Technical field
[0001]
The present invention, wires, steel wires produced by the wire, and the tensile strength produced by the steel wire is about 1200MPa following parts than 700 MPa. Incidentally, the component of interest in the present invention include mechanical parts and building parts.
BACKGROUND
[0002]
　Automobiles and various industrial machines, for the purpose of weight reduction and size reduction, high-strength machine parts having a tensile strength of not less than 700MPa is used. Conventionally, high strength mechanical parts of this type, sequentially performs Mn, Cr, Mo, and hot rolling with respect to steel comprising the alloy elements from the alloy steel added such as B, and spheroidizing annealing of carbon steel for machine structural use Te softened, then brought to a predetermined shape by performing cold forging and rolling, are produced by subsequently imparting strength subjected to quenching and tempering treatment.
[0003]
　However, manufacturing such a steel, steel prices have increased due to the high content of alloying elements, also spheroidizing annealing and before the part shape, because of the need for quenching and tempering treatment after the molding costly.
[0004]
　Under these circumstances, omitting the spheroidizing annealing and quenching and tempering treatment, subjected to wire drawing to rapid cooling and aging treatment the wire with increased strength go, there is known a technique for imparting a predetermined strength . This technique is utilized in machine parts and the like, mechanical parts, etc. manufactured using this technique is called non-heat treated mechanical parts.
[0005]
　JP-A-2-166229, C: 0.03 ~ 0.20%, Si: 0.10% or less, Mn: 0.7 ~ 2.5%, V, Nb, 1 kind of Ti or more total: 0.05 ~ 0.30%, B: a steel containing 0.0005 to 0.0050%, consisting of wire rolling after 5 ° C. / sec or more cooled bainite structure at a cooling rate non method for producing a tempering machine parts is disclosed.
[0006]
　JP-A-8-41537, C: 0.05 ~ 0.20%, Si: 0.01 ~ 1.0%, Mn: 1.0 ~ 2.0%, S: 0.015 % or less, Al: 0.01 ~ 0.05%, V: a steel containing 0.05 to 0.3% subjected to heating after hot rolling to a temperature of 900 ~ 1150 ° C., 800 after the finish rolling ° C. by cooling the temperature range of up to 500 ° C. at 2 ° C. / sec or more average cooling rate from, after the ferrite + bainite structure, a method of manufacturing high-strength machine parts performing annealing in a temperature range of 550 ~ 700 ° C. There has been disclosed.
[0007]
　However, in these production methods, it is necessary to strictly control the cooling rate and cooling termination temperature, manufacturing methods increase complexity and manufacturing cost. Further, the tissue becomes uneven, there is a case where the cold forgeability is degraded.
[0008]
　In contrast, JP 2000-144306, in C of 0.40 to 1.0 mass%, and, the component composition meet certain condition, the tissue cold forging consisting pearlite and pseudo pearlite use steel have been disclosed. This steel is often C amount, conventionally, as compared to the mechanical structural carbon steel or machine structural alloy steel is used in mechanical parts, poor cold forgeability.
[0009]
　As described above, in the non-heat treated wires according to the prior art, and machine parts with good cold forgeability an inexpensive manufacturing method, the steel wire and the wire for the manufacture of the components is not obtained. In particular, the prior art is omitted spheroidizing annealing and quenching and tempering, etc., since the organization can not be obtained an excellent cold forgeability becomes uneven, even skip these processes, excellent and for the development of components capable of realizing the mechanical properties, there is room for improvement.
Summary of the Invention
Problems that the Invention is to Solve
[0010]
　In view of the above problems in the prior art,
(a) which can be manufactured at low cost, tensile strength of 700 ~ 1200 MPa component,
(b) use in the production of the component, spheroidizing annealing and quenching and tempering, and omissions of bluing treatment after cold forging capable steel wire, and to provide a wire for producing the steel wire,
an object.
Means for Solving the Problems
[0011]
　The present inventors have found that in order to achieve the above object, be omitted spheroidizing annealing is capable of cold forging, and, even without refining of quenching and tempering, a tensile strength of more than 700MPa component composition of a steel material for obtaining a high-strength component of the investigated tissue relationships. The present invention has been made based on the metallurgical knowledge obtained in this study, its gist the following.
[0012]
　(1) in mass%, C: 0.15 ~ 0.30%, Si: 0.05 ~ 0.50%, Mn: 0.50 ~ 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 ~ 0.060%, Ti: 0.005 ~ 0.030%, B: 0.0003 ~ 0.0050%, N: the 0.001 to 0.010 percent contains, a wire and the balance Fe and unavoidable impurities, is 90% or more bainite metal structure with an area ratio, average block grain diameter of the surface layer of bainite were measured in cross-section is at 15μm or less, transverse an average block grain diameter of the surface layer of bainite measured in terms, is the ratio of the average block grain diameter of bainite was measured at the center portion, (average block grain diameter of the surface layer of bainite) / (average block of bainite at the center portion the value of particle diameter) is less than 1.0, and The average particle size of the dispersed cementite in the bainite is 0.1μm or less, the wire, characterized in that.
