JP4375149B2 - High strength low alloy steel wire - Google Patents

Description

  The present invention relates to a high-strength low-alloy steel wire excellent in wire drawing workability. More specifically, the present invention relates to a high-strength low-alloy steel wire material that has an oxide scale that easily peels during mechanical descaling and has excellent wire drawing workability.

  The wire for a cord wire is required to have excellent wire drawing workability because it is hot-rolled to a diameter of about 1 to 2 mm after being hot rolled to a diameter of about 5.5 mm. For this reason, it is manufactured with due consideration to the scale properties, microstructure, cleanliness, center segregation, and the like. In particular, care is required regarding the scale properties.

  Here, for the purpose of removing scale generated on the surface before wire drawing, cold bending is applied with a bending roll or the like, but at this time, if the scale properties are poor, the scale is difficult to peel off or drop off, At the time of wire drawing, die baking due to residual scale (die life reduction) and wire breakage are caused, and wire drawing workability is deteriorated.

  By the way, in recent years, due to the need for higher fuel consumption of automobiles, there has been an increasing demand for weight reduction and strength improvement of cord wires installed inside tires, and in order to satisfy these demands, high-carbon steel has been made. Furthermore, a wire rod to which an alloy element such as Si or Cr is added may be used.

  However, when producing high-carbon low-alloy steel (high-strength low-alloy) wire, wire breakage occurs due to pro-eutectoid cementite when it is high carbon, and the addition of alloy elements such as Si and Cr promotes martensitic transformation. Therefore, it is very difficult to control the wire so that it has a predetermined scale property and a predetermined microstructure. Therefore, even for a high-strength low-alloy wire, a wire having excellent wire drawing workability has been strongly desired.

On the other hand, the wire is kept or heated at 700 ° C. or higher to increase the thickness of the scale and increase the proportion of FeO in the scale, and then rapidly cool to transform eutectoid into Fe ₃ O ₄ . By suppressing Fe and making it rich in FeO, a technique for reducing the residual scale amount of the wire (see, for example, Patent Document 1), a wire containing 0.6% or more of C is targeted for bainite or By using a bainite and pseudo-pearlite structure, the ratio of the Fe ₃ O ₄ composition occupying the wire scale is less than 30%, thereby improving the wire drawing workability (for example, see Patent Document 2), and the upper bainite structure. For wires with a high area ratio, 1 to 5% of holes in the scale are present in an area ratio of 1 to 5%. Technology for improving the scale detachment during scaling (e.g., see Patent Document 3.) It has been proposed.

JP-A-52-10829 JP-A-8-295991 JP-A-10-324923

  However, if the scale rich in FeO is thick, it causes a decrease in yield during mechanical descaling, and it is difficult to remove the scale uniformly and stably over the entire surface of the wire, and the wire is kept warm at high temperatures. As a result, the productivity is remarkably deteriorated and the manufacturing cost is increased.

Further, in order to obtain a microstructure such as bainite or bainite and pseudo pearlite, special equipment for performing atmospheric control such as mist patenting is required in the hot rolling process, which increases the manufacturing cost. Even if the Fe ₃ O ₄ composition ratio is controlled to be less than 30%, that is, even if FeO rich with a porous structure is used, the dense Fe ₃ O ₄ generated in the outer layer causes a mechanical descaling. It was difficult to remove the scale uniformly and stably over the entire surface of the wire.

  Further, in order to make a few% of fine pores exist in the scale, it is necessary to adjust the oxygen concentration in the bubbling gas charged in the salt bath used for salt cooling after wire rod rolling. In other words, if a general Stealmore cooling method is used, it becomes an atmospheric atmosphere, and thus cannot be substantially controlled. Further, according to the investigation by the inventor, even if the oxygen in the bubbling gas in the salt bath is controlled and the fine vacancies can be controlled to several percent, coarse vacancies exist at the same time. It was difficult to remove the scale stably over the entire length.

  The present invention was devised in view of the above points, and provides a high-strength low-alloy steel wire that can remove scales uniformly and stably over the entire surface and has excellent wire drawing workability. Objective.

