JP5503516B2 - High carbon steel wire rod excellent in dry drawing and method for producing the same - Google Patents

Description

  The present invention relates to a high carbon steel wire rod excellent in drawability, which is used for dry drawing, and a method for producing the same.

  Ultra fine steel wires used for tire reinforcing steel cords, bead wires, saw wires, belt cords, and the like are manufactured by drawing a high-carbon steel wire, which is a material, into a necessary wire diameter. At this time, in order to obtain good wire drawing properties, intermediate patenting treatment is performed once or twice in the middle of the dry drawing step of the high carbon steel wire. This is because the dry wire drawing process including the intermediate patenting process is a process mainly for size adjustment, and if the wire is broken during the wire drawing, the productivity of the ultra fine steel wire is significantly hindered. After this dry drawing, the wet drawing performed through the final patenting process determines the required quality of the ultra fine steel wire.

  The high carbon steel wire, which is a material for such an ultra fine steel wire, is naturally required to have good drawability. The high-carbon steel wire that is this material is manufactured by hot rolling. Conventionally, after this hot rolling, the hot-rolled wire is water-cooled and blast-cooled, thereby improving the wire structure. To obtain fine pearlite.

  As described above, the dry wire drawing process, which is a process for adjusting the size, mainly has issues such as improvement in productivity and reduction in production cost rather than quality. Specifically, wire drawing to thinner wire diameter, suppression of wire breakage during wire drawing, omission of intermediate patenting, improvement of life of wire drawing dies, increase of wire drawing speed, power consumption by reducing load on wire drawing machine motor Reduction is a challenge. Accordingly, there has been a demand for a high carbon steel wire material that can realize these.

  In response to such demands, many techniques for improving the drawability by controlling the pearlite structure of a high carbon steel wire have been introduced.

  For example, Patent Document 1 discloses that the size of the pearlite block of the high carbon steel wire is 6 to 8 in terms of the austenite grain size number of the steel, and the amount of proeutectoid cementite produced is 0.2% or less by volume. Further, by adjusting the cementite thickness in the pearlite to 20 nm or less and the concentration of Cr contained in the cementite to 1.5% or less, the intermediate patenting process during dry drawing is omitted.

  Patent Document 2 is similar to Patent Document 1 in that it provides intermediate patenting during dry wire drawing by controlling the austenite grain size of high carbon steel wire rods, pearlite colony size control, proeutectoid ferrite, and proeutectoid cementite shape control. The process is omitted.

  In Patent Document 3, the production area of the intragranular transformed upper bainite existing in the cross section of the high carbon steel wire is set to 30% or more, and the growth size of the intragranular bainite is set to 2 μm or more. Improve.

  Here, the pearlite structure is well known, and FIG. 1 schematically shows the pearlite structure. This FIG. 1 is based on the pearlite structure chart disclosed in Takahashi et al., “Ductility controlling factor of eutectoid pearlite steel”, Journal of the Japan Institute of Metals, vol 42, 1978, 708. As shown in FIG. 1, the pearlite structure is a layered structure of a hard cementite phase and a soft ferrite phase that is born and grows from an austenite grain boundary, and has both workability and strength. The width (interval) between the cementite phases sandwiching the ferrite phase is the lamellar interval L, and as the lamellar interval is uniform and narrow, higher strength can be achieved. The pearlite colony refers to a region where the directions of the pearlite lamella are aligned (same). In this way, pearlite nodules (also referred to as pearlite blocks) having a constant ferrite crystal orientation are formed by a plurality of colonies adjacent to each other and having different lamella directions.

  In recent years, high carbon steel wire materials tend to be softened particularly as a characteristic response to problems such as omission of the intermediate patenting, improvement of the life of a wire drawing die, and increase of the wire drawing speed. This is because the steel wire drawing resistance increases due to dry wire drawing, the steel wire drawing resistance increases, the load on the wire drawing machine motor increases, and power consumption increases. This is because the tendency to break due to embrittlement of the steel wire is increased. Due to the softening of the high-carbon steel wire material = the reduction in steel wire strength, the processing heat generation during wire drawing decreases, and the embrittlement of the steel wire is suppressed. As a result, it is possible to omit the intermediate patenting and increase the wire drawing speed, which have been limited in the past.

  From this point of view, pay more attention to the pearlite structure of the high carbon steel wire, and control the lamella spacing and the nodule diameter of the pearlite to eliminate the intermediate patenting and to further increase the drawing speed. Techniques for increasing the price have been proposed.

  For example, Patent Document 4 introduces a technique for improving the drawability by setting the average colony diameter of pearlite of a high carbon steel wire to 150 μm or less and the average lamella spacing to 0.1 to 0.4 μm. In addition, as described in Patent Document 4, the high-carbon steel wire material after hot rolling is adjusted to the winding temperature by water cooling, and the amount of blast is subsequently adjusted by an adjustment cooling device such as a stealmore conveyor or a roller conveyor. Manufactured by adjusting.

  In Patent Document 5, the average lamella spacing S of pearlite having a structure of 95% by area or more is expanded to 100 nm or more, and the average nodule diameter P is set to 30 μm or less to prevent coarsening, thereby maintaining the disconnection resistance and drawing. Techniques for improving the die life and further increasing the drawing speed have been proposed.

  In patent document 6, the area fraction of pearlite is 95% or more, the lamellar spacing is 0.08 to 0.35 μm, and the amount of non-diffusible hydrogen is 0.5 ppm or less. In Japanese Patent Application Laid-Open No. H10-260, a technique has been proposed which enables omission of intermediate patenting and reduces the disconnection rate in the wire drawing process and the stranded wire process.

  In Patent Document 7, 80% or more of the metal structure is composed of a pearlite structure, and TS ≦ 8700 / √ (λ / Ceq) +290 between the average tensile strength TS and the average lamellar interval λ of the high-carbon steel wire rod. A technique for further softening the wire has been proposed by providing a relationship and softening the mechanical properties of lamellar cementite.