[0013]
　(2) the wire further contains, by mass%, Cr: 0 ~ 0.40%, Nb: 0 ~ 0.03%, V: 0 ~ 0.10%, containing one or two of to, wire according to (1).
[0014]
　(3) by mass%, C: 0.15 ~ 0.30%, Si: 0.05 ~ 0.50%, Mn: 0.50 ~ 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 ~ 0.060%, Ti: 0.005 ~ 0.030%, B: 0.0003 ~ 0.0050%, N: the 0.001 to 0.010 percent contains, a wire drawing steel wire the balance being Fe and unavoidable impurities, is 90% or more bainite metal structure with an area ratio in the surface layer of the steel wire, the bainite measured in longitudinal section block average aspect ratio R of the grain is 1.2 to 2.0 and an average block grain diameter of the surface layer of bainite measured in cross-section (15 / R) is in μm or less, the surface layer of the bainite measured in transverse section an average block grain diameter and the average block grain bainite were measured at the center portion The ratio of the value of (average block grain diameter of the surface layer of bainite) / (average block grain size of bainite in the center) is less than 1.0, and an average particle size of the dispersed cementite in the bainite There is 0.1μm or less, the steel wire, characterized in that.
[0015]
　(4) The steel wire is further, by mass%, Cr: 0 ~ 0.40%, Nb: 0 ~ 0.03%, V: 0 ~ 0.10%, one or two of containing steel wire according to (3).
[0016]
　(5) limits the compression ratio is 80% or more, the steel wire according to the above (3) or (4).
[0017]
　(6) mass%, C: 0.15 ~ 0.30%, Si: 0.05 ~ 0.50%, Mn: 0.50 ~ 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 ~ 0.060%, Ti: 0.005 ~ 0.030%, B: 0.0003 ~ 0.0050%, N: the 0.001 to 0.010 percent contained, a part comprising the balance of Fe and unavoidable impurities, is 90% or more bainite metal structure with an area ratio in the surface layer of the part, the average aspect ratio R of the block grain bainite measured in vertical section 1.2 ~ 2.0, and is an average block grain diameter of the surface layer of bainite measured in cross-section (15 / R) μm or less, and average block grain diameter of the surface layer of bainite measured in transverse section, the center is the average ratio of the block particle size measured bainite in parts, The value of the average block grain size) / the surface layer of bainite (average block grain size of bainite in the center) is less than 1.0, and an average particle size of the dispersed cementite in bainite at 0.1μm or less some, parts, characterized in that.
[0018]
　(7) the component further contains, by mass%, Cr: 0 ~ 0.40%, Nb: 0 ~ 0.03%, V: 0 ~ 0.10%, containing one or two of to component according to (6).
Effect of the invention
[0019]
　According to the present invention, machine parts used in automobiles and various industrial machines and the like, and contributes to weight reduction and miniaturization of the building components used in the construction site, the tensile strength is low cost provide a high-strength components 700 ~ 1200 MPa can do.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020]
[1] wire and the steel wire according to the present embodiment, and is an SEM photograph showing parts of the present embodiment, the metal structure.
DESCRIPTION OF THE INVENTION
[0021]
　The present inventors, as described above, be omitted spheroidizing annealing is capable of cold forging, and, even without refining of quenching and tempering, high tensile strength is more than 700MPa the organization relationship between chemical composition of the steel material for obtaining a strength component were investigated in detail. Then, the present inventors have found that high-strength parts to inexpensively produce, based on the metallurgical knowledge obtained in the survey, in-line heat treatment utilizing the potential heat of the hot rolling of wire rods, and, thereafter the steel wire , for a series of manufacturing method of up to parts, promoting a comprehensive study, we reached the following conclusions.
[0022]
　Strengthening the steel wire by (a) drawing and cold forging, poor processability, high deformation resistance, and processability cracking tends to occur.
[0023]
　(B) In order to improve the workability of high strength steel wire, in the tissue mainly composed of bainite, it is fine surface layer of blocks particle diameter, and an average particle size of dispersed cementite in the bainite 0 it is effective to .1μm less.
[0024]
　(C) i.e., the area ratio of bainite as 90% or more, when the average aspect ratio block grains of bainite measured in longitudinal section and is R, the mean value of the block grain diameter of the surface layer of bainite measured in transverse section and (15 / R) μm or less, when the ratio of the particle size of the average block of the average block grain diameter and the wire inside the bainite surface of bainite is less than 1.0, can significantly enhance the cold workability.
[0025]
　(D) Furthermore, if the tissue of the (b) and (c), after forming the part, be omitted bluing treatment, it is possible to increase the yield strength ratio.
[0026]
　Thus, by improving the organization and composition of the steel material, it is omitted quenching and tempering treatment can be high strength, and it becomes possible to improve the cold forgeability.
[0027]
　Such, be omitted spheroidizing annealing is capable of cold forging, and steel wire as a material for obtaining the component also becomes high strength without performing the refining of quenching and tempering, at the stage of steel wire, already assumed to have a microstructure having the above characteristics, it is effective to process the part without heat treatment prior to processing them.
