.01% or less, More preferably, it is 0.005% or less.

[O: 0.01% or less (excluding 0%)]
When the amount of O (oxygen) in the steel is too large, a coarse oxide is formed and the wire drawing property is deteriorated. Therefore, the O content is preferably 0.01% or less. More preferably, it is 0.004% or less, More preferably, it is 0.002% or less.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Example 1]
A steel billet having a chemical component composition shown in Table 1 and having a cross section of 150 mm × 150 mm was prepared, heated in a heating furnace, and the primary scale generated in the heating furnace was removed (descaling), followed by rolling. After rolling the rolled steel wire, it was oxidized in a wet atmosphere (water vapor atmosphere) at a predetermined temperature, dew point, and time, and then cooled to obtain a steel wire (diameter: 5.5 mm). The temperature of the steel wire was adjusted by heat generation immediately after rolling and air cooling, and the temperature was measured using a radiation thermometer.

  The scale properties (the thickness of the FeO in the scale, the thickness of the oxide layer having a void ratio of 10% or more and less than 50%) adhered to the surface of the obtained steel wire were measured by the following method.

[Measurement of FeO thickness in scale]
As for the thickness of FeO in the scale, samples with a length of 10 mm were taken from the front end, the center, and the rear end of the coil, and any three scale sections from each sample were observed with a scanning electron microscope (SEM). (Observation magnification: 1000 times) and measured as an average value of the thickness of FeO having a length of 100 μm in the circumferential direction of the steel wire.

[Measurement of thickness of oxide layer having void ratio of 10% or more and less than 50%]
The thickness of the oxide layer containing voids (void-containing oxide layer) was determined by taking samples with a length of 10 mm from the front end, center, and rear end of the coil. Is observed with a scanning electron microscope (SEM) (observation magnification: 2000 times), a line segment having a length of 100 μm is taken along the circumference of the steel wire surface, and the ratio of the length of the void portion in the line segment The thickness (average thickness) of layers having a thickness of 10% or more and less than 50% was measured.

  The results are shown in the following Table 2 (Test Nos. 1 to 28) together with rolling conditions (heating temperature, descaling pressure after rolling) and oxidation treatment conditions (starting temperature, finishing temperature, dew point, time).

  Moreover, the peeling state (scale adhesion) of the steel wire rod after hot rolling and the mechanical descaling property (MD property) after the oxidation treatment were also evaluated by the following methods. The results are shown in Table 3 below (Test Nos. 1 to 28).

[Evaluation of scale peeling state of steel wire after hot rolling]
The scale peeling state of the steel wire rod after hot rolling (the peeling state of the scale after rolling) is three samples each having a length of 500 mm from the front end, the center portion, and the rear end of the coil after hot rolling. The surface appearance of the outer peripheral surface and inner peripheral surface of each sample was photographed with a digital camera, and the area ratio (scale peeling area ratio) of the part where the scale peeled was calculated by image analysis, and the average value was obtained. . If this area ratio (corresponding to the area ratio of the tertiary scale) is 3% or less, it is judged as pass (evaluation “◯”), and if it exceeds 3%, it is rejected (evaluation “x”). When the area ratio is 3% or less, the wire drawing workability (surface wrinkles and die life) in the secondary working process becomes good.

[Evaluation of mechanical descaling property (MD property)]
The mechanical descaling property (MD property) was evaluated by cutting and collecting the oxidized steel wire to a length of 250 mm, applying a tensile load of up to 4% using a tensile tester, and taking it out from the chuck. After blowing wind on the sample to blow off the scale on the surface of the steel wire, the appearance was photographed with a digital camera, and the residual scale area ratio was calculated by image analysis. If the residual scale area ratio is 10% or less (corresponding to approximately 0.05 mass% or less of the scale residual amount per steel wire), it is accepted (evaluation “◯”), and the case where it exceeds 10% is rejected. (Evaluation "x"). When the area ratio is 10% or less, the wire drawing workability (surface wrinkles and die life) in the secondary working process becomes good.

