TW201812027A - Wire rod, steel wire, and part - Google Patents

Abstract

Provided are a wire rod, etc. for which spheroidize annealing and quenching/tempering heat treatment steps can be omitted. The wire rod has a specified chemical composition. In area ratio, at least 90% of the metal structure is bainite. The average bainite block grain diameter in the surface layer measured in a transverse cross-section is 15 [mu]m or less. The ratio between the average bainite block grain diameter in the surface layer measured in a transverse cross-section and the average bainite block grain diameter measured in the center section is less than 1.0, and the average grain diameter of cementite dispersed in the bainite is 0.1 [mu]m or less.

Description

Wire, steel wire and components

FIELD OF THE INVENTION The present invention relates to a wire, a steel wire manufactured from the wire, and a member having a tensile strength of 700 MPa to 1200 MPa manufactured from the steel wire. In addition, the target component in the present invention includes a mechanical component or a building component.

BACKGROUND OF THE INVENTION For the purpose of weight reduction or miniaturization of automobiles or various industrial machines, high-strength mechanical members having a tensile strength of 700 MPa or more are used. In the past, such high-strength mechanical members were softened by sequentially performing hot rolling, spheroidizing annealing on a steel material composed of an alloy steel in which alloy elements such as Mn, Cr, Mo, and B were added to carbon steel for mechanical structures. After tempering, cold forging and rolling are performed to form a predetermined shape, and then quenching and tempering are performed to impart strength and manufacture.

However, the steel materials described above are expensive due to the large content of alloying elements, and because spheroidizing annealing before forming the shape of the component, and quenching and tempering after forming must be performed, the manufacturing cost increases.

Due to the foregoing, there is a known technique in which a wire drawing process is performed on a wire whose spheroidizing annealing and quenching and tempering treatments are omitted, and rapid cooling or aging treatment is performed to increase the strength, so as to impart predetermined strength. This technology is used for mechanical components and the like, and mechanical components and the like manufactured using this technology are called non-quenched and tempered mechanical components.

Japanese Patent Laid-Open No. 2-166229 discloses a method for manufacturing a non-quenched and tempered mechanical member composed of a toughened iron structure. The manufacturing method is to roll a wire rod at a cooling rate of 5 ° C / sec or more. Cooling of steels with the following chemical composition: C: 0.03 ~ 0.20%; Si: 0.10% or less; Mn: 0.7 ~ 2.5%; one or more of V, Nb, Ti in total: 0.05 ~ 0.30%; and B : 0.0005 ~ 0.0050%.

Also, Japanese Patent Laid-Open No. 8-41537 discloses a method for manufacturing a high-strength mechanical member. The method includes C: 0.05 to 0.20%, Si: 0.01 to 1.0%, and Mn: 1.0 to 2.0%. , S: 0.015% or less, Al: 0.01 ~ 0.05%, and V: 0.05 ~ 0.3%, the steel is hot rolled after being heated to a temperature of 900 ~ 1150 ℃, and after finishing rolling, the steel is rolled at 2 ℃ / An average cooling rate of sec or more is performed in a temperature range from 800 ° C to 500 ° C, thereby forming ferrous iron + toughened iron structure, and then annealing in a temperature range of 550 to 700 ° C.

However, in these manufacturing methods, it is necessary to strictly control the cooling rate or the cooling end temperature, and the manufacturing method is complicated and the manufacturing cost increases. In addition, the structure may become uneven and the cold forgeability may be deteriorated.

In contrast, Japanese Patent Laid-Open No. 2000-144306 discloses that C is 0.40 to 1.0% by mass, and the composition of the composition satisfies a specific conditional expression, and the structure is degenerate pearlite or degenerate pearlite. ) Steel for cold forging. The above-mentioned steel has a large amount of C, and its cold forgeability is inferior to that of carbon steel for mechanical structure or alloy steel for mechanical structure that has been conventionally used for mechanical members.

As described above, among the non-quenched and tempered wire rods manufactured by the conventional technology, it is not possible to produce mechanical members with good cold forgeability, or steel wires and wire rods for manufacturing the members, using a low-cost manufacturing method. In particular, the conventional techniques such as spheroidizing annealing, quenching, and tempering treatments are omitted, and there may be cases where the structure is not uniform and excellent cold forgeability cannot be obtained. Therefore, even if these processes are omitted, excellent mechanical characteristics can be achieved. There is still room for improvement in the development of components.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention In view of the above-mentioned problems of the conventional technology, the present invention aims to provide: (a) a member capable of being manufactured at a low price and having a tensile strength of 700 to 1200 MPa; and (b) a method for manufacturing The component can omit spheroidizing annealing and quenching, tempering, and blueing steel wire after cold forging; and a wire material for manufacturing the steel wire.

Means for Solving the Problem In order to achieve the above-mentioned object, the present inventors investigated the relationship between the composition and structure of steel materials. The aforementioned steel materials can be cold-forged even if spheroidizing annealing is omitted, and they are not tempered or tempered. Quality treatment can still be used to make high-strength members with a tensile strength of 700 MPa or more. The present invention has been made based on the knowledge of metallurgical knowledge obtained through the above investigation, and the gist thereof is as follows.