JP 2004-91912 A JP 2001-181789 A JP-A-8-295930 JP 2000-63987 A Japanese Patent No. 3681712 JP 2008-261028 A Japanese Patent Laid-Open No. 2005-208553

  However, due to the recent trend of reducing the environmental load, the demand for improving the drawability of the high carbon steel wire is increasing in order to improve the productivity of the dry wire drawing process and reduce the cost. On the other hand, as in the prior art, the effect of improving the drawability is insufficient only with the softening technology of the high carbon steel wire.

  This invention is made | formed in view of this problem, and it aims at providing the high carbon steel wire which has the outstanding wire drawing property which improved the productivity of a dry wire drawing process, and its manufacturing method.

  The gist of the high carbon steel wire of the present invention for achieving the above object is a high carbon steel wire to be used for dry drawing, in mass%, C: 0.68 to 0.86%, Si: 0.05 to 0.5% and Mn: 0.1 to 0.8%, respectively, and the balance is composed of Fe and inevitable impurities, and the structure has pearlite of 95% by area or more. The distance L is in the range of 150 to 300 nm, the lamellae having a distance of 120 nm or less among the pearlite lamellae is 30% or less, and the standard deviation of the lamella distance is 50 nm or less. The average nodule diameter D of the pearlite is 40 μm or less, and the average nodule diameter D and the average lamella interval L satisfy the relationship D <−0.1 × L + 60.

  Moreover, the summary of the manufacturing method of this invention high carbon steel wire for achieving the said objective is a manufacturing method of the high carbon steel wire with which it uses for dry drawing, Comprising: In mass%, C: 0.68 Steel slabs each containing ˜0.86%, Si: 0.05-0.5%, Mn: 0.1-0.8%, the balance being Fe and inevitable impurities, are heated to a finishing temperature of 1,500 Hot-rolling is performed at 900 ° C., and after finishing the finish rolling in this hot rolling, immediately water-cooled to a temperature in the range of 950 to 800 ° C., and subsequently 640 to 580 ° C. at an average cooling rate of 30 ° C./s or more. After rapidly cooling to a temperature within the range, the temperature is rapidly heated to a temperature within the range of 650 to 720 ° C. within 2 seconds from this temperature range at an average heating rate of 50 ° C./s or more, and further, this 650 to 720 ° C. Within a temperature range of less than 0.5 ° C / s (including 0 ° C / s) Or a wire that has undergone pearlite transformation while being held at a temperature-decreasing rate, and the structure of this wire has 95% by area or more of pearlite, and the average lamella spacing L of this pearlite is in the range of 150 to 300 nm, and Of these pearlite lamellae, the lamella region having an interval of 120 nm or less is 30% or less, the standard deviation of the lamella interval is 50 nm or less, and the average nodule diameter D of this pearlite is 40 μm or less. In addition, the average nodule diameter D and the average lamella interval L are to satisfy the relationship of D <−0.1 × L + 60.

  In order to suppress disconnection in the dry wire drawing process, the present inventor widens the pearlite lamella spacing (average lamella spacing L) of the high carbon steel wire to some extent, lowers the strength of the wire, and reduces embrittlement. We recognized that it was essential. And based on such premise, the wire drawing property improvement measure which further reduces the embrittlement of the steel wire in the said dry-type wire drawing process was researched.

  As a result, under such a premise, the nodule diameter (average nodule diameter D) of the pearlite, which has a physical meaning as a crystal grain, greatly increases the drawability, in particular, the resistance to embrittlement during dry drawing. I found out that it had an effect. As a result, it was found that uniform coarsening of the pearlite lamella spacing is effective in reducing the embrittlement of the steel wire. That is, the embrittlement resistance of the steel wire is improved by appropriately controlling the pearlite average lamella spacing, suppressing the formation of fine lamella structures, and making the distribution of lamella spacing in the wire uniform.

  In other words, by reducing the area where the pearlite lamella spacing is fine and reducing the pearlite lamella spacing variation and making it uniform, the resistance to embrittlement is improved, and the breakage resistance during dry wire drawing is greatly improved. It has been found that improved and excellent wire drawing can be obtained. According to the present invention, the above-described improvement in productivity and reduction in production cost during dry drawing (drawing to a thinner wire diameter, suppression of disconnection during drawing, omission of intermediate patenting, life of a drawing die, etc. Improvement, increase in wire drawing speed, reduction of power consumption by reducing the load on the wire drawing machine motor, etc.).

It is a schematic diagram which shows the structure | tissue of a high carbon steel wire. It is explanatory drawing which shows the structure prescription | regulation of this invention high carbon steel wire. It is explanatory drawing which shows the manufacturing method of this invention high carbon steel wire.

Chemical composition:
First, the reason for limiting the chemical composition of the high carbon steel wire of the present invention will be described. The chemical component composition of the high carbon steel wire of the present invention is a premise for making the steel wire material structure described later as a high carbon steel wire used for dry drawing. Also, in order to ensure mechanical characteristics such as strength required for extra fine steel wire for improving the productivity of the dry wire drawing process, improving the wire drawing for cost reduction, and for reinforcing the tire of the present invention. It becomes the premise of.

  Therefore, the high carbon steel wire of the present invention includes, in mass%, C: 0.68 to 0.86%, Si: 0.05 to 0.5%, and Mn: 0.1 to 0.8%, respectively. The chemical composition is composed of the remaining Fe and inevitable impurities.

  In addition, although the unit of the following element content is all the mass%, it may only describe with%.

  Other elements other than these are basically unavoidable impurities, and the impurity content (allowable amount) level of ordinary high-carbon steel wire rods. However, when used as an ultra-fine steel wire for tire reinforcement or the like targeted by the present invention, P, S, N, Al, O, etc. for controlling oxides that cause disconnection in order to ensure ductility The lower the content, the better.

  Below, content of each main element and its limitation reason (meaning) are demonstrated.

