Seonjae

Abstract

Having a chemical composition of C: 0.10 to 0.60%, Si: 0.01 to 0.45%, Mn: 0.30 to 1.00%, P: 0.030% or less, S: 0.001 to 0.050%, Al: 0.001 to 0.080%, Cr: 0.85 to 1.50%, N: 0.0010 to 0.0200%, O: 0.004% or less, remainder: Fe and impurities, and having a metallic structure in the 1 / 4D portion containing 96% or more of bainite in terms of area percentage when the diameter is D, and the remainder being martensite when the area percentage of bainite is less than 100%, and the diffraction peak of the (211) plane when the X-ray diffraction pattern is measured by Cr-Kα rays using an X-ray stress diffraction device in the 1 / 4D portion of the longitudinal cross-section Wire rod having a half-width of 2.20° or more and a cross-sectional shrinkage rate of 30.0% or more.

Description

Technology Field

[0001] The present disclosure relates to wire material. Background Technology

[0002] Because low-alloy steel cold heading wires containing Mn and Cr have high deformation resistance and low ductility, wear or damage to the mold may occur during cold heading, or processing cracks may develop in the formed parts. To address these issues, a method of softening hot-rolled wires by performing spheroidizing annealing is being implemented.

[0003] Furthermore, in recent years, as part shapes have become more complex with the aim of reducing manufacturing costs and enhancing functionality, steel used for cold forging requires not only softness but also extremely high ductility.

[0004] When the content of Cr or Mo is high, sufficient processability is not obtained with a single spheroidizing annealing, so it is manufactured by performing two spheroidizing annealings. However, since an increase in the number of annealing cycles leads to increased manufacturing costs and CO2 emissions, technology for simplifying and eliminating spheroidizing annealing is required.

[0005] Patent Document 1 describes a cold-working machine structural steel capable of sufficiently softening even with a short spheroidizing treatment time at a relatively low spheroidizing annealing temperature, such as about 750°C, having a predetermined chemical composition, an area percentage of proeutectoid ferrite of 10% or more and 70% or less, and also containing one or more selected from the group consisting of bainite, martensite, and pearlite, and having a dislocation density of 3.5 × 10⁻⁶ 14 m -2 An ideal cold-worked machine structural steel has been proposed.

[0006] Patent Document 2 describes a steel wire containing alloying elements and having excellent cold forging properties, having a predetermined chemical composition, wherein 95% or more of the area of ​​the metallic structure consists of ferrite and spherical carbides, wherein the ferrite has an average grain size of 10.0 to 30.0 μm, and the spherical carbides have an average aspect ratio of spherical carbides with an equivalent diameter of 0.1 μm or more of 2.5 or less, and furthermore, when the content of C (mass%) included in the steel wire is expressed as [C], the number of spherical carbides with an equivalent diameter of 0.1 μm or more is 1.5 × 10⁻⁶ 6 ×[C] to 7.0×10 6 ×[C] pieces / mm 2 A steel wire has been proposed.

[0007] Patent Document 3 proposes a steel wire material that enables a reduction in the spheroidizing annealing time, achieves improved processing performance and reduced deformation resistance in cold forging after spheroidizing treatment, and realizes excellent cold forging properties, comprising C: 0.005 to 0.6 mass%, having a pseudo-pearlite of 10 area% or more, a bainite of 75 area% or less, a ferrite of 60 area% or less, and satisfying the relationship (pseudo-pearlite area% + bainite area% + ferrite area%) ≥ 90 area%.

[0008] Patent Document 4 proposes a steel wire rod having a predetermined chemical composition and cementite as an internal structure, in order to realize stable drawing and pressing performance, wherein at least 80% of the cementite in the cross-section perpendicular to the longitudinal direction of the wire rod has a short diameter of 0.1 μm or less and an aspect ratio of the ratio of the long diameter to the short diameter is 2.0 or less.

