JP6501036B2 - Steel wire - Google Patents

Description

  The present invention relates to a steel wire used for prestressed concrete and the like, which has a tensile strength of at least 1800 MPa and a hydrogen resistance without deteriorating the drawability and twisting characteristics without reducing the manufacturability. The present invention relates to a high strength steel wire with improved embrittlement characteristics.

The disclosure of Japanese Patent Application No. 2016-063666, filed March 28, 2016, is incorporated herein by reference in its entirety.
All documents, patent applications, and technical standards described herein are incorporated herein by reference as if each individual document, patent application, and technical standard were specifically and individually described. Captured by reference.

  The steel wire used for the tension of prestressed concrete mainly used for civil engineering and construction structures is called PC steel wire. Conventionally, a wire obtained by subjecting a piano wire to patenting to make the structure pearlite, and then performing wire drawing and stranding is called a strand. The pearlite steel wire for PC strand is manufactured by carrying out the aging heat treatment of this strand at the last process. Hereinafter, the “perlite steel wire for PC strand” may be simply described as “steel wire” or “perlite steel wire”.

  In recent years, a pearlite steel wire for PC strand having high tensile strength exceeding 1800 MPa is required for the purpose of reduction of construction cost and weight reduction of a structure. However, there is a problem that the resistance to hydrogen embrittlement of the steel wire decreases as the pearlite steel wire for PC strand is strengthened.

  As a technique for improving the resistance to hydrogen embrittlement of steel wire, Patent Document 1 discloses a plate-like shape in pearlite in a region at a depth of at least D / 10 (D is the diameter of the steel wire) of the surface of PC steel wire. A high strength PC steel wire having an average aspect ratio of cementite of 30 or less has been proposed. However, in order to obtain the PC steel wire described in Patent Document 1, processing such as rapid heating and holding for 5 seconds or more at 450 ° C. or more is required in the final step after cold drawing. The method becomes complicated.

  Moreover, in patent document 2, when the wire diameter of PC steel wire is set to D, the high strength PC steel wire which made the hardness of the area | region of 0.1 D from the surface 1.1 or less times the internal hardness is Proposed. However, in order to obtain the hardness as described in Patent Document 2, for example, after heating the wire to 900 ° C. to 1100 ° C., the partial pearlite transformation treatment is carried out by holding it in a temperature range of 600 ° C. to 650 ° C. After the application, the temperature range is kept below 540 ° C. to less than 600 ° C., the finish rolling is performed at 700 ° C. to 950 ° C. by hot rolling, and then the temperature range is cooled to 500 ° C. to 600 ° C. After drawing, a complicated manufacturing method is required such as holding in a temperature range of more than 450 ° C. to 650 ° C. for 2 seconds to 30 seconds and subsequently subjecting to a bluing treatment at 250 ° C. to 450 ° C.

Japanese Patent Laid-Open No. 2004-360005 JP, 2009-280836, A

  As described above, according to the conventional technique for improving the resistance to hydrogen embrittlement of high strength steel wire having wire drawability and twisting property, the manufacturing method becomes complicated and the productivity is lowered. It was difficult to achieve both the tensile strength of the wire and the resistance to hydrogen embrittlement.

  Therefore, in view of such circumstances, the present invention is a pearlite steel wire having high strength and excellent resistance to hydrogen embrittlement without deteriorating the drawability and twisting characteristics without reducing the productivity. Intended to provide.

  First, the structure of the steel wire needs to be pearlite which can be easily strengthened at the time of wire drawing. The present inventors investigated in detail the relationship between the chemical composition and the structure that affect the resistance to hydrogen embrittlement of a high strength steel wire having a tensile strength of 1800 MPa or more. As a result, the present inventors have found the following findings on a pearlite steel wire that can be manufactured inexpensively by improving the hydrogen embrittlement resistance of high strength steel wire and improving productivity. .

  Generally, high strength pearlite steel wire is manufactured by the following method. The wire is dipped in a Pb bath or a molten salt bath installed in a hot rolling line to perform patenting treatment to complete pearlite transformation, and then the patented wire is drawn. And the heat stretch process which heat-processes aging heat treatment and a tension | tensile_strength is performed with respect to the wire after wire-drawing.

  The present inventors have found that the following method is effective to improve the hydrogen embrittlement resistance of pearlite steel wire. It is a method of appropriately finely granulating lamellar cementite in the pearlite structure by devising the conditions of wire drawing. Lamellar cementite is a single crystal platy cementite among those in which platy ferrite and cementite after heat treatment are alternately arranged (perlite structure), and this lamellar cementite is refined by wire drawing. Be done. In the present specification, lamellar cementite that has been refined by wire drawing is referred to as “lamellar cementite”.