C: 0.68 to 0.86 mass%
C is a basic element for securing the strength of a high carbon steel wire. If the C content is too small, the strength required for an ultrafine steel wire for tire reinforcement or the like targeted by the present invention cannot be secured. In addition, the steel wire rod may not have a pearlite-based structure in the manufacturing process, and may adversely affect the drawability. On the other hand, an increase in the C content directly leads to an increase in strength. However, excessive addition deteriorates ductility, and proeutectoid cementite is generated, thereby inhibiting the drawability. For this reason, C content is taken as 0.68 to 0.86% of range.

Si: 0.05 to 0.5% by mass
Si contributes to deoxidation and stabilization of the pearlite structure. Moreover, it has the effect | action which raises an intensity | strength by solid solution strengthening. In an extra fine steel wire drawn to an extra fine diameter, such as for tire reinforcement, which is the subject of the present invention, Si is added as a deoxidizer to prevent the formation of hard alumina inclusions. It is important to ensure mechanical properties. If the Si content is too low, these effects will be insufficient, but excessive addition will cause coarsening of the oxide, increase in the strength of the wire, generation of a scale that is difficult to peel off, excessive strengthening of the solid solution of ferrite, etc. Inhibits wire drawing. Therefore, the Si content is in the range of 0.05 to 0.5%, and the lower limit is preferably 0.07%, more preferably 0.10%, and still more preferably 0.15%. The upper limit is preferably 0.45%, more preferably 0.40%, and still more preferably 0.35%.

Mn: 0.1 to 0.8% by mass
Mn has a deoxidizing and desulfurizing action, and a strength improving action by solid solution strengthening, and contributes to stabilization of the pearlite structure. If the Mn content is too small, these effects will be insufficient, but excessive addition will deteriorate the uniformity of the structure due to segregation (non-uniformity) and coarsening of the sulfide, thereby degrading the wire drawing. Let Also, the ferrite is excessively solid-solution strengthened to reduce the drawability. Therefore, the Mn content is in the range of 0.1 to 0.8% by mass, and the lower limit is preferably 0.15%, more preferably 0.20%, and even more preferably 0.25%. The upper limit is preferably 0.75%, more preferably 0.70%, and still more preferably 0.60%.

Al:
Al is known as an effective deoxidizing element, but hard alumina inclusions cause disconnection in a wire rod drawn to an extremely fine diameter, and lower the drawability. In addition, the mechanical properties of the fine steel wire are also reduced. For this reason, the Al content is preferably as small as inevitable impurities, and is specifically regulated to 0.0050% or less. In order to ensure drawability even under severe wire drawing conditions, further reduction is necessary, preferably 0.0035% or less, more preferably 0.0025% or less.

P:
P is an unavoidable impurity element, and particularly has a great influence, for example, remarkably deteriorates the wire drawing property in order to solid-solution strengthen ferrite. Moreover, since the toughness of a steel material will deteriorate when it contains excessively, it is so preferable that content is small. Specifically, the P content is 0.02% or less, and in order to ensure drawability even under severe wire drawing conditions, the P content is further reduced, preferably 0.010% or less, more preferably 0.007%. The following.

S:
S is also an unavoidable impurity element. If it is excessively contained, the size and amount of sulfide increase, and the ductility deteriorates. Moreover, inclusion MnS is produced and the drawability is inhibited. Therefore, the smaller the content, the better. More specifically, the S content is set to 0.020% or less, and in order to ensure the drawability even under severe wire drawing conditions, the content is further reduced, preferably 0.010. % Or less, more preferably 0.007% or less.

N:
N is also an unavoidable impurity element, and it dissolves in ferrite and causes age hardening or strain age hardening by heat generation during wire drawing, increasing the strength of the steel wire and degrading toughness, and affecting wire drawability. Is big. For this reason, the smaller the content, the better. Specifically, the content is made 0.0040% or less. In order to ensure the drawability even under severe wire drawing conditions, it is further reduced, preferably 0.0030% or less, more preferably 0.0025% or less.

O:
O is also an unavoidable impurity element, and an increase in the amount of oxygen in the steel leads to coarse oxides, causing disconnection. For this reason, the smaller the content, the better. Specifically, the content is made 0.0030% or less. In order to ensure drawability even under severe wire drawing conditions, further reduction is necessary, and the upper limit is preferably 0.0020% or less, more preferably 0.0015% or less.

Organization:
Next, the structure of the high carbon steel wire of the present invention will be described. First, the relationship between the structure and the drawability will be described to clarify the reasons for limiting each requirement of the structure of the present invention.

  In the present invention, as a premise, the structure of the high carbon steel wire having the above composition is a pearlite-based structure having pearlite of 95 area% or more. If it does not have pearlite of 95 area% or more, even with the above-described composition, the drawability that is a basic characteristic is lowered.

  As shown in FIG. 1, the pearlite structure is called pearlite lamella (hereinafter also simply referred to as lamella), pearlite colony (hereinafter also simply referred to as colony), pearlite nodule (or pearlite block, hereinafter also simply referred to as nodule). Has a hierarchical organizational structure composed of organizational units. The lamella is defined as a layered structure of cementite and ferrite, the colony is defined as a structural unit having lamellas arranged in the same direction, and the nodule is defined as a structural unit having the same crystal orientation of ferrite.

  In the pearlite structure of the high carbon steel wire of FIG. 1, the present invention has a pearlite average lamella spacing L in the range of 150 to 300 nm, and among the lamellae of this pearlite, a lamella having a spacing of 120 nm or less. The area is 30% or less, the standard deviation of the lamella interval is 50 nm or less, the average nodule diameter D of this pearlite is 40 μm or less, and the average nodule diameter D and the average lamella interval L are D <−0.1 × L + 60 is satisfied. The method for measuring the nodule diameter D will be described in the examples described later.

  FIG. 2 shows the structure of the high carbon steel wire according to the present invention. In FIG. 2, the vertical axis is the average nodule diameter D, and the horizontal axis is the average lamella spacing L of pearlite. 2 is a range satisfying the relationship of D <−0.1 × L + 60 defined by the present invention. The oblique line that is the upper limit of the oblique line range is a line that satisfies D = −0.1 × L + 60 in the relational expression.