(1) A wire rod characterized by containing C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, and Al: 0.005 ~ 0.060%, Ti: 0.005 ~ 0.030%, B: 0.0003 ~ 0.0050%, and N: 0.001 ~ 0.010%, and the remaining part is composed of Fe and unavoidable impurities; the metal structure of the wire is based on the area ratio More than 90% are toughened iron; the average grain size of the surface toughened iron measured at the cross section is 15 μm or less; the average grain size of the surface toughened iron measured at the cross section and the toughened iron measured at the center The ratio of the average block size, that is, the average block size of the toughened iron in the surface layer / the average block size of the toughened iron in the center, is less than 1.0; and the snow dispersed in the toughened iron The average particle diameter of bright carbon iron is 0.1 μm or less.

(2) According to the wire of (1) above, the wire further contains one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10% by mass%.

(3) A steel wire characterized in that the steel wire is processed by drawing and contains C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, and P: 0.030% by mass% Below, S: 0.030% or less, Al: 0.005 ~ 0.060%, Ti: 0.005 ~ 0.030%, B: 0.0003 ~ 0.0050%, and N: 0.001 ~ 0.010%, and the remainder is caused by Fe and unavoidable impurities Constitutes the steel wire; the metal structure of the steel wire is more than 90% of the area ratio of the toughened iron; on the surface of the steel wire, the average aspect ratio R of the toughened iron nuggets measured in the longitudinal section is 1.2 to 2.0; The average block size of the surface toughened iron measured at the cross section is (15 / R) μm or less; the average block size of the surface toughened iron measured at the cross section and the average block size of the toughened iron measured at the center The ratio of (the average particle diameter of the toughened iron in the surface layer) / (the average particle diameter of the toughened iron in the central portion) is less than 1.0; The particle diameter is 0.1 μm or less.

(4) The steel wire according to the above (3), wherein the steel wire further contains one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10% in terms of mass%.

(5) The critical compression ratio of the steel wire according to (3) or (4) above is 80% or more.

(6) A member characterized in that it contains C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, and Al: 0.005 ~ 0.060%, Ti: 0.005 ~ 0.030%, B: 0.0003 ~ 0.0050%, and N: 0.001 ~ 0.010%, and the remainder is composed of Fe and unavoidable impurities; the metal structure of the component is calculated by area ratio More than 90% is toughened iron. On the surface of the component, the average aspect ratio R of the toughened iron nuggets measured in the longitudinal section is 1.2 ~ 2.0. The average particle size of the toughened iron on the surface is (15 / R) μm or less; the ratio of the average block diameter of the surface toughened iron measured at the cross section to the average block diameter of the toughened iron measured at the center, that is, (the average particle diameter of the surface toughened iron) / The value of the (average particle diameter of the toughened iron in the central portion) is less than 1.0; and the average particle diameter of the citronite dispersed in the toughened iron is 0.1 μm or less.

(7) The component according to the above (6), wherein the component further includes one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10% by mass%.

ADVANTAGEOUS EFFECTS OF THE INVENTION According to the present invention, it is possible to provide a low-cost tensile strength of 700 to mechanical components used in automobiles, various industrial machinery, and the like, and construction components used in construction sites. 1200MPa high strength member.

The form for implementing the invention is as described above. The inventors have investigated the relationship between the composition and structure of the steel in detail. The steel can be cold-forged even if spheroidizing annealing is omitted, and it is tempered without quenching or tempering. Treatment can still be used to make high-strength members with a tensile strength of more than 700 MPa. In addition, in order to manufacture high-strength members at a low price, based on the metallurgical knowledge obtained from the investigation, the inventors have conducted a series of manufacturing of steel wires and members to the in-line heat treatment using the residual heat of the hot rolling delay time, Methods, comprehensive research was conducted, and the following conclusions were reached.

(a) A steel wire with high strength through wire drawing and cold forging has poor workability, high deformation resistance, and is prone to process cracking.

(b) In order to improve the workability of high-strength steel wire, it is made into a structure with toughened iron as the main body, and the particle size of the surface layer is made fine. It is effective to set the particle diameter to 0.1 μm or less.

(c) That is, when the area ratio of the toughened iron is set to 90% or more, and the average aspect ratio of the toughened iron nuggets measured in the longitudinal section is set to R, if measured in cross section, The average value of the bulk particle diameter of the surface toughened iron is (15 / R) μm or less, and the ratio of the average bulk particle diameter of the surface toughened iron to the average bulk particle diameter of the toughened iron inside the wire is less than 1.0, can significantly improve cold workability.

(d) Furthermore, by forming the structures described in (b) and (c) above, even if the bluing treatment is omitted after forming into a member, the yield strength ratio can be increased.

As described above, by improving the composition and structure of the steel material, even if the quenching and tempering treatments are omitted, the strength can be increased, and the cold forgeability can be improved.