(A) In general, Fe ₃ O ₄ is said to have poor scale peelability. When the proportion of FeO having a porous structure and softer than Fe ₃ O ₄ or Fe ₂ O ₃ is increased, mechanical descaling property is increased. Was thought to improve. However, it was not always sufficient as a means for removing the scale uniformly and stably over the entire surface.

(B) On the other hand, hard and dense Fe ₃ O ₄ may stop the progress of cracks generated during thermal shrinkage during mechanical descaling and increase the residual scale. However, it has been found that if an appropriate amount of vacancies can be present in Fe ₃ O ₄ , the vacancies become a propagation path of cracks, and the progress of the cracks can be promoted, which makes it easy to peel.

(C) As a result of investigating the scale peelability of _Fe 3 _{O 4,} the volume ratio of the _Fe 3 _{O 4} to a constant value or more, and, by controlling within a certain range pore area ratio in _Fe 3 _{O 4} For example, it has been found that the scale can be removed uniformly and stably over the entire surface, but it has been found that the internal structure tends to deteriorate the wire drawing workability when attempting to obtain such scale properties. did.

  (D) That is, although it is necessary to control the cooling rate after wire rod rolling in order to obtain such scale properties, in the case of high strength low alloy steel containing 0.8% or more of C, after wire rod rolling If the cooling rate is too fast, the internal structure of the wire will be mainly martensite or bainite, and conversely if the cooling rate is too slow, a large amount of proeutectoid cementite will be generated, which is too hard for wire drawing. Become an internal organization.

(E) Furthermore, as a result of repeated studies, the volume ratio of Fe ₃ O ₄ is 30% or more, the pore area ratio in Fe ₃ O ₄ is 20 to 80%, and the pearlite area ratio in the internal structure is 90%. It has been clarified that when the amount is not less than%, the scale can be removed uniformly and stably, and a high-strength low-alloy steel wire excellent in wire drawing workability can be obtained.

  The present invention has been completed based on the above findings.

In the high strength low alloy steel wire of the present invention, the composition inside the wire is mass%, C: 0.80 to 1.10%, Si: 0.10 to 0.70%, Mn: 0.10 to 0 80%, Cr: 0.02 to 0.70%, B: 0.005% or less, P: 0.012% or less, S: 0.012% or less, Al: 0.002% or less, N: 0 0.003% or less, O: 0.002% or less, the balance being Fe and impurities, the pearlite area ratio in the structure inside the wire is 90% or more, and Fe ₃ O _{4 in} the scale on the surface of the wire The volume ratio is 30% or more, and the pore area ratio in the Fe ₃ O ₄ is 20% or more and 80% or less.

  According to the high-strength low-alloy steel wire according to the present invention, the scale can be removed uniformly and stably over the entire surface, die baking and disconnection do not occur, and the wire drawing workability is excellent.

  Hereinafter, each requirement of the present invention will be described in detail. The inside of the wire refers to a metal portion excluding the scale on the surface of the wire, and “%” of the composition inside the wire means “mass%”.

(A) Composition inside wire C: 0.80 to 1.10%
C is an element effective for increasing the strength of the steel wire. However, when the content is less than 0.80%, it is difficult to give the final product a high strength of, for example, 4000 MPa in terms of TS (tensile strength). On the other hand, when there is too much content of C, steel materials will become hard and a wire drawing workability will fall. In particular, when the C content exceeds 1.10%, it becomes difficult to prevent the formation of proeutectoid cementite (that is, cementite along the prior austenite grain boundaries), and wire drawing workability is greatly reduced. Even if the scale property is within the specified range, the wire drawing limit is not more than 2.6 at true strain, and disconnection frequently occurs. Therefore, the content of C is set to 0.80 to 1.10%.

Si: 0.10 to 0.70%
Si is an element effective for increasing the strength. Further, it is an element necessary as a deoxidizer. However, if the content is less than 0.1%, the effect of addition is poor. On the other hand, if it exceeds 0.70%, hard FeO.SiO ₂ is generated at the interface between the ground iron and the scale, and the scale properties described later are within the specified range. Even so, the wire drawing limit cannot be 2.6 or more in true strain. Therefore, the content of Si is set to 0.10 to 0.70%.