  From the results shown in Tables 2 and 3, it can be considered as follows. In an example in which an oxidation treatment was performed for 1 second or more in a humid atmosphere having a dew point of 30 ° C. or higher in a temperature range of 530 to 720 ° C. (Test No. 3, 6, 10-13, 16-20, 23, 25), the thickness of the FeO layer is 5 μm or less, and the thickness of the oxide layer with a void ratio of 10% or more and less than 50% is 0.50 μm or more. It can be seen that the residual scale area ratio after the MD test is suppressed to 10% or less.

  On the other hand, in the examples (test Nos. 1, 2, 4, 5, 7 to 7) that are outside the appropriate production conditions defined in the present invention (that is, out of the temperature range of 530 to 720 ° C. or less than 30 ° C. dew point). 9, 14, 15, 21, 22, 24, 26-28), the thickness of the FeO layer is 5 μm or less, and the thickness of the oxide layer is 0.1% or more and less than 50%. It becomes thinner than 50 μm, and it can be seen that the residual scale after the MD test is not suppressed to 10% or less.

[Example 2]
Example 1 above shows the results when the oxidation treatment temperature is controlled to be constant (that is, the oxidation treatment start temperature and the end temperature are the same), but the steel manufactured under the condition that the temperature decreases during the oxidation treatment. The wire was evaluated in the same manner as in Example 1 (other conditions are the same as in Example 1).

  Scale properties (FeO thickness in the scale, thickness of oxide layer with a void ratio of 10% or more and less than 50%) attached to the surface of the steel wire rod, rolling conditions (heating temperature, descaling pressure after rolling) It is shown in the following Table 4 (test Nos. 29 to 40) together with the oxidation treatment conditions (start temperature, end temperature, dew point, time). Moreover, the peeling state (scale adhesion) of the steel wire rod after hot rolling and the mechanical descaling property (MD property) after the oxidation treatment are shown in Table 5 below.

  From the results shown in Tables 4 and 5, it can be considered as follows. In the example in which the oxidation treatment was performed while appropriately suppressing the time (stay time) in the temperature range exceeding 720 ° C. (Test Nos. 30, 32, 33, 35, 36, 39), the thickness of the FeO layer was 5 μm. The thickness of the oxide layer having a void ratio of 10% or more and less than 50% is formed to 0.50 μm or more, and the residual scale area ratio after the MD test is suppressed to 10% or less. I understand that.

  On the other hand, even when the oxidation process is outside the appropriate manufacturing conditions defined in the present invention, that is, when the oxidation process is started at a temperature exceeding 720 ° C. and the oxidation process is terminated at 720 ° C. or lower, the oxide layer containing voids Although the oxide layer is not formed in the example of test No. 29, the thickness of the FeO layer is increased at the same time (test Nos. 31, 34, 37, 38, 40) It can be seen that the scale peeling area ratio after rolling deteriorates (over 3%).

Claims (4)

  C: 0.50 to 2.0% (meaning by mass%, hereinafter the same), Si: 0.10 to 0.50% and Mn: 0.10 to 3.0%, respectively, with the balance being Fe and A steel wire made of unavoidable impurities and having a scale layer formed on the surface, wherein the scale layer includes an FeO layer having an average thickness of 5 μm or less, and a void formed below the FeO layer. When the oxide layer is observed with a microscope in a cross section perpendicular to the axis of the steel wire rod, the proportion of voids on the line segment concentric with the steel wire surface is 10%. As mentioned above, the steel wire rod excellent in mechanical descaling property characterized in that the thickness of the portion that becomes less than 50% is 0.50 μm or more on average.   The steel wire according to claim 1, which is applied to a tire cord, a hose wire or a saw wire.   In producing the steel wire rod according to claim 1 or 2, after finishing rolling, at a temperature higher than 720 ° C., the contact time with the humid atmosphere is 0 to 5 seconds while cooling to 720 ° C., and the dew point is 30 A method for producing a steel wire rod excellent in mechanical descaling property, wherein the contact is made at 530 to 720 ° C for 1 second or longer in a wet atmosphere at -80 ° C.   In producing the steel wire rod according to claim 1 or 2, after finish rolling, the steel wire is cooled to 720 ° C. or lower in an air atmosphere, and 1 at 530 to 720 ° C. in a humid atmosphere having a dew point of 30 to 80 ° C. A method for producing a steel wire rod excellent in mechanical descaling property, characterized by contacting for at least 2 seconds.

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