[0028]
　In this case, compared to the conventional manufacturing method of softening by performing spheroidizing annealing, but cold workability is deteriorated, it is possible to reduce the hardening and tempering costs after processing the spheroidizing annealing costs, in terms of cost, the present invention is advantageous.
[0029]
　Moreover, method for producing the wire as a steel wire material, by utilizing the residual heat of the hot rolling immediately after the rolling, by immersion in a molten salt bath, even without addition of a large amount of alloy elements, it can be obtained steel of the above-mentioned tissues.
[0030]
　That is, the present invention component, a steel having an adjusted chemical composition, by using the residual heat of the hot rolling is immersed in a molten salt bath, the bainite entities having a predetermined average block grain diameter and cementite particle size and wire which was wire drawing at specific conditions at room temperature, to adjust the high strength bainite, is prepared by a series of manufacturing method of molding a part.
[0031]
　Therefore, the present invention is a component of the tensile strength of 700 ~ 1200 MPa can be manufactured at low cost.
[0032]
　(Chemical composition)
　wire for the tensile strength according to the present embodiment is 700 ~ 1200 MPa components, and steel wire (hereinafter, simply are "wire" may be referred to as "steel wire"), as well as the embodiment component according to (hereinafter, simply referred to as "part") is described composition of. Steel wire according to the present embodiment, the wire according to the present embodiment is obtained by drawing. Also, parts of this embodiment, a steel wire according to the present embodiment is obtained by forging cold, or cold forging and rolling. Wire drawing, cold forging, and rolling does not affect the chemical composition of the steel. Therefore, description of the chemical composition described below are wire, it falls under both of the steel wire, and components. In the following description, "%" means "% by mass". Incidentally, the balance of the chemical composition is Fe and unavoidable impurities.
[0033]
　C: 0.15 ~ 0.30%
　C is an element necessary for securing a tensile strength. If C content is less than 0.15%, it is difficult to obtain a tensile strength of not less than 700 MPa. Preferably, C content is 0.20% or more. On the other hand, if the C content of 0.30 percent, the cold forgeability deteriorates. Preferably not more than 0.25%.
[0034]
　Si: 0.05 ~
　0.50% Si, together with a deoxidizing element, is an element to increase the tensile strength by solid solution strengthening. If Si content is less than 0.05%, the addition effect is not sufficiently exhibited. Preferably, Si content is 0.15% or more. On the other hand, if the Si content is 0.50 percent, with the addition effect is saturated, ductility during hot rolling is deteriorated, easily flaws occurred. Preferred Si content is 0.30% or less.
[0035]
　Mn: 0.50 ～ 1.50
　Pasento Mn is an element to increase the tensile strength of steel. When Mn content is less than 0.50%, the addition effect is not sufficiently exhibited. Preferably, Mn content is 0.70% or more. On the other hand, if the Mn content is 1.50 percent, with the addition effect is saturated, transformation completion time for the isothermal transformation treatment of the wire is long, productivity is deteriorated. Preferred Mn content is less 1.30%.
[0036]
　P: 0.030% or less
　P is an element deteriorating the cold workability segregated in the grain boundary. If P content is 0.030%, the cold workability of the degradation is significant. Preferred P content is 0.015% or less. Wire according to the present embodiment, steel wire, and so components need not contain P, the lower limit of the P content is 0%.
[0037]
　S: 0.030% or less
　S, like P, is an element which deteriorates the cold workability segregated in the grain boundary. When S content is 0.030%, the cold workability of the degradation is significant. Preferred S content is 0.015% or less, more preferably 0.010% or less. Wire according to the present embodiment, steel wire, and so equipment need not contain S, the lower limit of the S content is 0%.
[0038]
　Al: 0.005 ~
　0.060% Al is a deoxidizing element and also is an element which forms an AlN that functions as pinning particles. AlN is comminuted grain, thereby increasing the cold workability. Further, Al is an element having an effect of suppressing dynamic strain aging by reducing solid solution N. If Al content is less than 0.005%, the above effect can not be obtained. Preferred Al content is 0.020% or more. If the Al content is 0.060 percent, with the above-mentioned effect is saturated, flaws are likely to occur during hot rolling. Preferred Al content is 0.050% or less.
[0039]
　Ti: 0.005 ~
　0.030% Ti is a deoxidizing element and, TiN is formed and an element having an effect of suppressing dynamic strain aging by reducing solid solution N. When Ti content is less than 0.005%, the above effect can not be obtained. Preferred Ti content is 0.010% or more. When Ti content is 0.030 percent, with the above-mentioned effect is saturated, flaws are likely to occur during hot rolling. Preferred Ti content is 0.025% or less.
[0040]
　B: 0.0003 ~ 0.0050%
　B suppresses grain boundary ferrite, effect and to enhance the cold workability, an effect of improving the promoting strength bainite transformation. Effect is insufficient at less than 0.0003%, the effect is saturated if it exceeds 0.0050%.
[0041]
　N: 0.0010 ~ 0.0100%
　N is an element which can degrade the cold workability by dynamic strain aging. To avoid such adverse effects, the N content to 0.0100% or less. The N is to form AlN and TiN to refine the crystal grain size has an effect of increasing the cold workability. For this reason, the lower limit of 0.0010%. A preferable content of N is 0.0020 to 0.0040%.