Mn: 0.10 to 0.80%
Mn is an element necessary for ensuring the hardenability of steel. However, if the content is less than 0.1%, it is difficult to obtain the above effect. On the other hand, Mn is an element that easily segregates, and when its content exceeds 0.80%, segregation occurs particularly in the central part of the wire, and martensite and bainite are generated in the segregated part, so that wire drawing workability is reduced. Resulting in. Therefore, the content of Mn is set to 0.10 to 0.80%.

Cr: 0.02 to 0.70%
Cr has the effect of increasing the strength after rolling and patenting by reducing the lamella spacing of pearlite. In addition, it has the function of increasing the work hardening rate during cold working such as wire drawing. In order to reliably obtain such an effect, the Cr content is preferably 0.02% or more. However, if its content exceeds 0.70%, the time until the completion of pearlite transformation becomes long, and martensite and bainite are generated in the center of the wire after hot rolling. Hard FeO · Cr ₂ O ₃ remains at the scale interface, and the frequency of disconnection during wire drawing increases. Therefore, the content of Cr is set to 0.02 to 0.70%.

  In the present invention, the contents of B, P, S, Al, N, and O (oxygen) are limited as follows.

B: 0.005% or less B is an element mixed as an impurity element, but may not be added. If added, it forms BN by bonding with N dissolved in the steel, reduces the solid solution N, improves the wire drawing workability, and further generates vertical cracks in the twist test after the final wire drawing. There is an effect to suppress. In order to reliably obtain this effect, the content is preferably 0.0003% or more. However, if B is contained in excess of 0.005%, coarse BN is generated and wire drawing workability is lowered. Therefore, the B content is set to 0.005% or less.

P: 0.012% or less P segregates at the grain boundary and lowers the wire drawing workability. In particular, when the content exceeds 0.012%, the wire drawing workability deteriorates remarkably. Therefore, the content of P is set to 0.012% or less.

S: 0.012% or less S decreases wire drawing workability. In particular, when the content exceeds 0.012%, the wire drawing workability deteriorates remarkably. Therefore, the content of S is set to 0.012% or less.

Al: 0.002% or less Al forms oxide inclusions containing Al ₂ O ₃ as a main component and reduces wire drawing workability. In particular, when the content exceeds 0.002%, the oxide inclusions are coarsened, wire breakage occurs frequently during wire drawing, and wire drawing workability is significantly reduced. Therefore, the Al content is set to 0.002% or less.

N: 0.003% or less N is fixed to dislocations during cold wire drawing and increases the strength of the steel wire, but decreases the wire drawing workability. In particular, when the content exceeds 0.003%, the wire drawing workability deteriorates remarkably. Therefore, the N content is set to 0.003% or less.

O (oxygen): 0.002% or less O forms oxide inclusions and lowers the wire drawing workability. In particular, if the O content exceeds 0.002%, the oxide inclusions become coarse, so that the wire drawing workability deteriorates remarkably and wire breakage frequently occurs during the wire drawing. Therefore, the content of O is set to 0.002% or less.

  In addition, in many cases, the reduction of B, P, S, Al, N and O (oxygen) to the content is thorough management of the refractory so that the above-mentioned elements are not mixed due to refractory melting, It can be carried out by selective selection of steelmaking raw materials, low P or low S of the hot metal, sufficient implementation of de-S or de-P in the hot metal pretreatment, and thorough floating and removal of inclusions in the molten steel stage.

(B) Microstructure in wire material Pro-eutectoid cementite is a network-like carbide that precipitates at the prior austenite grain boundaries and has a very adverse effect on wire drawing workability. The same applies to martensite and lower bainite structures. It is known that the wire drawing workability is improved when the upper bainite single phase is used, but in order to obtain the structure, special equipment for controlling the atmosphere in the hot rolling process is necessary, and the manufacturing cost Will increase. Therefore, the microstructure of the wire rod of the present invention is mainly made of pearlite that can be generated without using special equipment for controlling the atmosphere. Here, the pearlite-based structure is defined as a microstructure having a pearlite area ratio of 90% or more.

