WO2018008698A1 - 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 µm 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 µm or less.

Description

Wire, steel wire and parts

The present invention relates to a wire, a steel wire produced from the wire, and a part having a tensile strength produced from the steel wire of 700 MPa to 1200 MPa. In addition, a machine part and a building part are contained in the components used as object in this invention.

High-strength mechanical parts having a tensile strength of 700 MPa or more are used for automobiles and various industrial machines for the purpose of reducing weight and size. Conventionally, this type of high-strength mechanical component is sequentially subjected to hot rolling and spheroidizing annealing on a steel material made of alloy steel obtained by adding alloy elements such as Mn, Cr, Mo, and B to carbon steel for mechanical structure. And then softened, and then cold forged or rolled into a predetermined shape, and then subjected to quenching and tempering treatment to impart strength.

However, such steel materials are manufactured because of the high alloying element content, which increases the price of the steel materials, and also requires spheroidizing annealing before forming the part shape and quenching / tempering processing after forming. Cost increases.

Under such circumstances, a technique for omitting the spheroidizing annealing, quenching and tempering treatment, performing wire drawing on the wire rod with increased strength by performing rapid cooling and aging treatment, and giving a predetermined strength is known. . This technique is used for machine parts and the like, and machine parts and the like manufactured using this technique are called non-tempered machine parts.

JP-A-2-166229 discloses that C: 0.03 to 0.20%, Si: 0.10% or less, Mn: 0.7 to 2.5%, one of V, Nb, and Ti, or A steel containing 2 or more types: 0.05 to 0.30%, B: 0.0005 to 0.0050%, a non-made bainite structure cooled at a cooling rate of 5 ° C / sec or more after wire rolling. A method for manufacturing a tempered machine part is disclosed.

JP-A-8-41537 discloses C: 0.05 to 0.20%, Si: 0.01 to 1.0%, Mn: 1.0 to 2.0%, S: 0.015. %, Al: 0.01-0.05%, V: 0.05-0.3% steel is heated to a temperature of 900-1150 ° C. and then hot-rolled. A method for producing a high-strength machine part in which a ferrite + bainite structure is formed by cooling in a temperature range from ℃ to 500 ℃ at an average cooling rate of 2 ℃ / sec or more, and then annealed in a temperature range of 550 to 700 ℃ Is disclosed.

However, these manufacturing methods require strict control of the cooling rate and the cooling end temperature, making the manufacturing method complicated and increasing the manufacturing cost. Further, the structure becomes non-uniform, and the cold forgeability may deteriorate.

In contrast, Japanese Patent Laid-Open No. 2000-144306 discloses cold forging in which C is 0.40 to 1.0% by mass, the component composition satisfies a specific conditional expression, and the structure is made of pearlite or pseudo-pearlite. Steel for use is disclosed. This steel has a large amount of C and is inferior in cold forgeability as compared with carbon steel for machine structure and alloy steel for machine structure conventionally used for machine parts.

As described above, with the non-heat treated wire according to the conventional technique, a mechanical part having good cold forgeability by an inexpensive manufacturing method, and a steel wire and a wire for producing the part are not obtained. In particular, with regard to the prior art that omits spheroidizing annealing and quenching / tempering treatment, etc., it is excellent even if these treatments are omitted because the structure becomes uneven and excellent cold forgeability cannot be obtained. There was room for improvement in the development of parts capable of realizing the mechanical characteristics.

In view of the above problems in the prior art, the present invention
(A) a component having a tensile strength of 700 to 1200 MPa that can be manufactured at low cost;
(B) To provide a steel wire capable of omitting the spheroidizing annealing, quenching / tempering treatment, and bluing treatment after cold forging, and a wire rod for producing the steel wire used for the production of the part. ,
With the goal.

In order to achieve the above object, the present inventors can perform cold forging even if spheroidizing annealing is omitted, and the tensile strength is 700 MPa or more without performing tempering treatment of quenching and tempering. The relationship between the composition of steel and the structure to obtain high strength parts was investigated. The present invention has been made on the basis of metallurgical findings obtained through such investigations, and the gist thereof is as follows.

(1) By mass%, 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%, N: 0.001 to 0.010% Containing wire and balance Fe and inevitable impurities, the area ratio is 90% or more of the metal structure is bainite, the average block particle size of the bainite of the surface layer measured in cross section is 15 μm or less, The average block particle size of bainite in the surface layer measured on the surface and the average block particle size of bainite measured in the center, (average block particle size of bainite in the surface layer) / (average block of bainite in the center) The value of particle size) is less than 1.0 , An average particle size of cementite dispersed in the bainite is 0.1μm or less, the wire, characterized in that.