[0042]
　In the present invention, Cr: 0.01 ~ 0.40%, Nb: 0 ~ 0.03%, V: 0 to ~ 0.10% may contain one or two. Cr, containing Nb and V is optional may be 0%. Cr is effective to improve the tensile strength of the steel, Nb, and V, the effect of suppressing or dynamic strain aging by reducing solid solution N, has the effect of increasing the strength by promoting bainite transformation.
[0043]
　Cr: 0.01 ～ 0.40
　Pasento Cr is an element to increase the tensile strength of steel. If Cr content is less than 0.01%, the above effect is not sufficiently obtained. On the other hand, if the Cr content is 0.40 percent, it tends to occur martensite, thereby drawing workability and cold forgeability deteriorates. The preferred content of Cr is from 0.03 to 0.30 percent.
[0044]
　Nb: 0 ~
　0.03% Nb forms a NbN, an element having an action to suppress the dynamic strain aging by reducing solid solution N. If the Nb content of 0.03 percent, with the above-mentioned effect is saturated, flaws are likely to occur during hot rolling. Nb content is preferably 0.025% or less.
[0045]
　V: 0 ~ 0.10%
　V forms a VN, which is an element having an effect of suppressing dynamic strain aging by reducing solid solution N. If the V content of 0.10 percent, with the above-mentioned effect is saturated, flaws are likely to occur during hot rolling. Preferred V content is 0.05% or less.
[0046]
　O: 0 ~ 0.0030% or less
　O is wire, steel wire, and in part (e.g. mechanical parts), as oxides, such as Al and Ti. If the O content exceeds 0.0030%, coarse oxides are generated in steel, it tends to occur fatigue failure. Preferred O content is 0.0020%. The lower limit of O content is 0%.
[0047]
　Above, the wire according to the present embodiment has described component composition of the steel wire and parts, the balance of the chemical composition is Fe and unavoidable impurities. Here, the inevitable impurities, contained in the raw material, or a component mixed in the manufacturing process, intentionally refer to components not intended to be contained in the steel. Moreover, the inevitable impurities, specifically, Sb, Sn, W, Co, As, Mg, Pb, Bi, and H and the like. Incidentally, Sb, Sn, W, Co, As, Mg, Pb, Bi, and H, respectively, in order to realize the effect of the present application, 0.010%, respectively, 0.10%, 0.50%, 0 .50%, 0.010%, 0.010%, 0.10%, acceptable to include up to 0.10%, and 0.0010%.
[0048]
　Then, the wire and the steel wire according to the present embodiment, as well as components according to the present embodiment, the metal structure will be described. Steel wire according to the present embodiment the wire according to the present embodiment is obtained by drawing, component according to the present embodiment by a steel wire according to the present embodiment will be cold-forged or cold forging and obtained by rolling. Cold forging and rolling exerts influence on the parts of the metal structure is small. For parts, the amount of cold forging and rolling have on working is because small.
[0049]
　(Bainite area ratio of 90% or more)
　on the bainite area ratio of the metal structure, drawing, because the influence of the cold forging and rolling are small, the following description, the wire, in any of steel wires and parts applicable. Wire according to the present embodiment, steel wire and parts of metal structure, an area ratio containing 90% or more of bainite. In the present embodiment, the bainite, as shown in FIG. 1, after etching object (wire, steel wire or parts) cross section of the (steel (cross section perpendicular to the axis of the steel wire)) in nital, the surface layer from a predetermined depth of the object to the position (e.g., 0.25 times the depth of the diameter from the surface layer), when taken with a scanning electron microscope (SEM), needle-shaped or granular cementite dispersed an organization that is recognized as are.
[0050]
　In this embodiment, wire, bainite area ratio of the steel wire and parts are determined by the following procedure. That is, first, is etched to revealing the tissue cross-section of the object with nital. Then, the diameter of the object when the D, 4 places depth from the surface of the object is determined by rotating at 90 ° intervals about the longitudinal axis of symmetry thereof at a depth position of 50μm If the four locations determined by rotating at 90 ° intervals about the said axis at a depth position of depth 0.25D from the surface of the object, the depth from the central portion (surface layer of the shaft identifying one place and determined a depth position) of 0.5 D, the total of 9 points. And photographing magnification 1000 times structural photograph using SEM for these nine positions. Furthermore, marking a non-bainite in tissues photos taken (ferrite, each tissue of pearlite and martensite) visually determining the area of ​​each tissue by image analysis. As a result, the region including a bainite is determined by subtracting the area of ​​the non-bainite from the entire observation field of view. The area ratio of this area to the area ratio of bainite. Note that this operation is measured for at least two samples, calculated to obtain the average value thereof, the bainite area ratio the mean value in the present embodiment.