The aforementioned steel wire is a material that can be cold forged even if spheroidizing annealing is omitted, and can be used to make high-strength components without quenching and tempering, so that the steel wire can be used at the steel wire stage. The microstructure that has the above characteristics can be effectively processed into components without heat treatment before processing.

At this time, although the cold workability is deteriorated compared with the conventional manufacturing method of softening by spheroidizing annealing, the cost of spheroidizing annealing and the cost of quenching and tempering after processing can be reduced. The invention is advantageous.

Furthermore, the manufacturing method of the wire material used as the material of the steel wire utilizes the residual heat of the hot rolling delay time and immediately immerses it in the molten salt bath after rolling, so that the above-mentioned can be obtained without adding a large amount of alloy elements. Organization of steel.

That is, the component of the present invention is manufactured by a series of manufacturing methods: using the residual heat of the hot rolling delay time to immerse the steel with the adjusted composition in a molten salt bath, and making it into a predetermined average block The toughened iron main body wire composed of the particle size and the size of the crystalline carbon iron is drawn at a room temperature under specific conditions, and the high-strength toughened iron is adjusted to form a component.

Therefore, the present invention can manufacture a member having a tensile strength of 700 to 1200 MPa at low cost.

(Composition of components) Wires for members and steel wires (hereinafter sometimes referred to as "wires" and "steel wires", respectively) for tensile strengths of 700 to 1200 MPa according to this embodiment, and members of this embodiment ( Hereinafter, the component composition may be simply referred to as a "member". The steel wire of this embodiment is produced by drawing the wire of this embodiment. In addition, the member of this embodiment is produced by cold forging, cold forging, and rolling of the steel wire of this embodiment. Wire drawing, cold forging and rolling will not affect the composition of the steel. Therefore, the following descriptions of the composition of the wires, steel wires, and components can be applied. In the following description, "%" means "mass%". The remainder of the component composition is Fe and unavoidable impurities.

C: 0.15 to 0.30% C is an element required for securing tensile strength. If the C content is less than 0.15%, it will be difficult to obtain a tensile strength of 700 MPa or more. Therefore, the C content should be above 0.20%. On the other hand, if the C content exceeds 0.30%, the cold forgeability is deteriorated. Therefore, it should be less than 0.25%.

Si: 0.05 to 0.50% Si is a deoxidizing element, and is an element that can improve tensile strength by solid-solution strengthening. If the Si content is less than 0.05%, the effect of addition cannot be fully exhibited. Therefore, the Si content should be above 0.15%. On the other hand, if the Si content exceeds 0.50%, the addition effect will be saturated, and the ductility during the hot rolling delay will be deteriorated, and defects will easily occur. Therefore, the Si content is preferably 0.30% or less.

Mn: 0.50 to 1.50% Mn is an element that can improve the tensile strength of steel. If the Mn content is less than 0.50%, the effect of addition cannot be fully exhibited. Therefore, the Mn content should be more than 0.70%. On the other hand, if the Mn content exceeds 1.50%, the addition effect will be saturated, and the metamorphic end time during the isothermal metamorphic treatment of the wire will be longer, and the manufacturability will be deteriorated. Therefore, the preferred Mn content is 1.30% or less.

P: 0.030% or less P is an element that segregates at crystal grain boundaries and deteriorates cold workability. If the P content exceeds 0.030%, the deterioration of cold workability becomes significant. Therefore, the preferred P content is 0.015% or less. Since the wires, steel wires, and members of this embodiment do not need to contain P, the lower limit of the P content is 0%.

S: 0.030% or less S, like P, is an element that segregates at crystal grain boundaries and deteriorates cold workability. If the S content exceeds 0.030%, the deterioration of cold workability becomes significant. Therefore, the S content is preferably 0.015% or less, and more preferably 0.010% or less. Since the wires, steel wires, and members of this embodiment do not need to contain S, the lower limit value of the S content is 0%.

Al: 0.005 to 0.060% Al is a deoxidizing element and an element capable of forming AlN that functions as a pinned particle. AlN makes the crystal grains finer, thereby improving cold workability. In addition, Al is an element capable of reducing solid solution N and suppressing dynamic strain aging. If the Al content is less than 0.005%, the above effects cannot be obtained. Therefore, the preferred Al content is 0.020% or more. If the Al content exceeds 0.060%, the above effects are saturated, and defects are liable to occur during the hot rolling delay. Therefore, the preferable Al content is 0.050% or less.

Ti: 0.005 ~ 0.030% Ti is a deoxidizing element and forms TiN. It is an element that can reduce the solid solution N and suppress the dynamic strain aging. If the Ti content is less than 0.005%, the above effects cannot be obtained. Therefore, the preferred Ti content is 0.010% or more. If the Ti content exceeds 0.030%, the above effects are saturated, and defects are liable to occur during the hot rolling delay. Therefore, the preferred Ti content is 0.025% or less.

B: 0.0003 to 0.0050% B suppresses grain boundary ferrous iron, and has the effect of improving cold workability, or promoting the transformation of toughened iron and increasing the strength. When less than 0.0003%, the effect is insufficient, and if it exceeds 0.0050%, the effect is saturated.