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

(3) By mass, 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%, N: 0.001 to 0.010% It is a steel wire that has been drawn and composed of the balance Fe and unavoidable impurities, and 90% or more of the metal structure is bainite by area ratio, and the bainite block measured in a longitudinal section in the surface layer of the steel wire The average aspect ratio R of the grains is 1.2 to 2.0, the average block particle size of the bainite of the surface layer measured in the cross section is (15 / R) μm or less, and the bainite of the surface layer measured in the cross section is Average block particle size and average block of bainite measured at the center The average particle size of cementite dispersed in the bainite, which is the ratio of the diameters (average block particle size of bainite on the surface layer) / (average block particle size of bainite at the center) is less than 1.0. A steel wire having a diameter of 0.1 μm or less.

(4) The above steel wire further comprises one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, V: 0 to 0.10% in mass%. The steel wire according to (3), which is contained.

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

(6) By mass, 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%, N: 0.001 to 0.010% It is a component comprising the balance Fe and unavoidable impurities, and 90% or more of the metal structure is bainite by area ratio, and the average aspect ratio R of the block grain of bainite measured in the longitudinal section in the surface layer of the component is The average block particle size of the bainite in the surface layer measured in the cross section is 1.2 to 2.0 or less, and the average block particle size of the bainite in the surface layer measured in the cross section and the center Is the ratio of the average block particle size of bainite measured in part The value of (average block particle size of bainite of the surface layer) / (average block particle size of bainite at the center) is less than 1.0, and the average particle size of cementite dispersed in bainite is 0.1 μm or less A part characterized by being.

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

According to the present invention, high-strength parts having a tensile strength of 700 to 1200 MPa that contribute to weight reduction and downsizing of machine parts used in automobiles and various industrial machines and construction parts used at construction sites are provided at low cost. can do.

It is a SEM photograph which shows the metal structure of the wire and steel wire which concern on this embodiment, and the component which concerns on this embodiment.

As described above, the present inventors can perform cold forging even if spheroidizing annealing is omitted, and a tensile strength exceeding 700 MPa without performing tempering treatment of quenching and tempering. The relationship between the composition of steel and the structure for obtaining high strength parts was investigated in detail. And in order to manufacture high-strength parts inexpensively, the present inventors based on the metallurgical knowledge obtained in the investigation, in-line heat treatment using the retained heat at the time of hot rolling of the wire, and the subsequent steel wire A comprehensive study of a series of manufacturing methods up to parts was conducted, and the following conclusions were reached.

(A) A steel wire that has been strengthened by wire drawing and cold forging is inferior in workability, has high deformation resistance, and is susceptible to work cracking.

(B) In order to improve the workability of the high-strength steel wire, the structure is mainly composed of bainite, the block particle size of the surface layer is made fine, and the average particle size of cementite dispersed in bainite is 0. It is effective to set the thickness to 1 μm or less.

(C) That is, assuming that the area ratio of bainite is 90% or more and the average aspect ratio of the block grains of bainite measured in the longitudinal section is R, the average value of the block grain size of bainite in the surface layer measured in the transverse section is When the ratio is set to (15 / R) μm or less and the ratio of the average block particle size of bainite in the surface layer to the average block particle size of bainite inside the wire is less than 1.0, cold workability can be remarkably improved.

(D) Furthermore, by forming the structures of the above (b) and (c), the yield ratio can be increased even if the bluing process is omitted after forming the part.

Thus, by improving the component composition and structure of the steel material, it was possible to increase the strength even if the quenching and tempering treatment was omitted, and to improve the cold forgeability.

Such a steel wire that can be cold forged even if spheroidizing annealing is omitted, and that is a material for obtaining a high-strength part without performing tempering treatment of quenching and tempering, It is effective to have a microstructure with the above characteristics at the stage of steel wire, and to process this into a part without performing heat treatment before processing.

In this case, compared with the conventional manufacturing method of softening by performing spheroidizing annealing, the cold workability deteriorates, but since the spheroidizing annealing cost and the quenching / tempering cost after processing can be reduced, in terms of cost, The present invention is advantageous.

Furthermore, about the manufacturing method of the wire used as the material of the steel wire, by using the residual heat at the time of hot rolling, immediately after rolling, it is immersed in a molten salt bath without adding a large amount of alloy elements, A steel material having the above-described structure can be obtained.