  The technical reason (significance) of the structure definition of the present invention will be described as follows from the viewpoint of combining embrittlement suppression of steel wire and prevention of deterioration of ductility.

  Steel wire embrittlement in dry wire drawing is known to be caused by so-called strain aging, in which a large amount of dislocations introduced by wire drawing strain are bonded to carbon and nitrogen by the temperature rise of the steel wire during wire drawing. ing. Since the temperature rise of the steel wire at the time of wire drawing is caused by heat generated by processing and friction, the softening (strength reduction) of the steel wire can reduce the temperature rise of the steel wire at the time of wire drawing, and the steel wire becomes brittle. Can be suppressed.

  For this reason, as described above, in the prior art, by softening the wire rod before wire drawing, the steel wire strength after wire drawing is reduced, drawing resistance is reduced, heat generation is reduced, and steel wire embrittlement is achieved. This has made it possible to increase the drawing speed. Here, the essential technical issue for the embrittlement control in the dry wire drawing process is to control the steel wire material that does not embrittle, and to suppress the aging embrittlement by suppressing the processing heat generation by softening the wire. Is one of the means.

  However, since the strength of the wire is determined by the lamella spacing (average lamella spacing L), which is the strength factor of pearlite, which is the main constituent structure of the wire, naturally the guideline for softening the wire is the lamella spacing ( The average lamella spacing L) is enlarged or optimized. However, since there is a limit to the expansion of the lamella spacing, there is a limit to the suppression of embrittlement of the steel wire due to the softening of the pearlite structure.

  Moreover, as the wire drawing speed increases for efficiency, even if the wire is softened, embrittlement occurs, and there is a problem that there is a big limit to improving the drawability of the high carbon steel wire. It was. These are the reasons why the effect of improving the drawability is insufficient in the conventional softening technology for high carbon steel wires.

  On the other hand, the present inventors consider that the fragile portion exists in the inside of the wire (structure), and even if the wire is softened, the fragile portion causes the wire to become brittle during wire drawing, and the pearlite structure factor We have conducted extensive research on the relationship between embrittlement and embrittlement.

  As a result, it has been found that the pearlite structure in the wire manufactured by the conventional manufacturing method has variations, and in particular, the variation in lamella spacing has a feature that correlates with the embrittlement of the wire.

  That is, the variation in lamella spacing causes microscopic material non-uniformity during wire drawing. In particular, even if the lamella spacing is increased for softening the wire, there are some portions where the lamella spacing is fine, and the presence of the portion where the lamella spacing is fine is the cause of embrittlement of the wire. It has become. Moreover, even if there are few locations where the lamella interval is fine, if the variation in the lamella interval is large, non-uniform plastic strain becomes prominent, which causes embrittlement. In other words, if there is such a variation in lamella spacing, even if the lamella spacing of the high carbon steel wire is enlarged and softened, embrittlement is promoted as the drawing speed increases.

Reduction of minute areas of lamella spacing, variation of lamella spacing:
Based on such an idea, the present inventors have increased the lamella spacing of the high carbon steel wire and softened it, resulting in non-uniform lamella spacing, that is, a region where the lamella spacing is fine. It has been found that embrittlement of the wire can be reduced by reducing the thickness of the wire and making the lamella spacing uniform.

  Therefore, in the present invention, among the perlite lamellae, the lamella spacing is 120 nm or less, and the presence of the lamellae is a factor causing embrittlement of the wire, and the lamella area is less than 30% on average. In addition, the standard deviation of the lamella interval is set to 50 nm or less.

  By reducing the small lamella area with a lamella spacing of 120 nm or less to 30% or less, the standard deviation of the lamella spacing is 50 nm or less, and uniformizing the lamella spacing of the high carbon steel wire, Brittleness can be reduced. The present invention can reduce embrittlement during dry wire drawing compared to the case of the prior art in which the lamella spacing is 120 nm or less (non-uniform lamella spacing locations) and the lamella spacing distribution is non-uniform. Is presumed to be due to the following reasons.

  One is that when the pearlite lamella spacing is as fine as 120 nm or less, the increase in the strength of the pearlite structure increases rapidly, and the nonuniformity of the strength in the wire structure increases. On the other hand, by reducing the fine lamella region having a pearlite lamella spacing of 120 nm or less, it is possible to reduce non-uniformity of strength in the wire structure, which causes embrittlement of the wire.

  Further, the amount of wire drawing strain of dry wire drawing is generally about 3 to 4, but when the lamella spacing of the wire is as fine as 120 nm or less, after wire drawing of such strain amount, the lamella spacing. However, it becomes as fine as about 20 nm. For this reason, along with the wire drawing process of the wire, cementite is likely to be locally decomposed and become brittle at these refined portions. On the other hand, by reducing the fine lamella region where the pearlite lamella spacing is 120 nm or less, it is possible to reduce the area where the lamella spacing is easily refined, which is likely to cause cementite decomposition locally. Can reduce the generation of the conversion part.

  Therefore, by reducing or eliminating a minute lamella region having a lamella spacing of 120 nm or less, 30% or less, preferably 20% or less, more preferably 10% or less, more preferably substantially 0%, Brittleness of the wire is suppressed. Further, even if there are few regions where the lamella spacing is extremely fine, if the variation in the lamella spacing is large, embrittlement is caused. It is thought that plastic strain concentrates in soft parts with relatively wide lamella spacing, promotes uneven distribution, and leads to embrittlement.

Average lamella spacing L:
On the other hand, the average lamella interval L is in the range of 150 to 300 nm. Even if the average lamella spacing L is reduced, the intensity of pearlite increases relatively moderately up to 150 nm. In addition, local cementite decomposition due to dry wire drawing hardly occurs. For this reason, even if the average lamella spacing L is reduced to 150 nm, the strength of the wire material becomes uneven and the generation of local embrittlement in the wire material structure becomes a factor of embrittlement of the wire material. Can be suppressed.