N: 0.0010 to 0.0100% N is an element that may deteriorate cold workability due to dynamic strain aging. In order to avoid the above-mentioned adverse effects, the N content should be 0.0100% or less. In addition, N forms AlN or TiN, which makes the crystal grain size fine, and has the effect of improving cold workability. Therefore, the lower limit should be 0.0010%. The preferred N content is 0.0020 to 0.0040%.

The present invention may also contain one or two of Cr: 0.01 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10%. Cr, Nb, and V may be optionally contained, and the percentage may be 0%. Cr has the effect of increasing the tensile strength of steel, Nb and V have the effect of reducing the solid solution N and suppressing the dynamic strain aging, or promoting the toughening of iron to improve the strength.

Cr: 0.01 to 0.40% Cr is an element that can improve the tensile strength of steel. If the Cr content is less than 0.01%, the above effects cannot be sufficiently obtained. On the other hand, if the Cr content exceeds 0.40%, loosening of Asada is likely to occur, and the wire drawability or cold forgeability is thereby deteriorated. Therefore, the Cr content should be 0.03 ~ 0.30%.

Nb: 0 to 0.03% Nb forms NbN, and is an element that has the effect of reducing solid solution N and suppressing dynamic strain aging. If the Nb content exceeds 0.03%, the above effects are saturated, and defects are liable to occur during the hot rolling delay. Therefore, the Nb content should be 0.025% or less.

V: 0 to 0.10% V forms VN and is an element that has the effect of reducing the solid solution N and suppressing dynamic strain aging. If the V content exceeds 0.10%, the above effects are saturated, and defects are liable to occur during the hot rolling delay. Therefore, the preferred V content is 0.05% or less.

O: 0 to 0.0030% or less O is present in wires, steel wires, and members (such as mechanical members) as oxides of Al and Ti. If the O content exceeds 0.0030%, coarse oxides are generated in the steel, and fatigue cracking is liable to occur. Therefore, the preferred O content is 0.0020% or less. And the lower limit of the O content is 0%.

As mentioned above, the component composition of the wire, the steel wire, and the member of the present embodiment has been described, and the remainder of the component composition is Fe and unavoidable impurities. Here, the unavoidable impurities are components contained in the raw materials or components mixed in the manufacturing process, and mean components that are not intentionally contained in the steel. In addition, the so-called unavoidable impurities include Sb, Sn, W, Co, As, Mg, Pb, Bi, and H. In addition, Sb, Sn, W, Co, As, Mg, Pb, Bi, and H can be allowed to contain 0.010%, 0.10%, 0.50%, 0.50%, 0.010%, 0.010%, and 0.10, respectively, to achieve the effects of this case. %, 0.10% and 0.0010%.

Next, the wire and steel wires of this embodiment and the metal structure of the member of this embodiment will be described. The steel wire of this embodiment is obtained by drawing the wire of this embodiment, and the member of this embodiment is obtained by cold forging, or cold forging and rolling of the steel wire of this embodiment. Cold forging and rolling have little effect on the metal structure of the component. This is because for the components, the amount of processing caused by cold forging and rolling is small.

(Area ratio of toughened iron: 90% or more) Wire drawing, cold forging, and rolling have little effect on the area ratio of toughened iron in metal structure. Therefore, the following descriptions can be applied to wire, steel wire and components. The metal structures of the wires, steel wires, and members of this embodiment contain 90% or more of toughened iron in terms of area ratio. In this embodiment, as shown in FIG. 1, the so-called toughened iron is a structure in which the cross-section of the object (wire, steel wire, or member) is etched with a nitric acid etchant (the axis is normal to the steel (steel wire)). Cross section), when scanning from the surface of the object to a predetermined depth (for example, from the surface to a depth of 0.25 times the diameter) with a scanning electron microscope (SEM), needle-shaped or granular Schiff carbon was identified. Iron dispersed organization.

In this embodiment, the toughened iron area ratio of the wire, steel wire, and member is determined by the following procedure. That is, the cross-section of the object is first etched with a nitric acid etchant to expose the tissue. Next, when the diameter of the object is D, a total of 9 points are specified: at a depth position of 50 μm from the surface layer of the object, centering on the longitudinal axis of the symmetrical object as a center, and rotating it every 90 °, 4 locations to be determined; 4 locations to be determined at a depth of 0.25D from the surface of the object, with the axis as the center and each rotation of 90 °; and the center of the axis (depth from the surface) Is 0.5D depth position). Then, for these 9 locations, a tissue photograph with a magnification of 1000 times was taken using SEM. Furthermore, the non-toughened iron (fat grain iron, boron iron, and loose iron of Asada) in the tissue photo taken was visually marked, and the area of each tissue was obtained by image analysis. As a result, the area containing the toughened iron can be obtained by subtracting the non-toughened iron area from the entire observation field. The area ratio of this area is defined as the area ratio of the toughened iron. In the above operation, at least two samples are measured and calculated, and then an average value is obtained, and the average value is used as the toughened iron area ratio of this embodiment.