  Here, FIG. 1 is a conceptual diagram for explaining the lamellar cementite, and schematically shows a structure enlarged in the order of (a), (b) and (c). As shown in FIG. 1 (a), within the former austenite grains (area surrounded by austenite grain boundaries), there are a plurality of pearlite blocks in which crystal orientations of ferrite are aligned. Do. The pearlite block is composed of a plurality of pearlite colonies having the same crystal orientation of ferrite but different directions of cementite (lamellar cementite).

  As shown in FIG. 1 (b), in appearance, the lamellar cementite looks like a plate-like cementite sandwiched between plate-like ferrites, like the lamellar cementite before wire drawing. If this lamellar cementite is further expanded, as shown in FIG. 1 (c), the lamellar cementite is composed of a large number of fine cementite grains divided by wire drawing. The present inventors have found that it is possible to improve the hydrogen embrittlement resistance of pearlite steel wire by appropriately finely granulating lamellar cementite in the pearlite structure. The reason why the method of finely graining lamellar cementite in the pearlite structure by wire drawing in this way is effective in improving the resistance to hydrogen embrittlement is not necessarily clear, but it is possible by wire forming and wire drawing It is presumed that the improvement effect of hydrogen embrittlement resistance can be obtained.

  That is, lamellar cementite, which is an embrittled structure in the structure of pearlite steel wire, causes hydrogen embrittlement when hydrogen intrudes under constant tensile stress. On the other hand, pearlitic cementite, which is the embrittlement structure, is pearlite by appropriately forming fine grained lamellar cementite by wire drawing, in addition to forming the texture by rolling or patenting the wire. It is possible to improve the ductility of the structure and to improve the hydrogen embrittlement resistance of the steel wire.

  The present inventors are able to improve the hydrogen embrittlement resistance of pearlite steel wire having high strength as compared with the prior art by appropriately forming fine grained lamellar cementite of lamellar cementite of steel wire. I found it to be.

The present invention has been made based on the above findings, and the summary thereof is as follows.
(1)
The steel wire according to one aspect of the present invention is, as a chemical component, C: 0.80% to 1.20%, Si: 0.10% to 2.00%, Mn: 0.20% by mass%. Component that contains 1.00%, P: 0.030% or less, S: 0.030% or less, O: 0.0100% or less and N: 0.0010% to 0.0100%, and is optionally contained Al: 0. 100% or less, Cr: 2.00% or less, Mo: 1.00% or less, V: 0.30% or less, B: 0.0050% or less, Ti: 0.050% or less, Nb: 0.050% or less, Zr: 0.050% or less, Ni: 2.00% or less, Cu: 1.00% or less, Ca: 0.010% or less and Mg: 0.010% or less, The balance is Fe and impurities, the structure contains pearlite, the area ratio of the pearlite is 90% or more, lamellar During cementite, it is the area ratio of particle size 10nm or less of cementite grains equivalent circle diameter of 50% or more, a tensile strength of at least 1800 MPa, less than 3000 MPa.
(2)
The steel wire described in the above (1) is, as a chemical component, in mass%, Al: 0.005% to 0.100%, Cr: 0.01% to 2.00%, Mo: 0.01% to 1.00%, V: 0.01% to 0.30%, B: 0.0001% to 0.0050%, Ti: 0.001% to 0.050%, Nb: 0.001% to 0.. 050%, Zr: 0.001% to 0.050%, Ni: 0.01% to 2.00%, Cu: 0.01% to 1.00%, Ca: 0.0001% to 0.010% And Mg: may contain one or more selected from the group consisting of 0.0001% to 0.010%.

  According to the present invention, it is possible to provide a high strength pearlitic steel wire excellent in hydrogen embrittlement resistance and having a tensile strength of at least 1800 MPa. And this high strength pearlite steel wire contributes to the reduction of construction cost of civil engineering and buildings and to weight reduction, and the industrial effect is extremely remarkable.

It is a conceptual diagram for demonstrating lamellar cementite, and has shown typically the structure expanded in order of (a), (b), and (c).

  As mentioned above, generally, high strength pearlite steel wire is manufactured as follows. First, a patenting treatment is performed on a wire manufactured by hot rolling in a molten salt tank installed in a Pb bath or a hot rolling line. Subsequently, this wire is drawn. After that, “heat stretch treatment” is performed on the wire, which is heat-treated while applying aging heat treatment or tension.

  In order to improve the hydrogen embrittlement resistance of high strength pearlite steel wire, as described above, lamellar cementite, which is an embrittled structure of pearlite steel wire, is averaged after wire rolling or reheating patenting treatment. It is effective to suppress the grain growth of cementite grains in lamellar cementite having a fine grain size by reducing the thickness of the lamellar cementite and suppressing the heat generation at the time of wire drawing. That is, it is possible to improve the ductility of lamellar cementite by suppressing lamellar cementite thickness before wire drawing and grain growth at the time of wire drawing and refining cementite, and to suppress the generation of hydrogen cracking. . Therefore, it is possible to suppress the deterioration of the hydrogen embrittlement resistance even with high strength.