  However, when the average lamella interval L exceeds the lower limit of 150 nm and is further refined, it is not possible to suppress the unevenness of the strength in the wire structure and the local embrittlement generation in the wire structure. The embrittlement of the wire cannot be suppressed. Therefore, the lower limit of the average lamella interval L is 150 nm, preferably 165 nm or more, more preferably 180 nm or more, and still more preferably 200 nm or more.

  On the other hand, in order to soften the wire and reduce the strength, as described above, the average lamella spacing L is preferably coarse (large), but if it is excessively coarse, voids are likely to be generated in the lamella, Ductility deteriorates and disconnection occurs due to insufficient ductility during wire drawing. Therefore, the upper limit of the average lamella interval L is 300 nm, preferably 280 nm or less, more preferably 260 nm or less, and still more preferably 240 nm or less. Thus, the wire drawing property is further improved by further limiting the upper and lower limit values of the average lamella interval L.

Nodule diameter (nodule size):
In the present invention, the average nodule diameter D of pearlite is 40 μm or less, and the average nodule diameter D and the average lamella spacing L satisfy the relationship of D <−0.1 × L + 60.

  As the lamella spacing L increases, the nodule diameter D generally tends to increase, but from the standpoint of disconnection due to insufficient ductility, the nodule diameter D is controlled as previously indicated. It is also important to suppress coarsening. The effect of the nodule diameter D on the ductility is different from the lamella interval L. When the voids generated in the lamella during processing are connected and grow as cracks, the nodule interface becomes resistance to crack growth. For this reason, the smaller the nodule diameter D of the wire, the more the growth of the cracks is suppressed and the ductility is excellent.

  Therefore, when considering the ductility of the wire, it is important to balance the lamella interval L in terms of suppressing void generation and the nodule diameter D in terms of suppressing crack growth.

  Therefore, in the present invention, the average nodule diameter D and the average lamella interval L satisfy the relationship (formula) of D <−0.1 × L + 60. Satisfying this relationship (formula) is a necessary condition for ensuring ductility when the lamella spacing is coarsened as described above. For this reason, this relationship (formula) is preferably D <−0.1 × L + 55, more preferably D <−0.1 × L + 45. Thus, the wire drawing property is further improved by further limiting this relationship (formula).

  Incidentally, the relational expression does not necessarily satisfy the relational expression between the average nodule diameter D and the average lamella interval L shown in the above-mentioned Patent Document 5. Nevertheless, the lack of ductility as pointed out in the case where the relational expression is not satisfied in the prior art will never fall if the relational expression of the present invention is satisfied, as will be shown later in the examples. This is not considered in these prior art, reducing the fine lamella spacing region, reducing the strength non-uniformity in the wire, local strain concentration, void, It is thought that the generation of cracks was suppressed, and that it greatly contributed to the improvement of ductility.

  However, when the average lamella interval L is close to the lower limit value, the fine lamella interval region is relatively large, and the ductility may be insufficient. For this reason, in addition to the relational expression, the upper limit of the average nodule diameter D is set to 40 μm or less. The upper limit of the average nodule diameter D is preferably 35 μm or less, more preferably 30 μm or less, and still more preferably 25 μm or less. Thus, the wire drawing property is further improved by further limiting the upper limit value of the average nodule diameter D.

Production method:
As described above, the gist of the present invention is to make the lamella spacing uniform by reducing the lamella spacing and reducing the fine region of the lamella spacing.

  The lamellar spacing changes depending on the transformation temperature of the steel wire, and the lamellar spacing becomes finer as the transformation temperature is lower. Therefore, it is necessary to stably grow pearlite having a large lamellar spacing by controlling the transformation of the steel wire in the low temperature region and controlling the transformation in the high temperature region. On the other hand, the nodule size is determined by the nucleation rate. The smaller the old γ particle size and the lower the temperature, the higher the nucleation rate and the smaller the nodule size. Therefore, in order to refine the nodule size, it is necessary to control to refine the old γ grain size and to lower the temperature for the transformation.

  In Patent Document 5, the former γ grain size is refined by the first stage cooling that is rapidly cooled after rolling, and the cooling rate and temperature range (620 to 680 ° C.) of the second stage cooling are controlled by increasing the lamellar spacing and the nodule size. The balance is achieved by adjusting the conditions necessary for the balance of miniaturization and growing by subsequent cooling of the third stage. According to this method, the average lamella spacing is certainly coarsened and the wire is softened. However, since the cooling is continuously performed in the pearlite growth process of the third stage cooling, the lamellar interval becomes finer as the transformation temperature decreases and the transformation progresses. Also, coupled with the relatively low transformation temperature range, the above-mentioned minute lamella region with a lamellar spacing of 120 nm or less increases, which cannot be reduced to 30% or less as in the present invention.

  Therefore, it is necessary to grow in a high temperature range where a fine lamellar structure is not generated in order to reduce the fine lamellar structure having a lamellar spacing of 120 nm or less. On the other hand, however, nodule size becomes coarse when nucleated at a high temperature, for example, isothermally maintained. In order to solve this problem, the present invention solves the problem by combining cooling and heating in the temperature control of the transformation. In other words, in order to refine the nodule size, it rapidly cools to prevent coarsening of the former γ grain size, and after cooling to a relatively low temperature to promote nucleation, the growth proceeds and the fine lamella increases. Before that, a coarse lamellar structure is stably grown by holding in a high temperature range by rapid heating.

  A specific manufacturing method of the present invention based on the above examination will be described below. Here, in FIG. 3, the heat pattern after hot rolling in the manufacturing method of this invention high carbon steel wire is shown.

  The steel slab having the composition described above is heated and hot-rolled at a finishing temperature of 1050 to 900 ° C. After finishing rolling in this hot rolling, the rolled wire is immediately cooled to a temperature in the range of 950 to 800 ° C. To do.

  Thereafter, from this temperature range, the rolled wire is subsequently rapidly cooled to a temperature in the range of 640 to 580 ° C. at an average cooling rate of 30 ° C./s or higher using cooling means such as blast cooling, mist cooling, and water cooling. Then, nucleation (transformation start) is performed.