However, there are cases where it is difficult to discriminate the toughened iron from a tissue photograph taken by the SEM. At this time, the electron backscatter diffraction device (EBSD) can be used and the KAM method (Kernel Average Misorientation) can be used for discrimination. The KAM method is a method of calculating the following for each pixel: the adjacent six of a regular hexagonal pixel in the measurement data are the first approximation, the outer 12 of them are the second approximation, or the more outer 18 are The azimuth difference between the pixels of the third approximation is averaged, and the value is the value of the center pixel. This calculation is carried out in such a way that it does not exceed the grain boundaries, whereby a distribution map showing the change in orientation of the grains can be made. Compared with the polygonal initial analysis of ferrous grain iron that deforms at high temperature, the difference in row density of the toughened iron is large and the strain in the grain is large, so the intra-grain difference in crystal orientation is large. Therefore, in the analysis of this embodiment, the third approximation is used as a condition for calculating the azimuth difference between adjacent pixels, and it is shown that the azimuth difference is 5 ° or less, and the crystal grains in which the azimuth difference exceeds 1 ° are toughened. iron.

Based on the above-mentioned method for determining toughened iron, in this embodiment, if the toughened iron area ratio of the wire is less than 90%, the steel wire obtained by drawing the wire or cold forging the wire The toughened iron area ratio of the component will be less than 90%. In this case, the strength of the component's yield strength ratio (= 0.2% off-site yield strength / tensile strength) is reduced, and the permanent elongation when used as, for example, a mechanical component is deteriorated. In addition to the toughened iron, steel wire may contain boron iron, pro-eutectoid grain iron, and Asada loose iron. However, as long as the area ratio of the toughened iron of the steel wire is more than 90%, the toughened iron may be allowed to be contained. Other metal structures. Furthermore, if the area ratio of the toughened iron of the steel wire is less than 90%, the strength (tensile strength, hardness, etc.) of the steel wire is not uniform, and therefore it is easy to crack when cold-worked into a member. Further, it is desirable that the steel wire does not contain a metal structure other than the toughened iron. Therefore, the upper limit of the area ratio of the toughened iron of the steel wire is 100%.

(The average bulk particle diameter of the toughened iron of the wire rod is 15 μm or less) In the wire rod of this embodiment, the average bulk particle diameter of the toughened iron measured at the cross section is 15 μm or less. Here, the cross-section means a surface perpendicular to the axial direction of the wire. If the average block size of the toughened iron measured in the cross section of the wire exceeds 15 μm, the ductility of the steel wire after the wire drawing process will be lowered, and the cold workability of the steel wire will be reduced accordingly. Furthermore, the average toughened iron block size of the member obtained by cold working the steel wire is coarsened. If the average particle size of the toughened iron is coarsened, the yield strength ratio will decrease. Moreover, the average particle size of the toughened iron of the wire is preferably smaller, so it is not necessary to specify a lower limit value.

(The average aspect ratio R of the toughened iron nuggets of steel wires and components is 1.2 ~ 2.0) In the steel wires and components of this embodiment, the toughened iron is measured at the position of the surface layer of the steel wires in the longitudinal section of the steel wires. The average aspect ratio R of lumps is 1.2 ~ 2.0. Here, the longitudinal section means a surface that is parallel to the axial direction of the wire and includes a central axis. If the average aspect ratio of the toughened iron block is less than 1.2, the hydrogen embrittlement resistance of a member manufactured by cold forging a steel wire may be deteriorated. In addition, if the average aspect ratio exceeds 2.0, the drop strength ratio decreases, and the permanent extension when used as a member deteriorates.

In this embodiment, the average aspect ratio R of the toughened iron nuggets of steel wires and members is determined as follows. First, EBSD is used for the longitudinal section of the steel wire to determine the grain boundary of the toughened ingot. At this time, the measurement step is set to 0.3 μm in two areas from 100 μm in the direction of the central axis of the steel wire and 500 μm in the direction of the central axis of the steel wire from the surfaces on both sides of the longitudinal section. The crystal orientation of bcc-Fe at each measurement point is defined as the boundary of the toughened iron nugget where the boundary with an azimuth difference of 15 degrees or more. Then, the area surrounded by the boundary is made into a toughened iron nugget. As described above, on one longitudinal section, the distribution map of the toughened iron nuggets is obtained in a total of 2 regions on both sides thereof. This was performed in 4 samples, and the distribution map of the toughened iron nuggets was obtained in a total of 8 regions. From the obtained distribution map of the toughened iron nuggets, ten toughened iron nuggets were selected in order from the largest circle equivalent diameter. For the selected ten toughened iron nuggets, measure the aspect ratio of the nuggets, and finally calculate their average value as the average aspect ratio R of the toughened iron nuggets.

(The average block size of the toughened iron of the steel wire is (15 / R) μm or less) In the steel wire of this embodiment, the average block size of the surface toughened iron measured at the cross section is (15 / R) μm the following. Here, the cross-section means a surface perpendicular to the axial direction of the steel wire. If the average block diameter of the toughened iron in the surface layer measured in the cross section of the steel wire exceeds (15 / R) μm, the ductility of the steel wire will be lowered, and the cold workability of the steel wire will be reduced accordingly. Furthermore, the average toughened iron block size of the member obtained by cold working the steel wire is coarsened, and the strength is reduced. In addition, since the average grain size of the toughened iron in the surface layer portion of the steel wire is preferably small, it is not necessary to specify a lower limit value.