That is, the component of the present invention is a bainite-based material composed of a predetermined average block particle size and cementite particle size by immersing a steel material having an adjusted composition in a molten salt bath using residual heat during hot rolling. It is manufactured by a series of manufacturing methods in which a wire is drawn at a specific temperature at room temperature, a high-strength bainite is adjusted, and molded into a part.

Therefore, according to the present invention, parts having a tensile strength of 700 to 1200 MPa can be manufactured at low cost.

(Component composition)
Wires for parts having a tensile strength of 700 to 1200 MPa according to the present embodiment, and steel wires (hereinafter sometimes simply referred to as “wires” and “steel wires”, respectively), and parts according to the present embodiment ( Hereinafter, the composition of the component (sometimes simply referred to as “component”) will be described. The steel wire according to the present embodiment is obtained by drawing the wire according to the present embodiment. Moreover, the component which concerns on this embodiment is obtained by cold forging the steel wire which concerns on this embodiment, or cold forging and rolling. Wire drawing, cold forging, and rolling do not affect the composition of the steel. Therefore, the description regarding the component composition described below applies to any of wire, steel wire, and parts. In the following description, “%” means “mass%”. The balance of the component composition is Fe and inevitable impurities.

C: 0.15-0.30%
C is an element necessary for ensuring tensile strength. When the C content is less than 0.15%, it is difficult to obtain a tensile strength of 700 MPa or more. Preferably, the C content is 0.20% or more. On the other hand, when the C content exceeds 0.30%, the cold forgeability deteriorates. Preferably it is 0.25% or less.

Si: 0.05 to 0.50%
Si is a deoxidizing element and is an element that increases the tensile strength by solid solution strengthening. When the Si content is less than 0.05%, the effect of addition is not sufficiently exhibited. Preferably, the Si content is 0.15% or more. On the other hand, when the Si content is more than 0.50%, the effect of addition is saturated, the ductility during hot rolling is deteriorated, and soot is easily generated. A preferable Si content is 0.30% or less.

Mn: 0.50 to 1.50%
Mn is an element that increases the tensile strength of steel. When the Mn content is less than 0.50%, the effect of addition is not sufficiently exhibited. Preferably, the Mn content is 0.70% or more. On the other hand, when the Mn content is more than 1.50%, the effect of addition is saturated, and the transformation completion time in the isothermal transformation treatment of the wire becomes long, and the productivity is deteriorated. A preferable Mn content is 1.30% or less.

P: 0.030% or less P is an element that segregates at a grain boundary and deteriorates cold workability. When the P content exceeds 0.030%, the cold workability is significantly deteriorated. A preferable P content is 0.015% or less. Since the wire, the steel wire, and the component according to the present embodiment do not need to contain P, the lower limit value of the P content is 0%.

S: 0.030% or less S, like P, is an element that segregates at the grain boundaries and degrades the cold workability. When the S content exceeds 0.030%, the cold workability is significantly deteriorated. A preferable S content is 0.015% or less, more preferably 0.010% or less. Since the wire, the steel wire, and the fixture which concern on this embodiment do not need to contain S, the lower limit of S content is 0%.

Al: 0.005 to 0.060%
Al is a deoxidizing element and an element that forms AlN that functions as pinning particles. AlN refines crystal grains, thereby improving cold workability. Moreover, Al is an element having an action of reducing solid solution N and suppressing dynamic strain aging. When the Al content is less than 0.005%, the above effects cannot be obtained. A preferable Al content is 0.020% or more. When the Al content is more than 0.060%, the above effect is saturated and wrinkles are likely to occur during hot rolling. A preferable Al content is 0.050% or less.

Ti: 0.005 to 0.030%
Ti is a deoxidizing element, and is an element that forms TiN and has an action of suppressing solid strain aging by reducing solid solution N. When the Ti content is less than 0.005%, the above effect cannot be obtained. A preferable Ti content is 0.010% or more. When the Ti content is more than 0.030%, the above effects are saturated and wrinkles are likely to occur during hot rolling. A preferable Ti content is 0.025% or less.

B: 0.0003 to 0.0050%
B has the effect of suppressing grain boundary ferrite and improving cold workability, and the effect of promoting bainite transformation and improving strength. If it is 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 such adverse effects, the N content is set to 0.0100% or less. N also has the effect of increasing the cold workability by forming AlN or TiN to reduce the crystal grain size. For this reason, the lower limit was made 0.0010%. A preferable N content is 0.0020 to 0.0040%.