  Subsequently, the rolled wire nucleated (starting transformation) by this rapid cooling is within the range of 650 to 720 ° C. from this temperature range within 2 seconds after reaching the temperature range of 640 to 580 ° C. (after this rapid cooling). Is rapidly heated to an average temperature increase rate of 50 ° C./s or more. Further, in this temperature range of 650 to 720 ° C., the nucleation temperature range is maintained at a gradual average heating rate or average cooling rate in the range of less than 0.5 ° C./s (isothermal holding), This temperature range is maintained until the transformation is completed. The pearlite transformation of the rolled wire rod is completed in the process of heat treatment of the heat pattern that combines such rapid heating and gradual heating or holding (isothermal holding).

  Holding until the completion of transformation by the combination of rapid heating and isothermal holding is a feature of the production method of the present invention. If it is a conventional method, conditions will overlap until nucleation (start of transformation) by rapid cooling to a temperature in the range of 640 to 580 ° C. at an average cooling rate of 30 ° C./s or more. However, after that, there is also the production efficiency of the wire, so that the combination of rapid heating and isothermal holding as in the present invention is not performed, and it is gradually cooled to room temperature as it is. Therefore, it is not necessary for the organization of the present invention.

  The reason why the above manufacturing method is more specific and the manufacturing conditions are limited will be described below.

Billet:
The steel slab (billet) is produced by continuous casting after melting the high-carbon steel having the above-mentioned chemical composition by a conventional method, or by further rolling the cast steel ingot.

Hot rolling:
When this steel slab is heated and hot rolled, by reducing the finishing temperature of the rolling to 1050 ° C. or lower, the recovery of austenite, recrystallization, and grain growth are suppressed, and the increase in strength is suppressed, Nodules can be made finer. The lower limit of the finishing temperature is set to 900 ° C. or higher because the load on the rolling mill becomes excessive if the temperature is too low.

Water cooling after hot rolling:
After the finish rolling in the hot rolling, the rolled wire is immediately water-cooled to a temperature in the range of 950 to 800 ° C. In this case, the average cooling rate is preferably 50 ° C./s or more. The temperature range achieved by this water cooling is a rule for providing the wire material with an appropriate descaling property. If the ultimate temperature is less than 800 ° C, the scale is difficult to peel off in the scale peeling step before wire drawing, and if it is 950 ° C or higher, the scale is easily peeled off. Thin, low-temperature scales and rust are generated. And since these low temperature scales and rust do not peel in the scale peeling process before wire drawing, they become a wrinkle of the drawn steel wire or cause wire breakage.

  In this respect, the lower limit of the temperature reached by water cooling is preferably 830 ° C, more preferably 850 ° C, and even more preferably 870 ° C. On the other hand, the upper limit of the temperature reached by water cooling is preferably 940 ° C, more preferably 930 ° C, and still more preferably 920 ° C.

Nucleation by rapid cooling:
Next, from this temperature range, the rolled wire is continuously cooled using a cooling means such as blast cooling, mist cooling, water cooling, etc., to cause nucleation (start of transformation). The average cooling rate during this rapid cooling is When it is less than 30 ° C./s, the old γ particle size becomes coarse. In this respect, the lower limit of the average cooling rate during the rapid cooling is preferably 40 ° C./s or more, more preferably 50 ° C./s or more, and further preferably 70 ° C./s or more.

And the ultimate temperature of the high carbon steel wire by this rapid cooling is set to a temperature within the range of 640 to 580 ° C., and the structure of the high carbon steel wire is nucleated to increase the nucleation rate, and the nodule size is refined. To do. If this reached temperature is less than 580 ° C., the nodule size is further refined. However, in the case of the above-described high carbon steel wire rods, the transformation rate is easy to proceed at a temperature lower than 580 ° C. because the pearlite growth rate is fast, and the nucleation rate is high, so it is difficult to avoid an increase in the fine lamellar region. is there. On the other hand, when the ultimate temperature exceeds 640 ° C., the nucleation rate becomes low and the nodule size becomes coarse (cannot be refined), so that the structure defined in the present invention cannot be obtained.

  In this respect, the lower limit of the reached temperature of the high carbon steel wire rod by this rapid cooling is preferably 590 ° C, more preferably 600 ° C, and even more preferably 610 ° C. On the other hand, the upper limit of the temperature reached by this rapid cooling is preferably 625 ° C, more preferably 630 ° C, and even more preferably 635 ° C.

Suppression of transformation progression by rapid heating:
Subsequently, the high carbon steel wire nucleated (starting transformation) by this rapid cooling is immediately heated rapidly to suppress (reduce) the progression of transformation at a low temperature. For this purpose, an average rate of temperature increase of 50 ° C./s or more from this temperature range to a temperature within the range of 650 to 720 ° C. within 2 seconds after reaching the temperature range of 640 to 580 ° C. (after this rapid cooling). It is necessary to heat the high carbon steel wire rapidly. In order to reduce minute lamellae having a lamella spacing of 120 nm or less, it is necessary to quickly raise the temperature to a high temperature range after nucleation (start of transformation).

  When the nucleation temperature range of 640 to 580 ° C. is maintained for more than 2 seconds after the start of the transformation by the rapid cooling, the transformation of the high carbon steel wire proceeds, and the lamella spacing is as small as 120 nm or less. Increases lamellae. In addition, even when the average rate of temperature increase is less than 50 ° C./s, transformation proceeds during temperature increase, and minute lamellae having a lamella interval of 120 nm or less increase. Moreover, if the temperature reached by the rapid heating is less than 650 ° C., the average lamella spacing cannot be increased to 150 nm or more. Moreover, when this ultimate temperature exceeds 720 degreeC, an average lamella space | interval will exceed 300 nm and will coarsen.