In this embodiment, the average grain size of the toughened iron in the surface layer of the wire rod (the same applies to the steel wire and the member) is determined as follows. First, from the cross section of the wire, an area having a width of 500 μm in the direction of the central axis and extending 500 μm in the circumferential direction was determined from the surface layer, and four regions that rotated the region around the central axis by 90 ° were specified. Then, for these four areas, the block size measured by the EBSD device is averaged to be the average block size of the toughened iron in the surface layer of the wire (the same is true for the steel wire and the member).

(The average bulk particle diameter of the toughened iron of the component is (15 / R) μm or less) In the member of this embodiment, the average bulk particle diameter of the surface toughened iron measured at the cross section is (15 / R) μm or less. Here, the cross-section means a surface perpendicular to the axial direction of the member. If the average bulk particle diameter of the surface toughened iron measured in the cross section of the component exceeds (15 / R) μm, the yield strength ratio will decrease. In addition, since the average grain size of the toughened iron in the surface layer portion of the steel wire is preferably small, it is not necessary to specify a lower limit value. The method for determining the average particle diameter of the toughened iron in the member is the same as the method for determining the average particle diameter of the toughened iron in the wire.

((Average particle size of the toughened iron on the surface of the wire, steel wire, and component) / (Average particle size of the toughened iron on the center) is less than 1.0) In the wire, steel wire, and component of this embodiment, The ratio of the average bulk particle diameter of the toughened iron measured on the cross section to the average bulk particle diameter of the toughened iron measured on the central portion of the cross section was less than 1.0. If the ratio exceeds 1.0, the cold forgeability of the steel wire is deteriorated, and the yield strength ratio of the member is deteriorated.

In the present embodiment, the average grain diameter of the toughened iron in the center of the wire rod (the same is true for steel wires and members) is determined as follows. First, in the cross section of the wire, a 500 μm × 500 μm area centered on the central axis is determined, and the block size is measured using the EBSD device for the area. Next, the same measurement was performed in three different cross sections, and the block sizes were averaged for the four samples to be the average block size of the toughened iron at the center of the wire (the same is true for steel wires and members).

Then, in this embodiment, (the average particle diameter of the toughened iron in the surface layer) / (average particle diameter of the toughened iron in the center portion) is used to obtain the particle diameter of the surface layer and the center portion. Diameter ratio.

(The average particle diameter of cis-carbon iron dispersed in the toughened iron is 0.1 μm or less) In the wire rod, steel wire, and member of this embodiment, the average particle diameter of cis-carbon iron dispersed in the toughened iron is 0.1. μm or less. If the average particle diameter of the Xueming carbon iron exceeds 0.1 μm, the cold forgeability of the steel wire is deteriorated. In addition, the falling strength ratio of the member is reduced, and the permanent extension when used as, for example, a mechanical member is deteriorated.

The average particle diameter of cis-carbon iron in the toughened iron of this embodiment is determined by the following procedure. First, a cross-section of an object (wire, steel wire, or member) is etched using a bitter acid etchant to expose the tissue. Next, when the diameter of the object is D, a total of 9 points are specified: at a depth position of 50 μm from the surface layer of the object, centering on the longitudinal axis of the symmetrical object as a center, and rotating it every 90 °, 4 locations to be determined; 4 locations to be determined at a depth of 0.25D from the surface of the object, with the axis as the center and each rotation of 90 °; and the center of the axis (depth from the surface) Is 0.5D depth position). Then, for these 9 locations, a field emission scanning electron microscope (FE-SEM) was used to take a photograph of the tissue at a magnification of 20,000 times. Finally, the captured image was binarized, and the equivalent diameter of the Xueming carbon iron circle was obtained by image analysis, and the average value of 9 samples was calculated as the average particle diameter of the Xueming carbon iron. .

(The critical compression ratio of the steel wire is 80% or more.) The steel wire obtained as described above exhibits good cold workability. In this embodiment, a critical compression ratio is used as an index showing cold workability. In this embodiment, the so-called critical compression ratio means that a sample having a height of 1.5 times the diameter is machined from a drawn steel wire by machining, and is compressed in the axial direction using a mold having concentric circular grooves. The maximum compression rate at which the end face of this sample does not crack. In addition, the so-called compression ratio is set to (H-H1) / H when the height (the dimension in the axial direction) before compression of the cable is H and the height (the dimension in the axial direction) after compression of the cable is H1. ) × 100 displayed value. In the steel wire of this embodiment, the critical compression ratio can be set to 80% or more, and excellent cold workability can be achieved.