(C) Scale on the surface of the wire The reason for limiting the volume ratio of Fe ₃ O ₄ to the scale on the surface of the wire to 30% or more is that if it is less than 30%, pores are used as crack propagation paths during mechanical descaling This is because it becomes difficult to remove the scale uniformly and stably over the entire surface of the wire. Moreover, the reason for limiting the hole area ratio in Fe ₃ O ₄ to 20 to 80% is that if it is less than 20%, the amount of residual scale increases, and die seizure or wire breakage occurs during the subsequent wire drawing. If it exceeds 80%, the scale is easily removed during the conveyance to the mechanical descaling line, and rusting occurs depending on the storage conditions thereafter.

  The wire of the present invention can be produced by melting the steel having the above-mentioned chemical composition, and rolling the wire after continuous casting or split rolling, and cooling to room temperature. In order to achieve the internal structure of the wire at the same time, the finish rolling temperature of wire rod rolling is set to 800 to 1000 ° C., the cutting temperature is set to 800 to 950 ° C., and the average cooling rate from the cutting temperature to 600 ° C. is set to 6 to 18 ° C. The average cooling rate from 600 ° C. to 300 ° C. is 4 to 15 ° C./s, and the average cooling rate from the scraping temperature to 200 ° C. is 7 to 13 ° C./s.

When the average cooling rate from the scraping temperature to 600 ° C. is less than 6 ° C./s, pro-eutectoid cementite precipitates, and when it exceeds 18 ° C./s, martensite and bainite are generated. Therefore, the area ratio of pearlite in the internal structure is less than 90%. When the average cooling rate from 600 ° C. to 300 ° C. is less than 4 ° C./s, the productivity at the time of rolling the wire is remarkably deteriorated, and when it exceeds 15 ° C./s, the Fe ₃ O ₄ occupies the scale on the surface of the wire. The volume ratio is less than 30%. Furthermore, when the average cooling rate from the scraping temperature to 200 ° C. is less than 7 ° C./s, the pore area ratio in Fe ₃ O ₄ is less than 20%, and when it exceeds 13 ° C./s, the porosity in Fe ₃ O ₄ The void area ratio exceeds 80%.

  The tensile strength of the high strength low alloy steel wire of the present invention thus obtained is 1150 MPa or more, and the drawing is 25% or more. Furthermore, the tensile strength of the filament after final drawing of such a wire becomes 4000 MPa or more.

Hereinafter, the present invention will be described with reference to examples.
Three types of steel having chemical compositions shown in Table 1 were melted in a 70-ton converter and subjected to out-furnace refining, and then continuous casting was performed. The obtained casting was hot-forged by a usual method to form a 140 mm square billet, then the finish rolling temperature was adjusted to 880 to 1000 ° C, and the milling temperature was adjusted to the range of 820 to 900 ° C. Hot rolled to a diameter of 5.5 mm. After cutting, the wire was cooled to room temperature by Stealmore cooling to obtain a wire. Table 2 shows the average cooling rate in each temperature zone during cooling.

In the analysis of the scale composition, the obtained wire was distorted, the scales were completely peeled off, the powders were pulverized, and the peak intensity ratio of FeO, Fe ₃ O ₄ , and Fe ₂ O ₃ by X-ray diffraction From this, the ratio of Fe ₃ O ₄ was calculated. The results are shown in Table 2.

The hole area ratio in Fe ₃ O ₄ is obtained by mirror-polishing the cross-section of the obtained wire (that is, the cut surface perpendicular to the length direction), and then using an optical microscope at a magnification of 500 times. Micro photography was performed, and the photographed image was calculated by image analysis. The photographed cross section was obtained by equally collecting 8 samples from each winding of the tip, middle and end of the 2-ton wire coil obtained, and dividing each of them into 8 equal parts in the circumferential direction. It has been done. Table 2 shows the average value of the calculated values.