  That is, in a steel wire, the area ratio of pearlite is 90% or more, and in lamellar cementite, the area ratio of cementite particles having a circle equivalent diameter and a particle diameter of 10 nm or less (hereinafter simply referred to as “area of cementite particles of 10 nm or less The strength of the steel wire after wire drawing and heat stretching can be made 1800 MPa or more by setting the ratio to 50% or more, and hydrogen embrittlement resistance of the steel wire It is possible not to degrade the

  First, the reason which limited the range of the chemical component which the steel wire in this embodiment contains is demonstrated. In addition, "%" showing content of each component in the following description means mass%.

C: 0.80% to 1.20%
C is an element necessary for making the structure of the steel wire pearlite and securing the tensile strength of the steel wire after wire drawing. If the C content is less than 0.80%, pro-eutectoid ferrite is formed in the wire, and it becomes difficult to secure a predetermined tensile strength of, for example, 1800 MPa. Therefore, the lower limit of the C content is set to 0.80%. In order to increase the tensile strength more stably, the C content is preferably 0.85% or more, more preferably 0.90% or more. On the other hand, when the C content exceeds 1.20%, proeutectoid cementite increases in the wire, and the wire drawability of the wire is degraded. Therefore, the upper limit of the C content is 1.20%. The C content is preferably 1.15% or less, more preferably 1.10% or less, in order to eliminate variations and obtain wire drawability more stably.

Si: 0.10% to 2.00%
Si is an element necessary to enhance tensile strength by solid solution strengthening, as well as to enhance relaxation characteristics. If the Si content is less than 0.10%, these effects are insufficient. Therefore, the lower limit of the Si content is 0.10%. In order to obtain higher relaxation properties, the Si content is preferably 0.30% or more, more preferably 0.50% or more. On the other hand, when the Si content exceeds 2.00%, these effects saturate and the wire drawability of the wire is deteriorated, and the productivity of the steel wire is lowered. Therefore, the upper limit of the Si content is 2.00%. The Si content is preferably 1.80% or less, more preferably 1.50% or less, in order to more stably prevent cracking during hot rolling.

Mn: 0.20% to 1.00%
Mn is an element necessary to increase the tensile strength of the steel after pearlite transformation. If the Mn content is less than 0.20%, this effect is insufficient. Therefore, the lower limit of the Mn content is 0.20%. In order to increase the tensile strength more stably, the Mn content is preferably 0.30% or more, more preferably 0.50% or more. On the other hand, if the Mn content exceeds 1.00%, the local strength increases due to segregation, and the twisting characteristics are degraded. Therefore, the upper limit of the Mn content is 1.00%. From the viewpoint of the alloy cost, the Mn content is preferably 0.90% or less, more preferably 0.80% or less.

P: 0.030% or less P is an element contained in the steel wire as an impurity and segregating at grain boundaries to deteriorate the hydrogen embrittlement resistance. In particular, when the P content exceeds 0.030%, the deterioration of the hydrogen embrittlement resistance becomes remarkable. Therefore, the P content is limited to 0.030% or less. The P content is preferably 0.015% or less, more preferably 0.010% or less. The lower limit of the P content includes 0%. However, the lower limit of the P content is preferably 0.0001% in consideration of the current refining technology and the production cost.

S: 0.030% or less S, like P, is an element contained in the steel wire as an impurity and segregating at grain boundaries to deteriorate the hydrogen embrittlement resistance. In particular, when the S content exceeds 0.030%, the deterioration of the hydrogen embrittlement resistance becomes remarkable. Therefore, the S content is limited to 0.030% or less. The S content is preferably 0.015% or less, more preferably 0.010% or less. The lower limit of the S content includes 0%. However, in view of the current refining technology and production cost, the lower limit of the S content is preferably 0.0001%.

O: 0.0100% or less O is an element which is unavoidably contained in a steel wire and exists as an oxide such as Al, Ti or Mn. In particular, when the O content exceeds 0.0100%, a coarse oxide is formed, which causes a break in wire drawing. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0080% or less, more preferably 0.0050% or less. The lower limit of the O content includes 0%. However, in view of the current refining technology and production cost, the lower limit of the O content is preferably 0.0001%.

N: 0.0010% to 0.0100%
N is an element necessary to form nitride / carbonitride with Al, Ti, Nb, and V, to refine the grain size, and to improve the ductility of the steel wire. If the N content is less than 0.0010%, this effect can not be obtained. Therefore, the lower limit of the N content is 0.0010%. In order to improve the ductility more stably, the N content is preferably 0.0015% or more, more preferably 0.0025% or more. On the other hand, when the N content exceeds 0.0100%, fine precipitates increase to lower the ductility of the steel wire. Therefore, the upper limit of the N content is 0.0100%. In order to obtain the ductility of the steel wire more stably, the N content is preferably 0.0070% or less, more preferably 0.0050% or less.