Next, an example of a method for manufacturing a wire, a steel wire, and a member will be described. First, prepare steel sheets with the following components: C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, and Al: 0.005 ~ 0.060%, Ti: 0.005 ~ 0.030%, B: 0.0003 ~ 0.0050% and N: 0.001 ~ 0.010%, and if necessary, Cr: 0 ~ 0.40%, Nb: 0 ~ 0.03%, and V: 0 ~ 0.10% One or two of them, and the remainder is composed of Fe and impurities. After heating the steel sheet to 1000 to 1150 ° C, hot rolling is performed at a finishing rolling temperature of 800 to 950 ° C, thereby obtaining a wire rod. Next, the wire of 800 to 950 ° C is cooled to 600 ° C at an average cooling rate of 40 ° C / s or more, and then cooled to 480 ° C at an average cooling rate of 25 ° C / s or more. Thereafter, the wire is maintained at a constant temperature for 15 seconds or more in the temperature range of 400 to 480 ° C (first constant temperature hold), and further immersed in the temperature range of 530 to 600 ° C for 25 seconds or more to perform constant temperature maintenance ( 2nd thermostat). Then, the wire is finally cooled by water cooling.

The two-stage cooling and the first constant temperature holding after finishing rolling are performed by immersing the wire in a molten salt at 400 to 480 ° C in the first molten salt tank. In addition, the second constant temperature holding is performed by immersing the wire in a molten salt at 530 to 600 ° C in a second molten salt tank.

Here, in the method for manufacturing a wire of this embodiment, cooling of the wire of 800 to 950 ° C is specifically divided into cooling to 600 ° C and cooling to 600 ° C to 480 ° C, and these are performed in two stages. In the subsequent cooling, the cooling rate is specifically set to 25 ° C./s or more, so that the average block size of the toughened iron can be controlled to 15 μm or less.

In the method for manufacturing a wire in this embodiment, the temperature of the molten salt bath in the first molten salt tank is set to 400 to 480 ° C, and the immersion time is set to 15 to 50 seconds. By setting the molten salt bath temperature to 400 ° C. or higher, it is possible to suppress the mixing of Asada loose iron and obtain excellent cold forgeability. On the other hand, by setting it to 480 ° C. or less, the average particle diameter of the citronite can be made small, and excellent cold forgeability can be obtained without the need for a bluing treatment. In addition, by setting the immersion time to 15 s or more, the non-toughened iron structure can be suppressed from being mixed, and excellent cold forgeability can be obtained. On the other hand, by setting it to 50 s or less, the average particle diameter of the citronite can be made small, excellent cold forgeability can be obtained, and the bluing treatment can be eliminated.

Similarly, in the method for manufacturing a wire of this embodiment, the temperature of the molten salt bath in the second molten salt tank is set to 530 to 600 ° C, and the immersion time is set to 25 to 80 s. By setting the molten salt bath temperature to 530 ° C or higher, it is possible to suppress the incorporation of loose iron in Asada and obtain excellent cold forgeability. On the other hand, by setting the temperature to 600 ° C. or lower, the average particle diameter of the citronite can be made small, and excellent cold forgeability can be obtained without the need for a bluing treatment. In addition, by setting the immersion time to 25 s or more, it is possible to suppress the mixing of Asada loose iron and obtain excellent cold forgeability. On the other hand, by setting it to 80 s or less, the average particle diameter of citronite can be made small, excellent cold forgeability can be obtained, and the bluing treatment can be eliminated.

Next, as an example, the steel wire of this embodiment can be manufactured by the following method. That is, the wire material obtained by the above method is subjected to wire drawing processing with a total reduction of 10 to 55%. The total reduction of 10 to 55% in wire drawing processing can be achieved by one wire drawing processing, or by multiple wire drawing processing. As described above, the steel wire of this embodiment can be obtained.

In addition, the members (mechanical members, building members, etc.) of this embodiment can be manufactured as follows by way of example. That is, the steel wire is processed into the shape of various members by cold forging, or cold forging, and rolling to obtain a member having a tensile strength of 700 to 1200 MPa.

Examples Next, the examples of the present invention will be described. However, the conditions in the examples are only one example of conditions used to confirm the feasibility and effect of the present invention, and the present invention is not limited by the one example of conditions. . As long as the purpose of the present invention can be achieved without departing from the gist of the present invention, the present invention can adopt various conditions.

Using 14 types of steel sheets with the composition shown in Table 1, and under the conditions of 28 modes shown in Table 2, heating, hot rolling, isothermal treatment and cooling were sequentially performed to obtain wire (level 1 ~ 28). Next, using each wire, wire drawing was performed at a reduction ratio shown in Table 2 to obtain a steel wire (levels 1 to 28). Furthermore, using each steel wire, a sample having a height 1.5 times the diameter was machined to form a component (level 1 to 28). Then, the end surface of each member is compressed in the axial direction by using a mold having concentric circular grooves, and the maximum compression rate at which cracking does not occur is set as the critical compression rate of the member. And the steel wire with a critical compression ratio of 80% or more was judged to be good in cold workability. In addition, a tensile test piece was taken from the shaft portion of each member, and a tensile test was performed. After measuring the tensile strength and 0.2% off-site yield strength, the yield strength ratio (0.2% off-site yield strength / tensile strength) was measured. A member having a value of 0.90 or more was judged to have a good drop strength ratio. Regarding any one of steel materials, steel wires, and components, levels 1 to 7 and levels 14 to 20 are invention examples, and levels 8 to 13 and levels 21 to 28 are comparative examples.