[0051]
　However, bainite it may be difficult to determine from the organization photograph by SEM. In that case, it is determined by KAM method (Kernel Average Misorientation) using an electron backscatter diffraction apparatus (EBSD). KAM method first approximation is six adjacent regular hexagonal pixels certain of the measured data, the second approximation is a 12 outside, or even 18 and is the third approximation of the outside pixels averaging the azimuth difference between the calculation of the value and the value of the center pixel is a method of performing for each pixel. This calculation, by carrying out so as not to exceed the grain boundary, it is possible to create a map representing the orientation change of the grains. Bainite, since strain in the grains larger dislocation density than the polygonal such pro-eutectoid ferrite transformed at a high temperature, the greater the intragranular difference in the crystal orientation. Thus, the analysis in this embodiment, conditions for calculating the orientation difference between adjacent pixels is the third approximation, the orientation differences shown above represent a 5 ° or less, of which the azimuth difference is 1 ° greater particle and bainite.
[0052]
　Assuming discriminating method such bainite, in the present embodiment, when the area ratio of bainite of the wire is less than 90%, or steel wire obtained by this wire to wire drawing, cold steel wire bainite area ratio of the part obtained by forging is less than 90%. In this case, it reduced yield strength ratio of the component (= 0.2% proof stress / tensile strength) strength, for example, a permanent elongation in use as machine parts deteriorates. In addition to bainite, pearlite, pro-eutectoid ferrite, and martensite or the like may be included in the steel wire, as long as the area ratio of bainite steel wire is 90% or more, content of the metal structure except bainite is acceptable It is. In the case where the area ratio of bainite steel wire is less than 90%, since the strength of the steel wire (tensile strength and hardness and the like) becomes uneven, cracked during cold working to the component is likely to occur . Note that the steel wire so it is desirable not contain metal structure other than bainitic, the upper limit of the area ratio of bainite steel wire is 100%.
[0053]
　(Average block grain diameter of bainite wire rod 15μm or less)
　in the wire according to the present embodiment, the average block grain diameter of bainite were measured in cross-section is 15μm or less. Here, the cross section refers to a plane perpendicular to the axial direction of the wire. If the average block grain diameter of bainite measured in the cross section of the wire is greater than 15 [mu] m, the ductility of the steel wire after drawing is lowered, thereby lowering the cold workability of the steel wire. Furthermore, the average block grain diameter of bainite part obtained the steel wire by cold working becomes coarse. If the average block grain diameter of bainite is coarse, proof stress ratio decreases. The average block grain size of bainite wire rod, since the smaller is preferred, it is not necessary to define the lower limit value.
[0054]
　(Steel wire and parts, an average aspect ratio R of the block grain bainite 1.2-2.0)
　The steel wire and parts of the present embodiment, at the location of the surface layer of the steel wire, in longitudinal section of the steel wire average aspect ratio R of the block grains measured bainite is 1.2-2.0. Here, the longitudinal section, parallel to the axis direction of the wire, and means a plane including the center axis. The average aspect ratio of bainite block is less than 1.2, the hydrogen embrittlement resistance of the parts to produce a steel wire with cold forging deteriorates. When the average aspect ratio exceeds 2.0, it reduces the yield strength ratio, permanent beauty deteriorates when used as a component.
[0055]
　In the present embodiment, the steel wire and parts, an average aspect ratio R of the block grain bainite is determined as follows. First, with respect to vertical surfaces of the steel wire, to determine the bainite block boundaries using EBSD. At this time, 100 [mu] m from the surfaces of both sides of the longitudinal section in the direction of the steel wire central axis in each of the two regions direction 500μm of steel wire central axis, at each measurement point in the region of measurement steps as 0.3μm the bcc-Fe in measuring the crystal orientation, orientation difference is defined as bainite block boundary boundaries than 15 degrees. Then, a region surrounded by the boundary between bainite block grains. Obtaining a map of the bainite block grains in this way in one vertical plane at both sides of a total of two regions. This carried out in four samples, obtain a map of the bainite block grains in total of eight regions. Circle-equivalent diameter from the bainite block grain map obtained is selected ten bainite block grains in order from largest. Selection has been ten bainite block particle measuring the aspect ratio of the block grains, finally calculates the average value thereof and the average aspect ratio R of the block grain bainite.
[0056]
　(Average block grain diameter of bainite steel wire (15 / R) [mu] m or less)
　in the steel wire according to the present embodiment, an average block grain diameter of the surface layer of bainite measured in cross-section (15 / R) [mu] m or less is there. Here, the cross section refers to a plane perpendicular to the axial direction of the steel wire. If the average block grain diameter of bainite of the surface layer were measured in cross-section of the steel wire is more than (15 / R) μm, ductility of the steel wire is lowered, thereby lowering the cold workability of the steel wire. Furthermore, the average block grain diameter of bainite part obtained a steel wire by cold working are coarsened, strength is lowered. The average block grain size of bainite in the surface layer of the steel wire, so is preferably small, it is not necessary to define the lower limit value.
[0057]
　In the present embodiment, the average block grain size of bainite in the surface layer of the wire (the same is true for steel wires and components) is determined as follows. First, in cross-section of the wire, to determine the region of 500μm extending with a width of 500μm in the center axis direction from the surface in the circumferential direction, the four regions is rotated every 90 ° around the central axis of this area Identify. Then, for these four areas, and the average block grain size measured by EBSD apparatus, the average block grain size of bainite in the surface layer of the wire (the same is true for steel wires and parts).