  The above is the basic composition of the steel wire according to the present embodiment, and the balance is iron and impurities. In addition, the "impurity" in "the balance is Fe and an impurity" refers to what is unavoidably mixed from the ore as a raw material, scrap, or a manufacturing environment etc., when manufacturing steel industrially.

  In addition to the above-described basic components and impurities, the steel wire according to the present embodiment is further selectively selected from Al, Cr, Mo, V, B, Ti, Nb, Zr, Ni, Cu, Ca and Mg. It may contain one or more selected from the group consisting of

  Below, the numerical limitation range of the component contained arbitrarily and the reason for limitation are demonstrated.

Al: not more than 0.100% Al functions as a deoxidizing element, and at the same time, by forming AlN, the crystal grains are refined. As a result, it has the effect of improving the ductility of the steel wire. In addition, it has the effect of refining the crystal grains to improve the hydrogen embrittlement resistance of the steel wire. In order to obtain such an effect, the Al content is preferably made 0.005% or more. On the other hand, when the Al content exceeds 0.100%, these effects may be saturated and the productivity may be deteriorated. Therefore, 0.100% or less of Al content is preferable. The Al content is more preferably 0.008% to 0.070%, still more preferably 0.010% to 0.050%.

Cr: 2.00% or less Cr has the effect of enhancing the tensile strength of the steel after pearlite transformation. In order to obtain such an effect, the Cr content is preferably 0.01% or more. On the other hand, when the Cr content exceeds 2.00%, not only the alloy cost is increased, but also the unnecessary martensitic structure is easily generated in the steel wire according to the present embodiment, wire drawability and steel There is a risk of deteriorating the hydrogen embrittlement resistance of the wire. Therefore, the Cr content is preferably 2.00% or less. The Cr content is more preferably 0.05% to 1.00%, and still more preferably 0.10% to 0.50%.

Mo: 1.00% or less Mo has the effect of enhancing the tensile strength of the steel wire after aging heat treatment. When it is desired to obtain such an effect, the Mo content is preferably 0.01% or more. On the other hand, when the Mo content exceeds 1.00%, not only the alloy cost increases, but also the steel wire according to the present embodiment is likely to have an unnecessary martensitic structure, resulting in wire drawability and steel There is a risk of deteriorating the hydrogen embrittlement resistance of the wire. Therefore, the Mo content is preferably 1.00% or less. The Mo content is more preferably 0.03% to 0.50%, still more preferably 0.05% to 0.30%.

V: 0.30% or less V precipitates as carbide VC, has an effect of improving the hydrogen embrittlement resistance of the steel wire as well as enhancing the tensile strength. In order to obtain such an effect, the V content is preferably 0.01% or more. On the other hand, when the V content exceeds 0.30%, the alloy cost is increased and the productivity is lowered. Therefore, the V content is preferably 0.30% or less. The V content is more preferably 0.03% to 0.20%, still more preferably 0.05% to 0.15%.

B: 0.0050% or less B has the effect of enhancing the tensile strength of the steel wire after aging heat treatment and the effect of improving the hydrogen embrittlement resistance of the steel wire. In order to obtain such an effect, the B content is preferably made 0.0001% or more. On the other hand, when the B content exceeds 0.0050%, these effects saturate and the manufacturability is lowered. Therefore, the B content is preferably 0.0050% or less. The B content is more preferably 0.0003% to 0.0040%, and still more preferably 0.0005% to 0.0020%.

Ti: 0.050% or less Ti functions as a deoxidizing element and also precipitates carbides and nitrides to increase the tensile strength of the steel wire, and refines the crystal grains to improve the ductility of the steel wire. Have an effect. In order to obtain such an effect, the Ti content is preferably 0.001% or more. On the other hand, when the Ti content exceeds 0.050%, these effects saturate, and coarse oxides may be generated to deteriorate the wire drawability of the steel wire. Therefore, the Ti content is preferably 0.050% or less. The Ti content is more preferably 0.003% to 0.040%, still more preferably 0.005% to 0.030%.

Nb: 0.050% or less Nb has the effect of increasing the tensile strength of the steel wire by depositing carbides and nitrides, and the effect of refining the crystal grains and improving the ductility of the steel wire. In order to obtain such an effect, the Nb content is preferably 0.001% or more. On the other hand, if the Nb content exceeds 0.050%, these effects may be saturated and the twisting characteristics of the steel wire may be degraded. Therefore, the Nb content is preferably 0.050% or less. The Nb content is more preferably 0.003% to 0.040%, still more preferably 0.005% to 0.030%.

Zr: 0.050% or less Zr functions as a deoxidizing element, and also has the effect of reducing the solid solution S by forming a sulfide and improving the hydrogen embrittlement resistance of the steel wire. In order to obtain such an effect, it is preferable to make the Zr content 0.001% or more. On the other hand, if the Zr content exceeds 0.050%, these effects saturate and coarse oxides may be formed to deteriorate the wire drawability of the steel wire. Therefore, the Zr content is preferably 0.050% or less. The Zr content is more preferably 0.003% to 0.040%, still more preferably 0.005% to 0.030%.