[Table 1]

[Table 2]

In addition, each level including the blank columns in Table 2 will be described. For example, level 10 is an example of being manufactured by being immersed in a boiling water tank without being subjected to a thermostatic treatment after hot rolling. The level 11 is an example which is manufactured by cooling with air cooling without performing constant temperature abnormality treatment after hot rolling. The level 13 is an example in which the hot-rolled wire is temporarily cooled to room temperature, then heated to 1000 ° C., and immersed in a single-tank molten salt bath to produce it.

Next, the results regarding the wire structure are shown in Table 3, the results regarding the steel wire structure are shown in Table 4, and the results related to the cold forgeability of the steel wire and the characteristics of the member are shown in Table 5.

[table 3]

[Table 4]

[table 5]

As can be seen from Tables 2 to 5, all the manufacturing conditions specified in this case are within the range of 1 to 7 and 14 to 20 (invention examples) within the predetermined range. No matter which one can obtain the cold forgeability of steel wires and the characteristics of the components are good the result of. That is, it can be known that the tensile strength of the component is 700 to 1200 MPa regardless of the level 1 to 7 and the level 14 to 20, and even if the component is not subjected to the so-called bluing treatment, a drop strength ratio of 0.90 or more can be obtained.

On the other hand, it can be seen that any of the manufacturing conditions stipulated in this case are outside the predetermined range of levels 8 to 13 and levels 21 to 28 (comparative examples). At least one of the cold forgeability of steel wires and the characteristics of members Not showing good results.

Industrial Applicability As shown above, according to the present invention, a member having a tensile strength of 700 to 1200 MPa, which can be manufactured at a low price, can be obtained, and a spheroidizing annealing or quenching for manufacturing the member can be obtained. , Tempering, and blued steel wire after cold forging, and the wire used to make the steel wire. Therefore, the present invention has high applicability in the steel structure material manufacturing industry, and it seems that it is very promising.

FIG. 1 is an SEM photograph showing the wire and steel wires of this embodiment, and the metal structure of the member of this embodiment.

Claims (7)

A wire rod characterized by containing C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, and Al: 0.005 to 0.060 in mass% %, Ti: 0.005 ~ 0.030%, B: 0.0003 ~ 0.0050%, and N: 0.001 ~ 0.010%, and the rest is composed of Fe and unavoidable impurities; the metal structure of the wire is 90% by area ratio The above is the toughened iron; the average grain size of the surface toughened iron measured at the cross section is 15 μm or less; the average grain size of the surface toughened iron measured at the cross section and the average mass of the toughened iron measured at the center The ratio of the particle size, that is, the average particle size of the toughened iron in the surface layer / the average particle size of the toughened iron in the center, is less than 1.0; The average particle diameter is 0.1 μm or less. For example, the wire rod of claim 1, the aforementioned wire rod further includes one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10% in terms of mass%. A steel wire characterized by: The steel wire is processed by drawing and contains C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S : 0.030% or less, Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, and N: 0.001 to 0.010%, and the remainder is composed of Fe and unavoidable impurities; the The metal structure of the steel wire is more than 90% in terms of area ratio. It is toughened iron. On the surface of the steel wire, the average aspect ratio R of the toughened iron pellets measured in the longitudinal section is 1.2 ~ 2.0; The average block diameter of ductile iron is (15 / R) μm or less; the ratio of the average block diameter of the toughened iron measured at the cross section to the average block diameter of the toughened iron measured at the center, that is, (the surface layer The value of the average particle diameter of the toughened iron) / (the average particle diameter of the toughened iron at the center) is less than 1.0; and the average particle diameter of the skeletal carbon iron dispersed in the toughened iron is 0.1 μm or less . As for the steel wire of claim 3, the aforementioned steel wire further contains one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10% in terms of mass%. For steel wire of item 3 or 4, the critical compression rate is above 80%. A component characterized by containing C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, and Al: 0.005 to 0.060 in mass% %, Ti: 0.005 ~ 0.030%, B: 0.0003 ~ 0.0050% and N: 0.001 ~ 0.010%, and the remaining part is composed of Fe and unavoidable impurities; the metal structure of the component is more than 90% by area ratio It is toughened iron. On the surface of the component, the average aspect ratio R of the toughened iron particles measured in the longitudinal section is 1.2 ~ 2.0. The average particle size of the toughened iron measured on the cross section is (15 / R). μm or less; the ratio of the average block size of the toughened iron measured at the cross section to the average block size of the toughened iron measured at the center, that is, the (average block size of the toughened iron at the surface) / (central part The average particle size of the toughened iron) is less than 1.0; and the average particle diameter of the skeletal carbon iron dispersed in the toughened iron is 0.1 μm or less. As for the component of claim 6, the foregoing component further includes one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10% in terms of mass%.