In the present invention, one or two of Cr: 0.01 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10% may be contained. The content of Cr, Nb and V is arbitrary and may be 0%. Cr has the effect of increasing the tensile strength of the steel, and Nb and V have the effect of reducing the solid solution N to suppress dynamic strain aging, and the effect of increasing the strength by promoting bainite transformation.

Cr: 0.01-0.40%
Cr is an element that increases the tensile strength of steel. When the Cr content is less than 0.01%, the above effects cannot be obtained sufficiently. On the other hand, when the Cr content is more than 0.40%, martensite is liable to occur, thereby deteriorating the wire drawing workability and the cold forgeability. A preferable content of Cr is 0.03 to 0.30%.

Nb: 0 to 0.03%
Nb is an element which has the effect | action which forms NbN, reduces the solid solution N, and suppresses dynamic strain aging. When the Nb content is more than 0.03%, the above effect is saturated and wrinkles are likely to occur during hot rolling. The Nb content is preferably 0.025% or less.

V: 0 to 0.10%
V is an element that has the function of forming VN, reducing solid solution N, and suppressing dynamic strain aging. When the V content exceeds 0.10%, the above-described effects are saturated and wrinkles are likely to occur during hot rolling. A preferable V content is 0.05% or less.

O: 0 to 0.0030% or less O is present as an oxide such as Al and Ti in wire rods, steel wires, and parts (for example, machine parts). When the O content exceeds 0.0030%, coarse oxides are generated in the steel, and fatigue failure is likely to occur. A preferable O content is 0.0020% or less. The lower limit of the O content is 0%.

As mentioned above, although the component composition of the wire, the steel wire, and components which concern on this embodiment was demonstrated, the remainder of a component composition is Fe and an unavoidable impurity. Here, the inevitable impurities are components that are included in raw materials or mixed in during the manufacturing process, and are components that are not intentionally included in steel. Inevitable impurities are specifically Sb, Sn, W, Co, As, Mg, Pb, Bi, and H. Note that Sb, Sn, W, Co, As, Mg, Pb, Bi, and H are 0.010%, 0.10%, 0.50%, 0, respectively, for realizing the effects of the present application. It is acceptable to include up to .50%, 0.010%, 0.010%, 0.10%, 0.10%, and 0.0010%.

Next, the metal structure of the wire and steel wire according to this embodiment and the parts according to this embodiment will be described. The steel wire according to the present embodiment is obtained by drawing the wire according to the present embodiment, and the component according to the present embodiment is obtained by cold forging the steel wire according to the present embodiment, or cold forging. And obtained by rolling. The effect of cold forging and rolling on the metal structure of the part is small. This is because the amount of processing that cold forging and rolling exert on parts is small.

(Bainite area ratio: 90% or more)
Since the effects of wire drawing, cold forging, and rolling on the bainite area ratio of the metal structure are small, the following description applies to any of wire, steel wire, and parts. The metal structures of the wire, the steel wire, and the component according to the present embodiment include bainite having an area ratio of 90% or more. In this embodiment, as shown in FIG. 1, bainite is obtained by etching a cross section (cross section perpendicular to the axis of a steel material (steel wire)) of an object (wire material, steel wire or component) with nital, When a position of a predetermined depth from the surface layer of the object (for example, a depth of 0.25 times the diameter from the surface layer) is photographed with a scanning electron microscope (SEM), acicular or granular cementite is dispersed. It is a recognized organization.

In the present embodiment, the bainite area ratio of the wire, steel wire, and parts is determined by the following procedure. That is, first, the cross section of the object is etched with nital to reveal the structure. Next, assuming that the diameter of the object is D, the four positions determined by rotating every 90 ° about the longitudinal axis of the symmetrical object at a depth position of 50 μm from the surface layer of the object. And four locations determined by rotating the object about 90 ° around the axis at a depth position of 0.25D, and the center part of the axis (the depth from the surface layer is A total of nine locations are identified, including one location determined to a depth position of 0.5D. Then, tissue photographs at a magnification of 1000 times are taken using SEM at these nine locations. Furthermore, the non-bainite (ferrite, pearlite, and martensite structures) in the photographed structure photograph is visually marked, and the area of each structure is obtained by image analysis. As a result, the bainite-containing region can be obtained by subtracting the non-bainite region from the entire observation field. The area ratio of this region is defined as the area ratio of bainite. In addition, this operation measures and calculates about at least 2 samples, calculates | requires those average values, and makes the said average value the bainite area rate in this embodiment.