  Samples similar to those used in the above calculation of the void area ratio were collected, each sample was mirror-polished, then corroded with nital, and using a scanning electron microscope (SEM) at a magnification of 2500 times Photographed 4 surface parts of wire coil, 4 R / 2 parts (midpoint between radial surface and center), and 1 center part, and analyzed the resulting image for perlite area of each part The average value was calculated from the rate to obtain the pearlite area rate. The results are shown in Table 2.

  The amount of residual scale is 100 mm when the tensile strength is applied to the wire with a tensile tester, the distance between the chucks is 200 mm, and the displacement of the crosshead is 12 mm (6%). It cut | disconnected to length, the wire weight was measured (W1), this wire sample was immersed in hydrochloric acid, the scale adhering to the wire surface was peeled completely, and the wire weight was measured again (W2). From this weight measurement result, the residual scale amount was calculated by the following formula (1). The results are shown in Table 2.

Residual scale amount (% by weight) = (W1-W2) / W2 × 100 (1)

Each 2-ton hot-rolled wire manufactured under the conditions shown in Table 2 was subjected to dry wire drawing with a pass schedule in which the area reduction rate of each die was 15 to 25% after mechanical descaling. At this time, even when the wire drawing was performed to a diameter of 1.5 mm or less, it was evaluated that the wire drawing workability was good when the wire was not broken even once. Further, when the wire was disconnected five times, the wire drawing work for the wire was stopped. In Table 2, the wire drawing limit strain is represented by the value of true strain (ε), and the true strain (ε) is represented by the following formula (2). Here, d ₀ represents the diameter of the wire before drawing, and d represents the diameter of the wire after drawing. The true strain (ε) when drawn from 5.5 mm to 1.5 mm in diameter is 2.6, and the true strain (ε) value increases when the wire is drawn more thinly.

ε = 2ln (d ₀ / d) (2)

As is clear from Table 2, the specific volume ratio range of Fe ₃ O ₄ occupying the scale, the specific pore area ratio range in the Fe ₃ O ₄ , and the specific structure of the internal structure of the wire as defined in the present invention A wire that falls outside one of the pearlite area ratio ranges cannot be drawn from a diameter of 5.5 mm to a diameter of 1.5 mm (the value of the drawing process limit strain is less than 2.6), and the drawing workability is poor. it is obvious. Incidentally, Comparative Examples 4 and 5, the respective wire rods of Comparative Example 10 and Comparative Example 11 and Comparative Example 16 and Comparative Example 17, the pore area ratio in Fe ₃ O ₄ is the upper limit defined in the present invention Since (80%) is exceeded, the scale peels off during the conveyance of the wire coil, and the occurrence of rusting is severe, so that it is splashed (rejected) as a defective product.

  In contrast, the wire of the present invention does not break even when drawn from a diameter of 5.5 mm to a diameter of 1.5 mm (drawing limit strain value is 2.6 or more), and has good drawing workability. It turns out that it has. Further, it can be seen that the wire of the present invention has a small amount of residual scale, and the scale is removed uniformly and stably over the entire surface of the wire. Furthermore, it can be seen that a wire rod having excellent wire drawing workability as in the present invention can be obtained by the stealmore cooling without using a special cooling method.

Claims (1)

The composition inside the wire is% by mass, C: 0.80 to 1.10%, Si: 0.10 to 0.70%, Mn: 0.10 to 0.80%, Cr: 0.02 to 0 70%, B: 0.005% or less, P: 0.012% or less, S: 0.012% or less, Al: 0.002% or less, N: 0.003% or less, O: 0.002% Hereinafter, the balance is Fe and impurities, the pearlite area ratio in the structure inside the wire is 90% or more,
A high-strength low-alloy steel characterized in that the volume ratio of Fe ₃ O ₄ in the scale on the surface of the wire is 30% or more and the void area ratio in the Fe ₃ O ₄ is 20% or more and 80% or less. wire.