[0058]
　(Average block grain diameter of the part of bainite (15 / R) [mu] m or less)
　in the component according to the present embodiment, the average block grain diameter of the surface layer of bainite measured in cross-section (15 / R) [mu] m or less. Here, the cross section refers to a plane perpendicular to the axial direction of the component. If the average block grain diameter of the surface layer of bainite measured in the cross section of the part exceeds (15 / R) μm, strength ratio decreases. The average block grain size of bainite in the surface layer of the steel wire, so is preferably small, it is not necessary to define the lower limit value. Further, the method of determining the average block grain diameter of bainite parts is the same as the method of determining the average block grain diameter of bainite wire rod described above.
[0059]
　((The wire, the steel wire and components, the average of the surface layer of the bainite block particle diameter) / (average block grain diameter) of less than 1.0 bainite at the central portion)
　wire according to the present embodiment, steel wire, and goods in the average block grain diameter of the surface layer of bainite were measured in cross-section, the ratio of the average block grain diameter of bainite at the center portion as measured in cross-section is less than 1.0. When the ratio exceeds 1.0, the cold forgeability of the steel wire is deteriorated, and the strength ratio of the components is degraded.
[0060]
　In the present embodiment, the average block grain size of bainite at the central portion of the wire (the same is true for steel wires and components) is determined as follows. First, in cross-section of the wire, to determine the region of 500 [mu] m × 500 [mu] m centered on the central axis, to measure the block particle size for this region by EBSD device. Then, after the same measurement at three different cross-sections, and the average block grain size for the four samples, the average block grain size of bainite at the central portion of the wire (the same is true for steel wires and parts).
[0061]
　In the present embodiment, the ratio of the block grain diameter of the surface layer of the block particle diameter and the central portion, obtained by (average block grain diameter of the surface layer of bainite) / (average block grain size of bainite in the center).
[0062]
　(Average particle size of cementite dispersed in bainite 0.1μm or less)
　wire according to the present embodiment, steel wire and components, the average particle size of the dispersed cementite in the bainite is 0.1μm or less. When the average particle diameter of cementite exceeds 0.1 [mu] m, cold forging of the steel wire is deteriorated. Furthermore, the reduced yield strength ratio of at parts, for example, the permanent elongation in use as machine parts deteriorates.
[0063]
　The average particle size of cementite in the bainite according to the present embodiment is determined by the following procedure. First, the object with picral (wire, steel wire or component) the cross section of the etched to revealing the tissue. Then, the diameter of the object when the D, 4 places depth from the surface of the object is determined by rotating at 90 ° intervals about the longitudinal axis of symmetry thereof at a depth position of 50μm If the four locations determined by rotating at 90 ° intervals about the said axis at a depth position of depth 0.25D from the surface of the object, the depth from the central portion (surface layer of the shaft identifying one place and determined a depth position) of 0.5 D, the total of 9 points. And photographing magnification 20000 times structural photograph using for these nine field emission scanning electron microscope (FE-SEM). Finally, the captured image is binarized to obtain the circle equivalent diameter of cementite by image analysis, and calculates the average value of the nine samples, and the average particle size of cementite.
[0064]
　(Critical compressibility of 80% or more of the steel wires)
　steel wire obtained as described above exhibit good cold workability. In this embodiment, a limit compression ratio as an indicator of cold workability. In the present embodiment, the limit compression ratio, the steel wire after drawing, the sample height is 1.5 times the diameter created by machining, an end surface of the sample, a groove concentrically when compressed in the axial direction with attached mold, it means the maximum compression ratio that cracking does not occur. Note that the compression ratio before compression drawing height (axial dimension) H, after compression of drawing height (axial dimension) in the case of the H1, ((H-H1) / H) is a value indicated by × 100. The steel wire according to the present embodiment, the limit compression ratio can be 80% or more, it is possible to achieve excellent cold workability.
[0065]
　Next, wire, steel wire, and a method for manufacturing the component illustrating an example thereof. First, component composition, by mass%, C: 0.15 ~ 0.30%, Si: 0.05 ~ 0.50%, Mn: 0.50 ~ 1.50%, P: 0.030% or less , S: 0.030% or less, Al: 0.005 ~ 0.060%, Ti: 0.005 ~ 0.030%, B: 0.0003 ~ 0.0050%, N: 0.001 ~ 0. containing 010% as necessary, Cr: 0 ~ 0.40%, Nb: 0 ~ 0.03%, V: 0 contain one or two of the ~ 0.10%, the balance There is prepared a billet consisting of Fe and impurities. The billet was heated to 1000 ~ 1150 ° C., to obtain a wire by hot rolling at a finish rolling temperature of 800 ~ 950 ° C.. Then, the wire of the 800 ~ 950 ° C., cooled to 600 ° C. at an average cooling rate 40 ° C. / s or more, then cooled to 480 ° C. at an average cooling rate of 25 ° C. / s or higher. Thereafter, this wire to 400 ~ 480 ° C. in a temperature range of 15 seconds or more isothermal holding (first isothermal holding), further, by dipping or 25 seconds at a temperature range of 530 ~ 600 ° C. isothermal holding (second constant temperature for holding). And finally, get a wire and water-cooled.