However, bainite may be difficult to distinguish from the SEM micrograph. In that case, it discriminate | determines by KAM method (Kernel | Average | region | Misorientation) using an electron beam backscattering diffraction apparatus (EBSD). The KAM method is a pixel of the first approximation that is six adjacent hexagonal pixels in the measurement data, the second approximation that is 12 outside the pixel, or the third approximation that is 18 outside the pixel. This is a method of performing calculation for each pixel by averaging the azimuth differences between them and setting the value as the value of the center pixel. By performing this calculation so as not to cross the grain boundary, a map expressing the orientation change in the grain can be created. Since bainite has a high dislocation density and large intra-granular strain compared to polygonal pro-eutectoid ferrite transformed at high temperature, the intra-granular difference in crystal orientation is large. Therefore, in the analysis in the present embodiment, the condition for calculating the azimuth difference between adjacent pixels is the third approximation, and the one whose azimuth difference is 5 ° or less is displayed, of which the grains whose azimuth difference exceeds 1 ° are displayed. It shall be bainite.

On the premise of such a bainite discriminating method, in this embodiment, when the area ratio of the bainite of the wire is less than 90%, a steel wire obtained by drawing this wire, or a steel wire cold The area ratio of the bainite of the part obtained by forging is less than 90%. In this case, the strength ratio (= 0.2% proof stress / tensile strength) strength of the component is reduced, and the permanent elongation when used as a mechanical component is deteriorated, for example. In addition to bainite, pearlite, proeutectoid ferrite, martensite, etc. may be contained in the steel wire, but as long as the area ratio of bainite in the steel wire is 90% or more, inclusion of a metal structure other than bainite is acceptable. Is done. In addition, when the area ratio of the bainite of the steel wire is less than 90%, the strength (tensile strength, hardness, etc.) of the steel wire becomes non-uniform, so that cracking is likely to occur during cold working of parts. . In addition, since it is desirable that a steel wire does not contain metal structures other than bainite, the upper limit of the area ratio of the bainite of a steel wire is 100%.

(The average block particle size of the bainite wire is 15 μm or less)
In the wire according to the present embodiment, the average block particle size of bainite measured in a cross section is 15 μm or less. Here, the cross section means a plane perpendicular to the axial direction of the wire. When the average block particle size of bainite measured in the cross section of the wire exceeds 15 μm, the ductility of the steel wire after the wire drawing process is lowered, thereby reducing the cold workability of the steel wire. Furthermore, the average block particle size of the bainite of the part obtained by cold-working this steel wire becomes coarse. When the average block particle size of bainite becomes coarse, the yield strength ratio decreases. In addition, since the one where the average block particle diameter of the bainite of a wire is smaller is preferable, it is not necessary to prescribe | regulate the lower limit.

(Average aspect ratio R of bainite block grains of steel wires and parts is 1.2 to 2.0)
In the steel wire and parts according to the present embodiment, the average aspect ratio R of the bainite block grains measured in the longitudinal section of the steel wire is 1.2 to 2.0 at the surface layer position of the steel wire. Here, the longitudinal section means a plane that is parallel to the axial direction of the wire and includes the central axis. When the average aspect ratio of the bainite block is less than 1.2, the hydrogen embrittlement resistance of parts manufactured by cold forging a steel wire deteriorates. On the other hand, when the average aspect ratio exceeds 2.0, the proof stress ratio decreases, and the permanent spread deteriorates when used as a part.

In this embodiment, the average aspect ratio R of the bainite block grains of the steel wire and parts is determined as follows. First, a bainite block grain boundary is determined using EBSD with respect to the longitudinal section of a steel wire. At this time, in each of the two regions of 100 μm in the direction of the steel wire central axis and 500 μm in the direction of the steel wire central axis from each surface on both sides of the longitudinal section, the measurement step is set to 0.3 μm at each measurement point in the region. The crystal orientation of bcc-Fe was measured, and a boundary having an orientation difference of 15 degrees or more is defined as a bainite block boundary. And the area | region enclosed by this boundary is made into a bainite block grain. Thus, a map of bainite block grains is obtained in a total of two regions on both sides of one longitudinal section. This is done on 4 samples to obtain a map of bainite block grains in a total of 8 regions. From the obtained map of bainite block grains, 10 bainite block grains are selected in order from the largest equivalent circle diameter. The aspect ratio of the block grains is measured for the selected 10 bainite block grains, and finally, the average value thereof is calculated as the average aspect ratio R of the bainite block grains.