Ni: 2.00% or less Ni has the effect of suppressing the entry of hydrogen. In order to obtain such an effect, the Ni content is preferably 0.01% or more. On the other hand, when the Ni content exceeds 2.00%, not only the alloy cost is increased, but also the unnecessary martensitic structure is easily generated in the steel wire according to the present embodiment, and wire drawing of the steel wire And hydrogen embrittlement resistance may be degraded. Therefore, the Ni content is preferably 2.00% or less. The Ni content is more preferably 0.04% to 1.00%, still more preferably 0.06% to 0.60%.

Cu: 1.00% or less Cu has the effect of suppressing the penetration of hydrogen. In order to obtain such an effect, the Cu content is preferably 0.01% or more. On the other hand, when the Cu content exceeds 1.00%, the hot ductility is inhibited and the productivity is deteriorated, and an unnecessary martensitic structure is easily generated in the steel wire according to the present embodiment, and the steel There is a possibility that the wire drawability and hydrogen embrittlement resistance of the wire may be deteriorated. Therefore, the Cu content is preferably 1.00% or less. The Cu content is more preferably 0.02% to 0.50%, still more preferably 0.03% to 0.30%.

Ca: 0.010% or less Ca functions as a deoxidizing element and also has the effect of reducing solid solution S by forming a sulfide and improving the hydrogen embrittlement resistance. In order to obtain such an effect, the Ca content is preferably made 0.0001% or more. On the other hand, when the Ca content exceeds 0.010%, these effects saturate and coarse oxides are formed, which may deteriorate wire drawability. Therefore, the Ca content is preferably 0.010% or less. The Ca content is more preferably 0.0003% to 0.0050%, and still more preferably 0.0010% to 0.0030%.

Mg: 0.010% or less Mg functions as a deoxidizing element, and also has the effect of reducing solid solution S by forming a sulfide and improving hydrogen embrittlement resistance. In order to obtain such an effect, the Mg content is preferably made 0.0001% or more. On the other hand, when the Mg content exceeds 0.010%, these effects saturate and coarse oxides are formed, which may deteriorate wire drawability. Therefore, the Mg content is preferably 0.010% or less. The Mg content is more preferably 0.0003% to 0.0050%, still more preferably 0.0010% to 0.0030%.

  Next, the structure of the steel wire according to the present embodiment will be described.

  The structure of the steel wire according to the present embodiment includes pearlite. When the area ratio of this pearlite is less than 90%, the tensile strength of the steel wire after wire drawing and aging heat treatment is lowered, and the twisting property is deteriorated. Therefore, the area ratio of this perlite is made 90% or more. Preferably, the area ratio of perlite is 95% or more. The area ratio of pearlite may be 100%. On the other hand, the remaining part of the structure of the steel wire according to the present embodiment, that is, the structures other than pearlite is non-pearlite structures of ferrite, bainite, pseudo pearlite, proeutectoid cementite and martensite. These non-pearlite structures degrade the occurrence of cracking during wire drawing and hydrogen embrittlement resistance of the steel wire after wire drawing and aging heat treatment. Therefore, the area ratio of non-perlite structure is 10% or less. The area ratio of pearlite can be determined by subtracting the area ratio of non-perlite structure from 100%.

  Specifically, the area ratio of pearlite can be determined by the following method. In addition, D shows the diameter (unit mm) of a steel wire. In a steel wire sample (sample), an L cross section parallel to the longitudinal direction of the steel wire is mirror-polished and then picral etched. Then, five fields of view are taken at a magnification of 2000 × using an SEM at three positions 50 μm deep, D / 4, and D / 2 from the surface of the etched L cross section. The area per field of view is 60 μm × 40 μm. Mark the area of non-pearlite structure such as martensite, bainite, ferrite, etc. using the SEM photograph of each field of view obtained, obtain the area ratio by binarizing with image analysis software, remove them from the whole, That is, the area ratio of the non-perlite structure minus 100% can be obtained as the area ratio of the perlite.

  The pearlite steel wire is subjected to patenting after wire rolling or reheating, and the grain diameter of the fine cementite particles present in the fine lamellar cementite obtained by subsequent wire drawing becomes 10 nm or less in equivalent circle diameter . When the temperature of the steel wire rises due to heat generation during wire drawing, the finely divided cementite particles grow, and the area ratio of cementite particles of 10 nm or less becomes less than 50% of the total lamellar cementite and the ductility decreases. Lower the hydrogen embrittlement resistance. Therefore, the area ratio of cementite grains of 10 nm or less is set to 50% or more with respect to the area of all lamellar cementite. However, in order to make the area ratio of cementite particles of 10 nm or less more than 90%, it is necessary to increase drawing strain or to enhance heat generation during drawing and cooling after drawing, resulting in high cost. The area ratio of cementite particles of 10 nm or less is preferably 90% or less.