[0066]
　2-step cooling after finish rolling, and the first constant-temperature holding is carried out by immersing the wire in a molten salt of 400 ~ 480 ° C. in the first molten salt bath. The second constant-temperature holding is carried out by immersing the wire in the second molten salt bath to 530 ~ 600 ° C. molten salt.
[0067]
　Performed here, in the manufacturing method of the wire according to the present embodiment, in particular, the cooling of the wire of 800 ~ 950 ° C., and cooling to 600 ° C., in two stages of cooling and, up to 600 ° C. ~ 480 ° C. . In particular, in the subsequent cooling by a cooling rate 25 ° C. / s or higher, it is possible to control the average block grain size of bainite 15μm or less.
[0068]
　In the method of manufacturing a wire according to the present embodiment, the molten salt bath temperature in the first molten salt bath and 400 ~ 480 ° C., the immersion time to 15 ~ 50s. The molten salt bath temperature by a 400 ° C. or higher, to suppress the contamination of the martensite, resulting excellent cold forgeability. On the other hand, by a 480 ° C. or less, it is possible to reduce the average particle size of the cementite, and unnecessary bluing treatment with excellent cold forgeability can be obtained. Moreover, the immersion time With more than 15s, to suppress the contamination of non-bainite structure, excellent cold forgeability obtained. On the other hand, it is set to lower than or equal to 50s, can be to reduce the average particle size of the cementite, and unnecessary bluing treatment with excellent cold forgeability can be obtained.
[0069]
　Similarly, in the manufacturing method of the wire according to the present embodiment, the molten salt bath temperature in the second molten salt bath and 530 ~ 600 ° C., the immersion time can be set to 25 ~ 80s. The molten salt bath temperature by a 530 ° C. or higher, to suppress the contamination of the martensite, resulting excellent cold forgeability. On the other hand, by a 600 ° C. or less, it is possible to reduce the average particle size of the cementite, and unnecessary bluing treatment with excellent cold forgeability can be obtained. Moreover, the immersion time With more than 25s, to suppress the contamination of the martensite, resulting excellent cold forgeability. On the other hand, it is set to lower than or equal to 80s, can be to reduce the average particle size of the cementite, and unnecessary bluing treatment with excellent cold forgeability can be obtained.
[0070]
　Then, the steel wire according to the present embodiment can be produced by the following method as an example. That is, the wire material manufactured in the manner described above, to wire drawing at a total reduction of area of ​​10 to 55%. The total reduction of area of ​​10 to 55% in the drawing may be accomplished by a single wire drawing may be achieved by a plurality of times of drawing. Thus the steel wire is obtained according to the present embodiment.
[0071]
　Moreover, parts of the embodiment (machine parts, building parts and the like) can be prepared by the following method as an example. That is, the steel wire mentioned above, by cold forging, or by cold forging and form rolling were processed into the shape of various parts, components tensile strength is 700 ~ 1200 MPa is obtained.
Example
[0072]
　Next, a description will be given of an embodiment of the present invention, conditions in examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is, in this single condition example the present invention is not limited. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention, it is capable of adopting various conditions.
[0073]
　With billet 14 kinds of compositions shown in Table 1, under the conditions of 28 patterns shown in Table 2, heating, hot rolling, isothermal transformation process, sequentially subjected to cooling, the wire (the levels 1 to 28) It was produced. Then, using the respective wire rods, the wire was drawn at a reduction of area ratio shown in Table 2, were produced steel wire (levels 1-28). Further, using each steel wire, the sample height is 1.5 times the diameter created by machining, to produce a component (levels 1-28). Then, the end faces of the parts, and axially compressed by using a mold with groove concentrically, the maximum compression ratio that cracks do not occur, the limit compression ratio of the part. Then, the limit compression ratio is more than 80% of the steel wire is cold workability was judged to be good. Further, a tensile test piece from the shaft portion of each component was collected and subjected to tensile test, the tensile strength and 0.2% proof stress in terms of measured yield strength ratio (0.2% proof stress / tensile strength) is 0 .90 or more components strength ratio is determined to be good. Incidentally, steel, for any of the steel wires and parts, levels 1-7 and level 14-20 are invention examples, levels 8-13 and levels 21-28 are comparative examples.
[0074]
[Table 1]

[0075]
[Table 2]

[0076]
　Incidentally, to describe each level containing the blanks in Table 2, for example, level 10, after hot rolling, without isothermal transformation process is an example produced by immersing in boiling water bath. Level 11, after hot rolling, without isothermal transformation process is an example produced by cooling by air-cooling. Level 13, after once cooled to room temperature hot rolled wire rod was reheated to 1000 ° C., an example produced by immersion in a molten salt bath of 1 tank.
[0077]
　Next, the results for wire organization in Table 3, the results for the tissue of the steel wire in Table 4, and the results for the cold forgeability and the component characteristics of the steel wire in Table 5, respectively.