(Average block particle size of bainite of steel wire is (15 / R) μm or less)
In the steel wire according to the present embodiment, the average block particle size of the surface layer bainite measured in the cross section is (15 / R) μm or less. Here, the cross section means a plane perpendicular to the axial direction of the steel wire. When the average block particle size of the bainite on the surface layer measured in the cross section of the steel wire exceeds (15 / R) μm, the ductility of the steel wire is lowered, thereby reducing the cold workability of the steel wire. Furthermore, the average block particle size of the bainite of the part obtained by cold-working this steel wire becomes coarse, and the proof stress decreases. In addition, since the one where the average block particle size of the bainite in the surface layer part of a steel wire is smaller is preferable, it is not necessary to prescribe | regulate the lower limit.

In this embodiment, the average block particle size of bainite in the surface layer of the wire (the same applies to steel wires and parts) is determined as follows. First, in the cross section of the wire, a region extending 500 μm in the circumferential direction with a width of 500 μm from the surface layer in the central axis direction is determined, and four regions obtained by rotating this region every 90 ° around the central axis are determined. Identify. And about these four area | regions, the block particle diameter measured with the EBSD apparatus is averaged, and it is set as the average block particle diameter of the bainite in the surface layer of a wire (a steel wire and components are the same).

(The average block particle size of the bainite of the part is (15 / R) μm or less)
In the component according to the present embodiment, the average block particle size of the surface layer bainite measured in the cross section is (15 / R) μm or less. Here, the cross section means a plane perpendicular to the axial direction of the component. When the average block particle size of the bainite on the surface layer measured in the cross section of the component exceeds (15 / R) μm, the yield strength ratio decreases. In addition, since the one where the average block particle size of the bainite in the surface layer part of a steel wire is smaller is preferable, it is not necessary to prescribe | regulate the lower limit. Moreover, the determination method of the average block particle size of the bainite of components is the same as the determination method of the average block particle size of the bainite of a wire mentioned above.

((Average block particle size of bainite on the surface layer of wire rods, steel wires and parts) / (average block particle size of bainite at the center) is less than 1.0)
In the wire rod, steel wire, and article according to the present embodiment, the ratio of the average block particle size of bainite in the surface layer measured in the cross section and the average block particle size of bainite in the center portion measured in the cross section is 1.0. Is less than. When the ratio exceeds 1.0, the cold forgeability of the steel wire deteriorates and the yield strength ratio of the parts deteriorates.

In the present embodiment, the average block particle size of bainite at the center of the wire (the same applies to steel wires and parts) is determined as follows. First, an area of 500 μm × 500 μm centering on the central axis is determined in the cross section of the wire, and the block particle diameter is measured with an EBSD apparatus in this area. Next, after the same measurement was performed on three different cross sections, the block particle diameters of the four samples were averaged to obtain the average block particle diameter of bainite at the center of the wire (the same applies to steel wires and parts).

In this embodiment, the ratio between the block particle size of the surface layer and the block particle size of the central portion is obtained by (average block particle size of bainite in the surface layer) / (average block particle size of bainite in the central portion).

(Average particle size of cementite dispersed in bainite is 0.1 μm or less)
In the wire, steel wire, and component according to the present embodiment, the average particle size of cementite dispersed in bainite is 0.1 μm or less. When the average particle diameter of cementite exceeds 0.1 μm, the cold forgeability of the steel wire deteriorates. Further, the yield strength ratio of the parts is lowered, and for example, the permanent elongation when used as a machine part is deteriorated.

The average particle size of cementite in bainite according to this embodiment is determined by the following procedure. First, the cross-section of the object (wire, steel wire or part) is etched using picral to reveal the structure. Next, assuming that the diameter of the object is D, the four positions determined by rotating every 90 ° about the longitudinal axis of the symmetrical object at a depth position of 50 μm from the surface layer of the object. And four locations determined by rotating the object about 90 ° around the axis at a depth position of 0.25D, and the center part of the axis (the depth from the surface layer is A total of nine locations are identified, including one location determined to a depth position of 0.5D. Then, tissue photographs at a magnification of 20000 times are taken at these nine locations using a field emission scanning electron microscope (FE-SEM). Finally, the photographed image is binarized, the equivalent circle diameter of cementite is obtained by image analysis, the average value of nine samples is calculated, and the average particle diameter of cementite is obtained.

(The limit compression rate of steel wire is 80% or more)
The steel wire obtained as described above exhibits good cold workability. In the present embodiment, the critical compression ratio is used as an index indicating the cold workability. In the present embodiment, the critical compression ratio is a sample whose height is 1.5 times the diameter from a steel wire after wire drawing by machining, and concentric grooves are formed on the end surface of this sample. When compressing in the axial direction using the attached mold, it means the maximum compression ratio at which no cracks occur. The compression ratio is ((H−H1) /) where H is the height before drawing (axial dimension) before drawing, and H1 is the height after drawing (axial dimension) after drawing. H) A value indicated by x100. In the steel wire according to the present embodiment, the critical compression ratio can be 80% or more, and excellent cold workability can be realized.