  In this respect, the lower limit of the reached temperature of the high carbon steel wire by the rapid heating is preferably 660 ° C. or higher, more preferably 670 ° C. or higher, more preferably 680 ° C. or higher. On the other hand, the upper limit of the temperature reached by rapid heating is preferably 710 ° C. or lower, more preferably 700 ° C. or lower, and even more preferably 690 ° C. or lower.

Temperature hold until transformation is complete:
Further, while holding the high carbon steel wire at a moderate average temperature rising rate or an average temperature decreasing rate in the range of less than 0.5 ° C./s within the temperature range of 650 to 720 ° C., the temperature range is maintained. Until the transformation is complete (in the temperature range). The pearlite transformation of the rolled wire rod is completed in the process of heat treatment with a heat pattern combining such rapid heating and isothermal holding. Here, when the average heating rate exceeds 0.5 ° C./s, the variation in lamella spacing increases (standard deviation increases). This is because the lamella interval changes remarkably at a high temperature with a slight temperature change. From the viewpoint of reducing variation in lamella spacing, it is preferable that the change in transformation temperature during lamella growth is small.

  In addition, in maintaining the moderate average heating rate or average cooling rate within the range of less than 0.5 ° C./s, the average heating rate and the average cooling rate are set to 0, and the temperature is 650 to 720 ° C. You may hold | maintain within the temperature range.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated more concretely, this invention is not limitedly interpreted by this Example.

  By changing the heat treatment conditions after hot rolling of the high carbon steel hot-rolled wire having the composition shown in Table 1 as shown in Table 2, the pearlite structure is commonly used, but the average lamella spacing L and the spacing of 120 nm among the lamellae are 120 nm. The following minute lamella regions, average nodule diameter D, and wires having different relations between average nodule diameter D and average lamella spacing L were produced in the laboratory. And these wire-drawing property was evaluated from the presence or absence of a disconnection, deformation resistance, etc. The results are shown in Table 2.

  Specific production conditions for these high carbon steel hot-rolled wires will be described below. The high-carbon steel is melted in a converter so as to have the chemical composition of the wire shown in Table 1, and the steel ingot is cracked and rolled to produce a 155 mm square billet. After heating to about 1150 ° C., Hot rolling was performed at a finishing (end) temperature of 1050 to 1000 ° C. to obtain a wire having a diameter of 5.5 mm. In addition, in the chemical component composition of the wire of Table 1, the impurity elements of Mg, Ca, and REM were less than the detection limit amount of less than 0.007% in the total content in each example.

The wire rod after completion of the hot rolling was immediately cooled in the range of 950 to 800 ° C. by nozzle injection of cooling water in a cooling zone provided on the rolling line. At this time, the reached temperature was controlled by changing the amount of water and the cooling time. Further, the wire was subsequently cooled in a range of 640 to 580 ° C. by blast cooling or mist cooling (cooling water was sprayed in mist form). The cooling rate and the ultimate temperature were controlled by changing the air volume in blast cooling and changing the air / water ratio (air to water ratio) and spraying time in mist cooling. Although blast cooling and mist cooling were used this time, other cooling methods may be used. Thereafter, a heat treatment was performed in combination with the rapid heating and the moderate temperature increase, which is a feature of the production method of the present invention, to obtain a wire material in which the pearlite transformation was completed. In the heating / temperature raising step, after the air cooling on the conveyor was stopped, the wire rod was heated using a heater installed on the wire rod conveyer, and the rate of temperature rise and the temperature reached were controlled according to the heater conditions.
As a result, the pearlite structure has an average lamella interval L, a minute lamella region having an interval of 120 nm or less among the lamellae, an average nodule diameter D, and a wire having a different relationship between the average nodule diameter D and the average lamella interval L. Produced.

Tissue measurement:
Sample materials were sampled from the wire, and the ratio of the area of pearlite, the average nodule diameter D of pearlite, the average lamella spacing L, and the ratio of lamellae having a spacing of 120 nm or less among pearlite lamellae were measured. Observation was performed at any 10 locations of the test material, and the average of the measured values was taken as the respective value (average value).

  The pearlite area ratio is determined by etching a sample obtained by cutting a wire and mirror-polishing the cross section with a mixed solution of nitric acid and ethanol, and analyzing the structure at the center position between the surface and the center of the wire cross section with an SEM (scanning electron microscope). , Magnification was 1500), and arbitrary five visual fields were observed and determined by point calculation. As a result, the structure of all examples except for No. 8 (steel type I) including the invention examples and comparative examples, the area ratio of pearlite is 95% by area or more, other pro-eutectoid ferrite, pro-eutectoid cementite, residual The structure other than pearlite such as austenite, bainite or martensite was less than 5 area%.

For the average nodule diameter D, the sample was prepared in the same manner as described above, the structure was observed with an optical microscope (magnification 200), and the equivalent circle diameter of the nodule at the center position between the surface and the center of the wire cross section was determined. The particle size number G was determined to the first decimal place in accordance with the ferrite particle size measurement method (JISG 0552), and was converted to μm units by the following formula.
Nodule diameter D (μm) = 10 × 2 ^{(10-G) / 2}

  The average lamella spacing L was mirror-polished in the same manner as above, and the center position between the surface and the center of the cross section of the wire rod etched by the same method as above was observed with SEM, and a 3000 times photograph was taken with 6 fields of view. In order to measure at a location where the lamella is nearly perpendicular to the observation surface, a line segment is drawn at right angles to the lamella in five colonies from the first to the fifth in the field of view using the photographs of each field of view. The lamella interval (nm) was obtained from the length of the line segment and the number of lamellae crossing the line segment, and the lamella interval of all the line segments was averaged. At the same time, the ratio of fine lamella is obtained by dividing the number of data of 120 nm or less by the total number of data (N = 30) based on the measured lamella spacing data (N = 30) at the time of measuring the lamella spacing. Asked.

  The wire drawability of each of the above wire rods was evaluated based on the degree of increase in the breakage resistance and the pull-out resistance by conducting a wire drawing test under the following wire drawing conditions. In the wire drawing test, 1 ton of 5.5 mmφ rolled material was dry drawn to 1.0 mmφ. The disconnection resistance was evaluated by the presence or absence of disconnection when drawn by 1 ton.