In addition, the size of cementite particles in lamellar cementite, when the diameter (wire diameter) of the steel wire is D in unit mm, the L cross section of the steel wire is polished and the depth from the surface is 0.01 Using a transmission electron microscope at × D, 0.25 × D, photograph a dark field formed by inserting a limited field stop into a cementite diffraction spot in TEM at an accelerating voltage of 200 kV and using this as an image Obtained by analysis. First, in the imaging of a TEM dark field image, an area of 500 nm × 500 nm is imaged 10 fields at a depth from each surface. Next, the area of cementite particles with a particle size of 10 nm or less is calculated by analyzing the photographed photograph and determining the total area and equivalent circle diameter (diameter of a circle having the same area as the cementite particle area) of lamellar cementite. Do. The total area of the lamellar cementite obtained in each view and the area of cementite particles having a particle diameter of 10 nm or less are totaled, and the area ratio of the cementite particles of 10 nm or less is calculated by the following equation. The area ratio of cementite particles of 10 nm or less in lamellar cementite of a steel wire according to the present invention can be obtained.
Area ratio of cementite particles of 10 nm or less (%) = (total area of cementite particles having a circle equivalent diameter of 10 nm or less / total area of lamellar cementite) × 100

  By satisfying the above-described chemical composition and structure, it is possible to obtain a steel wire suitable as a PC steel wire having a tensile strength of 1800 MPa or more and excellent in hydrogen embrittlement resistance. If the tensile strength is less than 1800 MPa, it is impossible to achieve the purpose of reducing the construction cost and reducing the weight of the structure as a pearlite steel wire for PC strands. In addition, when the tensile strength is 3000 MPa or more, hydrogen embrittlement becomes remarkable. Therefore, the tensile strength of the steel wire according to the present embodiment is set to 1800 MPa or more and less than 3000 MPa. In addition, the tensile strength of the steel wire according to the present embodiment is measured by a test method according to JIS Z 2241: 2011, and the test piece shape is cut into a 300 mm length steel wire as drawn, and the distance between chucks It is a value of breaking strength at the time of a tensile test at 250 mm. In order to obtain the above-described steel wire, the steel wire may be manufactured by a manufacturing method described later.

  Next, the preferable manufacturing method of the steel wire which concerns on this embodiment is demonstrated.

  The steel wire according to the present embodiment can be manufactured as follows. In addition, the manufacturing method of the steel wire demonstrated below is an example for obtaining the steel wire which concerns on this embodiment, It does not limit by the following procedures and methods, It is a method which can implement | achieve the structure of this invention For example, any method can be adopted.

  First, steel is melted so as to have the above-mentioned chemical components, and then steel slabs are manufactured by continuous casting. Slab rolling may be performed on steel slabs after continuous casting. Next, the obtained billet is heated to a temperature of 1050 ° C. or higher, and hot rolled at a finish rolling temperature of 850 ° C. or higher to obtain a wire rod. Thereafter, the wire rod obtained after finish rolling is wound into a ring. At this time, the winding temperature is set to 950 ° C. or less. The coiling temperature is preferably lower than 740 ° C., since hardenability is significantly reduced.

  In order to make the area ratio of cementite particles with a size (round equivalent diameter) of 10 nm or less of lamellar cementite of steel wire 50% or more, the average lamellar cementite thickness after patenting treatment needs to be 120 nm or less . In order to make the average lamellar cementite thickness after this patenting, the wire rod after winding is immersed in a molten salt bath at 450 ° C. to 570 ° C. or lower, and the cooling rate to 850 ° C. to 650 ° C. is 15 ° C. It cools with the cooling rate of 2 / s or more, and performs a pearlite transformation process at the temperature of 500 degreeC-600 degreeC. This process is generally referred to as patenting process. At this time, if the temperature of the molten salt tank is too low, the structure of the steel wire becomes bainite, so the temperature of the molten salt tank needs to be 450 ° C. or higher.

  In order to reduce the average lamellar cementite thickness, lowering the temperature of the molten salt bath is effective, and it is preferable to set the molten salt bath temperature to 550 ° C. or less. In the pearlite transformation treatment, after hot rolling, the wire rod cooled at a cooling rate of 5 ° C./s to 30 ° C./s to room temperature is reheated to a temperature range of 950 ° C. or higher and then dipped in a Pb bath or a salt bath Then, the cooling rate to 850 ° C. to 650 ° C. may be cooled at a cooling rate of 15 ° C./s or more and kept at 500 ° C. to 600 ° C.