Next, an example of a method for manufacturing a wire, a steel wire, and a part will be described. First, the component composition is mass%, 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: 0.005-0.060%, Ti: 0.005-0.030%, B: 0.0003-0.0050%, N: 0.001-0. 010%, optionally containing one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, V: 0 to 0.10%, the balance A steel slab comprising Fe and impurities is prepared. The steel slab is heated to 1000 to 1150 ° C. and then hot rolled at a finish rolling temperature of 800 to 950 ° C. to obtain a wire. Next, the wire at 800 to 950 ° C. is cooled to 600 ° C. at an average cooling rate of 40 ° C./s or higher, and then cooled to 480 ° C. at an average cooling rate of 25 ° C./s or higher. Thereafter, the wire is held at a temperature range of 400 to 480 ° C. for 15 seconds or more (first constant temperature hold), and further immersed in a temperature range of 530 to 600 ° C. for 25 seconds or more to hold a constant temperature (second constant temperature). Hold). Finally, the wire is cooled with water.

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

Here, in the method of manufacturing the wire according to the present embodiment, the cooling of the wire at 800 to 950 ° C. is performed in two stages of cooling to 600 ° C. and cooling to 600 ° C. to 480 ° C. . In particular, in the latter stage cooling, the average block particle size of bainite can be controlled to 15 μm or less by setting the cooling rate to 25 ° C./s or more.

Further, in the method for manufacturing the wire according to the present embodiment, the molten salt bath temperature in the first molten salt bath is set to 400 to 480 ° C., and the immersion time is set to 15 to 50 s. By setting the molten salt bath temperature to 400 ° C. or higher, mixing of martensite is suppressed, and excellent cold forgeability is obtained. On the other hand, by setting it as 480 degrees C or less, the average particle diameter of cementite can be made small, the outstanding cold forgeability can be obtained, and a blueing process can be made unnecessary. Further, by setting the immersion time to 15 s or longer, mixing of non-bainite structure is suppressed, and excellent cold forgeability is obtained. On the other hand, by setting it as 50 s or less, the average particle diameter of cementite can be reduced, and excellent cold forgeability can be obtained and the bluing treatment can be made unnecessary.

Similarly, in the method for manufacturing a wire according to this embodiment, the molten salt bath temperature in the second molten salt bath can be set to 530 to 600 ° C., and the immersion time can be set to 25 to 80 s. By setting the molten salt bath temperature to 530 ° C. or higher, mixing of martensite is suppressed, and excellent cold forgeability is obtained. On the other hand, by setting the temperature to 600 ° C. or less, the average particle diameter of cementite can be reduced, and excellent cold forgeability can be obtained and the bluing treatment can be made unnecessary. Further, by setting the immersion time to 25 s or longer, mixing of martensite is suppressed, and excellent cold forgeability is obtained. On the other hand, by setting it to 80 s or less, the average particle diameter of cementite can be reduced, and excellent cold forgeability can be obtained, and the bluing treatment can be made unnecessary.

Next, the steel wire according to the present embodiment can be manufactured by the following method as an example. That is, the wire manufactured by the above method is drawn at a total area reduction of 10 to 55%. The total area reduction ratio of 10 to 55% in the wire drawing process may be achieved by a single wire drawing process or may be achieved by a plurality of wire drawing processes. Thus, the steel wire according to the present embodiment is obtained.

Furthermore, the parts (machine parts, building parts, etc.) of this embodiment can be manufactured by the following method as an example. That is, the above steel wire is processed into various parts by cold forging or by cold forging and rolling to obtain a part having a tensile strength of 700 to 1200 MPa.

Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Using steel slabs of 14 kinds of composition shown in Table 1, heating, hot rolling, isothermal transformation treatment, and cooling were sequentially performed under the conditions of 28 patterns shown in Table 2 to form a wire (level 1 to 28). Manufactured. Next, using each wire, wire drawing was performed at a surface reduction rate shown in Table 2 to produce steel wires (levels 1 to 28). Furthermore, using each steel wire, a sample having a height 1.5 times the diameter was prepared by machining to produce parts (levels 1 to 28). The end face of each part was compressed in the axial direction using a concentric grooved mold, and the maximum compression ratio at which no cracks occurred was defined as the limit compression ratio of the part. And the steel wire whose limit compressibility was 80% or more was judged that cold work property was favorable. In addition, a tensile test piece is taken from the shaft portion of each part, a tensile test is performed, and the tensile strength and the 0.2% proof stress are measured, and then the proof stress ratio (0.2% proof stress / tensile strength) is 0. .90 parts or more were judged to have good yield strength ratio. For any steel materials, steel wires and parts, 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.