  In the wire drawing test, each wire is immersed in hydrochloric acid to remove scale beforehand, and then a lubrication treatment is performed to form a phosphate film on the surface of the wire, and then a multistage dry wire drawing machine (tester) is used. The wire was drawn to a diameter of 1.0 mm. Drawing was performed by high-speed drawing with a final drawing speed of 1000 m / min.

  In each case, a winding test was conducted after the wire drawing test to investigate the presence of delamination.

  In each invention example, as shown in Tables 1 and 2, the chemical component composition, the processing conditions of cooling after hot rolling and pearlite transformation are performed within a preferable range. Therefore, as shown in Table 2, each of Invention Examples 1 to 5 has a pearlite (fraction) of 95 area% or more in the structure of the wire, the average lamella spacing L of pearlite is in the range of 150 to 300 nm, and Of the lamellae of this pearlite, the lamellae region having an interval of 120 nm or less is 30% or less, the average nodule diameter D of this pearlite is 40 μm or less, and the average nodule diameter D and the average lamella interval L And have a pearlite structure satisfying the relationship of D <−0.1 × L + 60. As a result, each of the above invention examples has excellent dry drawing properties such that the steel wire is not embrittled and can be drawn without disconnection.

  On the other hand, as shown in Tables 1 and 2, the chemical composition of each of Comparative Examples Nos. 6 to 11 is out of the scope of the invention, and disconnection occurs regardless of whether the organization requirements are satisfied.

  In Comparative Examples No. 12 to 20, although the chemical composition is within the scope of the invention, the manufacturing conditions are out of the scope of the invention, and although the structure of the wire is pearlite of 95 area% or more, among the average lamella spacing L and lamella This is a pearlite structure in which any one of the above-described relationships between a minute lamella region having an interval of 120 nm or less, a standard deviation of the lamella interval, an average nodule diameter D, and an average nodule diameter D and an average lamella interval L is removed. As a result, each of these comparative examples is inferior in dry wire drawing such as delamination or disconnection.

  That is, in Comparative Example No12, the average cooling rate of rapid cooling after water cooling after the finish rolling of hot rolling is less than 30 ° C./s. In Comparative Example No13, the temperature reached by this rapid cooling exceeds 640 ° C., which is too high. In Comparative Example No. 14, the temperature reached by this rapid cooling is less than 580 ° C., which is too low.

  In Comparative Examples No. 15 and 16, the stay (holding) time in the temperature range of 640 to 580 ° C. of the rolled wire nucleated (starting transformation) by rapid cooling is longer than 5 seconds. In Comparative Example No17, the average rate of temperature increase in rapid heating from the temperature range of 640 to 580 ° C. is too small at less than 50 ° C./s. In Comparative Example No18, the temperature reached by this rapid heating is less than 650 ° C., which is too low. In Comparative Example No19, the temperature reached by this rapid heating exceeds 720 ° C., which is too high. In Comparative Example No20, in the subsequent heating at a moderate average heating rate in the range of 0.5 to 2 ° C / s within the temperature range of 650 to 720 ° C, the average heating rate exceeds 2 ° C / s. It ’s too big.

  The chemical component composition of the present invention, the processing conditions for cooling and pearlite transformation after hot rolling, and the specific significance of the specific pearlite structure consisting of the above-mentioned provisions for the excellent dry-drawing properties are supported.

  ADVANTAGE OF THE INVENTION According to this invention, the high carbon steel wire which has the outstanding wire drawing property which improved remarkably the dry wire drawing process, and its manufacturing method can be provided. For this reason, the present invention can be suitably used as a high carbon steel wire for ultra fine steel wires used for tire reinforcing steel cords, bead wires, saw wires, belt cords and the like.

Claims (2)

  It is a high carbon steel wire used for dry drawing, and in mass%, C: 0.68 to 0.86%, Si: 0.05 to 0.5%, Mn: 0.1 to 0.8 Each of which is composed of Fe and unavoidable impurities, the structure has 95% or more of pearlite, the average lamella spacing L of this pearlite is in the range of 150 to 300 nm, and the lamellae of this pearlite Among them, the area of the lamella having an interval of 120 nm or less is 30% or less, the standard deviation of the lamella interval is 50 nm or less, the average nodule diameter D of this pearlite is 40 μm or less, and the average nodule A high carbon steel wire wire excellent in dry drawing, characterized in that the diameter D and the average lamella spacing L satisfy a relationship of D <−0.1 × L + 60.   It is a manufacturing method of the high carbon steel wire with which it uses for dry drawing, Comprising: By mass%, C: 0.68-0.86%, Si: 0.05-0.5%, Mn: 0.1 Each steel slab comprising 0.8% and the balance Fe and inevitable impurities is heated and hot-rolled at a finishing temperature of 1050 to 900 ° C. Immediately after finishing rolling in this hot rolling, 950 to 800 Water-cooled to a temperature within the range of ℃, and then rapidly cooled to a temperature within the range of 640-580 ° C at an average cooling rate of 30 ° C / s or higher, and then within a range of 650-720 ° C within 2 seconds from this temperature range The temperature is rapidly heated to an average temperature increase rate of 50 ° C./s or more, and the temperature rise is less than 0.5 ° C./s (including 0 ° C./s) within the temperature range of 650 to 720 ° C. A wire rod that has completed the pearlite transformation while being held at the speed or temperature drop rate. The pearlite has a pearlite of 95 area% or more, the average lamella spacing L of this pearlite is in the range of 150 to 300 nm, and the lamellae having a spacing of 120 nm or less among the lamellae of this pearlite is 30% In addition, the standard deviation of the lamella spacing is 50 nm or less, the average nodule diameter D of the pearlite is 40 μm or less, and the average nodule diameter D and the average lamella spacing L are D <−. The manufacturing method of the high carbon steel wire excellent in the dry-drawing property characterized by setting it as the structure | tissue which satisfy | fills the relationship of 0.1 * L + 60.

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