  And these wire rods are dry drawn to a steel wire of 4 mmφ to 6 mmφ with a total reduction of area of 75% to 90% to give tensile strength. In order to make the area ratio of cementite particles having a size of 10 nm or less in lamellar cementite of a steel wire 50% or more, it is necessary to suppress heat generation at the time of wire drawing. In order to suppress this heat generation, the heat generation at the time of wire drawing by the wire drawing processing which raised the lubricity using a pressure die etc. is suppressed. In addition, forced water cooling is performed after wire drawing, and the temperature of the steel wire is reduced to 170 ° C. or less within 5 seconds after wire drawing. When the steel wire after drawing exceeds 170 ° C for 5 seconds or more, finely divided cementite particles grow, and the area ratio of cementite particles having a particle diameter of 10 nm or less becomes less than 50%, and hydrogen embrittlement resistance The characteristics are degraded.

  By the above-mentioned manufacturing method, a high strength steel wire having a tensile strength of at least 1800 MPa and excellent in hydrogen embrittlement resistance characteristics is produced without deteriorating the drawability and twisting characteristics without reducing the manufacturability. can do.

  Hereinafter, the example of the steel wire of this invention is given and the effect of the steel wire concerning this embodiment is demonstrated more concretely. However, the conditions in the examples are one condition example adopted to confirm the practicability and effects of the present invention, and the present invention is not limited to the following examples. Without departing from the scope of the present invention, as long as the object of the present invention is achieved, the present invention can be implemented with appropriate modifications as long as the scope is met. Therefore, the present invention can adopt various conditions, which are all included in the technical features of the present invention.

  Hereinafter, the effects of the present invention will be more specifically described by way of examples.

  Using a steel material consisting of chemical components (unit: mass%) shown in Table 1 and Table 2, hot rolling is performed by heating to the heating temperature shown in Table 3 and Table 4, and coiling shown in Table 3 and Table 4 It wound up at temperature.

  Then, it was immersed in a molten salt tank at the rear of the hot rolling line to perform patenting treatment, and the obtained wire was subjected to wire drawing processing to produce a steel wire. At the time of wire drawing, heat generation was suppressed using a presser die or the like, and the steel wire was cooled by water cooling after wire drawing. In addition, the temperature of the steel wire after wire drawing was measured by the contact-type thermometer.

  Tables 3 and 4 show the technical characteristics of the steel wire, that is, the area ratio of pearlite, the area ratio of cementite grains of 10 nm or less in lamellar cementite, and the tensile strength of the steel wire. Each of these values is a value measured by the method described above.

  Test No. 2 (Level 2) is a steel wire obtained by blast cooling using Stelmore after hot rolling, without patenting in a molten salt bath, and then wire drawing and aging heat treatment is there. In this test No. 2, the cooling rate is slower than 15 ° C./s, the area ratio of pearlite is out of the range of the present invention, and the tensile strength of the steel wire after drawing and aging heat treatment does not satisfy 1800 MPa. It is a comparative example. Test No. 7 (Level 7) has a high molten salt temperature, the area ratio of pearlite is outside the range of the present invention, and the tensile strength of the steel wire after drawing and aging heat treatment does not satisfy 1800 MPa. It is an example. Test No. 10 (Level 10) is a comparative example in which the area ratio of cementite particles of 10 nm or less did not satisfy the range of the present invention because the aging heat treatment time after wire drawing was long. In test number 12 (level 12), since the molten salt temperature after rolling was low, the area ratio of pearlite was out of the range of the present invention, and the tensile strength of the steel wire after wire drawing and aging heat treatment was 1800 MPa. It is a comparative example which was not satisfied. Test number 43 (level 43) is a comparative example in which the tensile strength of the steel wire after aging heat treatment was less than 1800 MPa because the C content was below the range of the present invention. Test No. 46 (level 46) is a comparative example in which the tensile strength of the steel wire after aging heat treatment was less than 1800 MPa because the Si content was below the range of the present invention. Test No. 53 (level 53) is a comparative example in which the tensile strength of the steel wire after the aging heat treatment was less than 1800 MPa because the Mn content was below the range of the present invention.

  Next, the hydrogen embrittlement resistance was evaluated using the steel wire of each test number described in Table 3 and Table 4. In addition, about the steel wire whose tensile strength is less than 1800 MPa, since the required strength was not satisfied, hydrogen embrittlement resistance was not evaluated. In addition, for steel wires that did not satisfy one or both of drawability and twisting characteristics as the characteristics of the steel wire, except for test numbers 50, 51, 64 (levels 50, 51, 64) No evaluation of hydrogen embrittlement resistance was made. Further, with regard to a steel wire which satisfied both the wire drawing workability and the twisting property, the hydrogen embrittlement resistance was not evaluated for a steel wire whose manufacturability was lowered such as an increase in alloy cost.