In addition, each level including the blank in Table 2 will be described. For example, level 10 is an example of manufacturing by immersing in a boiling water tank without performing isothermal transformation after hot rolling. Level 11 is an example of manufacturing by cooling with air cooling without performing a constant temperature transformation treatment after hot rolling. Level 13 is an example in which the hot-rolled wire was once cooled to room temperature, reheated to 1000 ° C., and immersed in one tank of molten salt.

Next, Table 3 shows the results relating to the structure of the wire, Table 4 shows the results relating to the structure of the steel wire, and Table 5 shows the results relating to the cold forgeability of the steel wire and the characteristics of the parts.

As is clear from Tables 2 to 5, for all of the levels 1 to 7 and levels 14 to 20 (invention examples) in which all the production conditions specified in the present application are within a predetermined range, cold forging of steel wire Good results have been obtained with respect to properties and part properties. That is, in each of Levels 1 to 7 and Levels 14 to 20, the tensile strength of the parts is 700 to 1200 MPa, and the proof stress ratio is 0.90 or more without performing so-called blueing treatment after the parts are molded. It turns out that it is obtained.

On the other hand, for levels 8 to 13 and levels 21 to 28 (comparative examples) in which any of the manufacturing conditions specified in the present application is outside the predetermined range, the cold forgeability of the steel wire and the characteristics of the parts are all included. It can be seen that at least one of the above does not show good results.

As described above, according to the present invention, a part having a tensile strength of 700 to 1200 MPa that can be manufactured at low cost can be obtained, and spheroidizing annealing, quenching and tempering used for manufacturing the part can be obtained. The steel wire which can abbreviate | omit the blueing process after a process and cold forging, and the wire for manufacturing the steel wire can be obtained. Accordingly, the present invention is promising because of its high availability in the steel member manufacturing industry.

Claims (7)

% By mass, 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 %: Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, N: 0.001 to 0.010%, A wire consisting of the remaining Fe and inevitable impurities,
90% or more of the metal structure in the area ratio is bainite, the average block particle size of the bainite of the surface layer measured in the cross section is 15 μm or less, the average block particle size of the bainite of the surface layer measured in the cross section, and the central portion The ratio of the average block particle size of bainite measured in step (the average block particle size of bainite on the surface layer) / (average block particle size of bainite at the center) is less than 1.0, and An average particle diameter of cementite dispersed in bainite is 0.1 μm or less. 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%. Item 2. The wire according to Item 1. % By mass, 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 %: Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, N: 0.001 to 0.010%, A drawn steel wire consisting of the balance Fe and inevitable impurities,
The bainite has a metal structure of 90% or more in area ratio, and the average aspect ratio R of the bainite block grains measured in the longitudinal section is 1.2 to 2.0 in the surface layer of the steel wire, and the surface layer measured in the transverse section. The average block particle size of bainite is (15 / R) μm or less, and is the ratio of the average block particle size of bainite on the surface layer measured in the cross section to the average block particle size of bainite measured in the center portion. The average block particle size of bainite in the surface layer) / (average block particle size of bainite at the center) is less than 1.0, and the average particle size of cementite dispersed in bainite is 0.1 μm or less. A steel wire characterized by being. 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% by mass%. The steel wire according to claim 3. The steel wire according to claim 3 or 4, wherein the critical compression ratio is 80% or more. % By mass, 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 %: Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, N: 0.001 to 0.010%, A component comprising the balance Fe and inevitable impurities,
90% or more of the metal structure in area ratio is bainite, and the average aspect ratio R of the block grain of bainite measured in the longitudinal section is 1.2 to 2.0 in the surface layer of the part, and the surface layer measured in the transverse section The average block particle size of bainite is (15 / R) μm or less, and is the ratio of the average block particle size of bainite on the surface layer measured in the cross section to the average block particle size of bainite measured in the center portion. The average block particle size of bainite in the surface layer) / (average block particle size of bainite at the center) is less than 1.0, and the average particle size of cementite dispersed in bainite is 0.1 μm or less. A part characterized by being. The component further contains one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10% by mass%. Item according to Item 6.

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