[0078]
[table 3]

[0079]
[Table 4]

[0080]
[table 5]

[0081]
　Tables from 2-5 as apparent, for all production conditions specified in the present application levels 1-7 and level 14-20 is within a predetermined range (invention examples), both, cold forging of the steel wire good results have been obtained on the characteristics of sex and components. That is, for the levels 1-7 and level 14-20, both the tensile strength of the part is 700 ~ 1200 MPa, even without a so-called bluing treatment after part molding, 0.90 or more proof stress ratio obtained it can be seen that is.
[0082]
　In contrast, the levels 8-13 and level 21-28 either manufacturing conditions defined by the present is out of the predetermined range (Comparative Example) are both characteristic of the cold forgeability and components of the steel wire it can be seen that at least one of does not show good results.
Industrial Applicability
[0083]
　As described above, according to the present invention, which can be manufactured at low cost, tensile strength obtained components 700 ~ 1200 MPa, also used in the manufacture of the parts, spheroidizing annealing and quenching and tempering processing, and omissions of bluing treatment after cold forging can be steel wire, and it is possible to obtain a wire rod for the manufacture of the steel wire. Accordingly, the present invention, since it is likely used in the steel member preparation industry, is promising.

The scope of the claims
[Requested item 1]
　By mass%, C: 0.15 ~ 0.30% , Si: 0.05 ~ 0.50%, Mn: 0.50 ~ 1.50%, P: 0.030% or less, S: 0.030 % or less, Al: 0.005 ~ 0.060%, Ti: 0.005 ~ 0.030%, B: 0.0003 ~ 0.0050%, N: 0.001 to 0.010%, a wire and the balance Fe and unavoidable impurities,
　is 90% or more bainite metal structure with an area ratio, average block grain diameter of the surface layer of bainite were measured in cross-section is at 15μm or less, measured in cross section an average block grain diameter of the surface layer of bainite that is the ratio of the average block grain size of bainite was measured at the center portion, (average block grain diameter of the surface layer of bainite) / (average block grain size of bainite at the center portion value of) is less than 1.0, and, Baie The average particle size of the dispersed cementite during night is 0.1μm or less
wires, characterized in that.
[Requested item 2]
　The wire further contains, by mass%, Cr: 0 ~ 0.40%, Nb: 0 ~ 0.03%, V: 0 ~ 0.10%, containing one or two of, claims wire according to claim 1.
[Requested item 3]
　By mass%, C: 0.15 ~ 0.30% , Si: 0.05 ~ 0.50%, Mn: 0.50 ~ 1.50%, P: 0.030% or less, S: 0.030 % or less, Al: 0.005 ~ 0.060%, Ti: 0.005 ~ 0.030%, B: 0.0003 ~ 0.0050%, N: 0.001 to 0.010%, a wire drawing steel wire the balance being Fe and unavoidable impurities,
　a metal structure 90% bainite area ratio in the surface layer of the steel wire an average aspect blocks grain bainite measured in longitudinal section the ratio R is 1.2 to 2.0 and an average block grain diameter of the surface layer of bainite measured in cross-section (15 / R) [mu] m or less, the average block grain diameter of the surface layer of bainite measured in transverse section When, in the ratio of the average block grain diameter of bainite was measured at the center portion That the value of (average block grain diameter of the surface layer of bainite) / (average block grain size of bainite in the center) is less than 1.0, and the average particle size of the dispersed cementite in the bainite 0. it is 1μm or less
steel wire, characterized in that.
[Requested item 4]
　The steel wire is further, by mass%, Cr: 0 ~ 0.40%, Nb: 0 ~ 0.03%, V: 0 ~ 0.10%, containing one or two of, steel wire according to claim 3.
[Requested item 5]
　Limit compression ratio is 80% or more, the steel wire according to claim 3 or 4.
[Requested item 6]
　By mass%, C: 0.15 ~ 0.30% , Si: 0.05 ~ 0.50%, Mn: 0.50 ~ 1.50%, P: 0.030% or less, S: 0.030 % or less, Al: 0.005 ~ 0.060%, Ti: 0.005 ~ 0.030%, B: 0.0003 ~ 0.0050%, N: 0.001 to 0.010%, the component comprising the balance of Fe and unavoidable impurities,
　is 90% or more bainite metal structure with an area ratio in the surface layer of the part, the average aspect ratio R of the block grain bainite measured in longitudinal section 1.2 ~ 2.0, and it is an average block grain diameter of the surface layer of bainite measured in cross-section (15 / R) μm or less, and average block grain diameter of the surface layer of bainite measured in transverse section, measured at the center portion is the average block particle diameter ratio of the bainite, (surface layer The value of the average block grain diameter of bainite) / (average block grain size of bainite in the center) is less than 1.0, and an average particle size of cementite dispersed in the bainite is 0.1μm or less
that parts according to claim.
[Requested item 7]
　The component further contains, by mass%, Cr: 0 ~ 0.40%, Nb: 0 ~ 0.03%, V: 0 ~ 0.10%, containing one or two of, claims component according to claim 6.