The hydrogen embrittlement resistance was evaluated by the FIP test. The steel wire of each test number was immersed in a 50% 20% NH ₄ SCN solution, and a load of 0.8 times the breaking load was applied to evaluate the breaking time. The specific liquid volume was 12 cc / cm ² . The FIP test was evaluated 12 for each test number, and the average value was taken as the hydrogen embrittlement rupture time. Since the hydrogen embrittlement resistance depends on the tensile strength of the steel wire, it is judged that the hydrogen embrittlement resistance is good for 20 hours or more for a steel wire with a tensile strength of 1800 MPa or more. displayed. With regard to wire drawability, it is determined that “drawability is good” when no longitudinal cracks occur that result in breakage or cracks on the surface in the longitudinal direction up to the target wire diameter during wire drawing. It displayed as "good" in Table 5. Torsion characteristics are evaluated by a torsion test under the condition that the distance between chucks is 100 times the diameter and the torsional rotation speed is 5 to 60 rpm, and the case where no delamination occurs is "the torsion characteristics are good". It judged and displayed as "good" in Table 5.

  In addition, the manufacturing cost evaluation is based on the steel making cost including the alloy cost and the rolling cost, and is equal to or less than the alloy cost calculated based on the central value of the component range of the element including the selected element The thing was made into "the manufacturing cost is low", and it displayed as "low" in Table 5. On the other hand, in Table 5, when manufacturing cost, such as alloy cost, increased, it displayed as "high" and determined that the manufacturability of the steel wire fell. These results are shown in Table 5.

  Test No. 44 (Level 44) is a comparative example in which the hydrogen embrittlement resistance is poor because the P content exceeds the range of the present invention. Test No. 48 (level 48) is a comparative example in which the hydrogen embrittlement resistance is poor because the Cr content exceeds the range of the present invention. Test No. 49 (level 49) is a comparative example in which the hydrogen embrittlement resistance is poor since the N content exceeds the range of the present invention. Test No. 52 (level 52) is a comparative example in which the hydrogen embrittlement resistance is poor since the S content exceeds the range of the present invention. Test No. 54 (Level 54) is a comparative example in which the hydrogen embrittlement resistance is poor because the Mo content exceeds the range of the present invention. Test No. 60 (Level 60) is a comparative example in which the hydrogen embrittlement resistance is poor since the Ni content exceeds the range of the present invention. Test No. 61 (level 61) is a comparative example in which the hydrogen embrittlement resistance is poor since the Cu content exceeds the range of the present invention. The test numbers 47, 50, 51, 56, 57, 58, 59, 62, 63 (levels 47, 50, 51, 56, 57, 58, 59, 62, 63) respectively have a Mn content, an O content, Because the Si content, C content and B content, Ti content, Nb content, Nb content, Zr content, Ca content, and Mg content exceeded the scope of the present invention, wire drawability and twist characteristics I did not meet one or both of them. Therefore, it was not possible to evaluate the hydrogen embrittlement resistance. In addition, test numbers 45, 55 and 56 (levels 45, 55 and 56) respectively produce the steel wire because the Al content, V content, C content and B content exceeded the scope of the present invention. The sex was reduced. Test No. 64 (level 64) is an example in which the resistance to hydrogen embrittlement decreases because the strength is 3000 MPa or more.

  According to the present invention, a high strength steel wire used for prestressed concrete or the like which has a tensile strength of at least 1800 MPa and which has improved resistance to hydrogen embrittlement without lowering the drawability and twisting characteristics. Can be obtained without reducing the manufacturability, and the industrial contribution is extremely remarkable.

Claims (2)

As a chemical component, in mass%,
C: 0.80% to 1.20%,
Si: 0.10% to 2.00%,
Mn: 0.20% to 1.00%,
P: 0.030% or less,
S: 0.030% or less,
O: 0.0100% or less and N: 0.0010% to 0.0100%
The components containing and optionally contained
Al: 0. 100% or less,
Cr: 2.00% or less,
Mo: 1.00% or less,
V: 0.30% or less,
B: 0.0050% or less,
Ti: 0.050% or less,
Nb: 0.050% or less,
Zr: 0.050% or less,
Ni: 2.00% or less,
Cu: 1.00% or less,
Ca: 0.010% or less and Mg: 0.010% or less,
The balance is Fe and impurities,
Organization contains perlite and
The area ratio of the perlite is 90% or more,
In lamellar cementite, the area ratio of cementite particles having a circle equivalent diameter and a particle diameter of 10 nm or less is 50% or more,
A steel wire characterized by having a tensile strength of at least 1800 MPa and less than 3000 MPa. As a chemical component, in mass%,
Al: 0.005% to 0.100%,
Cr: 0.01% to 2.00%,
Mo: 0.01% to 1.00%,
V: 0.01% to 0.30%,
B: 0.0001% to 0.0050%,
Ti: 0.001% to 0.050%,
Nb: 0.001% to 0.050%,
Zr: 0.001% to 0.050%,
Ni: 0.01% to 2.00%,
Cu: 0.01% to 1.00%,
Ca: 0.0001% to 0.010% and Mg: 0.0001% to 0.010%
The steel wire according to claim 1, characterized in that it contains one or more selected from the group consisting of

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