[0009] Patent Document 5 proposes a wire rod capable of achieving reduced deformation resistance and improved ductility after spheroidizing annealing, comprising Cr: 0.85 to 1.50%, etc., wherein the content of each of C, Si, Mn, Cr, Cu, Mo, and V satisfies a specific relationship, the metal structure comprises at least bainite and martensite, the total area ratio of bainite, martensite, and ferrite measured in a cross-section perpendicular to the longitudinal direction of the wire rod is 98% or more, the area ratio of ferrite is less than 5%, the area ratio of martensite satisfies a specific relationship, the average circle equivalent diameter of the bainite block is 15 μm or less, the bainite block satisfies a specific relationship, and the tensile strength satisfies a specific relationship. Prior art literature

[0010] International Publication No. 2022 / 181272, Japanese Patent No. 7151885, Japanese Patent Publication No. 2006-225701, International Publication No. 2015 / 141840, Japanese Patent Publication No. 2020-186446 The problem to be solved

[0011] The present disclosure aims to provide a wire rod capable of producing a steel wire with good workability and having strength equivalent to that of two annealing cycles, even when the annealing cycle is performed once, when the steel wire is produced by performing drawing and annealing treatments on a low-alloy steel wire rod containing Mn and Cr. means of solving the problem

[0012] The above problem is solved by the following means.

[0013] <1> In mass %,

[0014] C: 0.10 to 0.60%,

[0015] Si: 0.01 to 0.45%,

[0016] Mn: 0.30 to 1.00%,

[0017] P: 0.030% or less,

[0018] S: 0.001 to 0.050%,

[0019] Al: 0.001 to 0.080%,

[0020] Cr: 0.85 to 1.50%,

[0021] N: 0.0010 to 0.0200%,

[0022] O: 0.004% or less,

[0023] Mo: 0 to 1.00%,

[0024] V: 0 to 0.50%,

[0025] Nb: 0 to 0.050%,

[0026] Ti: 0 to 0.100%,

[0027] Cu: 0 to 0.40%,

[0028] Ni: 0 to 0.30%,

[0029] Sn: 0 to 0.10%,

[0030] B: 0 to 0.0200%,

[0031] Ca: 0 to 0.0050%,

[0032] Mg: 0 to 0.0050%,

[0033] Remainder: Having a chemical composition of Fe and impurities,

[0034] When the diameter of the wire is D, the metal structure at the 1 / 4D portion at a depth of 1 / 4D from the surface of the wire in a cross-section perpendicular to the longitudinal direction of the wire comprises bainite with an area percentage of 96% or more and 100% or less, and the remainder when the area percentage of the bainite is less than 100% is martensite.

[0035] In the 1 / 4D portion of the cross-section parallel to the length direction, which includes the central axis of the above wire, the half-width of the diffraction peak of the (211) plane, measured by Cr-Kα rays using an X-ray stress diffraction device, is 2.20° or greater, and

[0036] Wire rod with a cross-sectional shrinkage rate of 30.0% or more.

[0037] <2> In mass %,

[0038] Mo: 0.01 to 1.00%,

[0039] V: 0.01 to 0.50%,

[0040] Nb: 0.001 to 0.050%, and

[0041] Ti: 0.005 to 0.100%

[0042] One or more types selected from the group consisting of

[0043] including, <1> Wire material listed in

[0044] <3> In mass %,

[0045] Cu: 0.02 to 0.40%,

[0046] Ni: 0.02 to 0.30%,

[0047] Sn: 0.01 to 0.10%, and

[0048] B: 0.0003 to 0.0200%

[0049] One or more types selected from the group consisting of

[0050] including, <1> or <2> Wire material listed in

[0051] <4> In mass %,

[0052] Ca: 0.0001 to 0.0050%, and

[0053] Mg: 0.0001 to 0.0050%

[0054] Type 1 or Type 2 selected from the group consisting of

[0055] including, <1> inside <3> Wire material listed in any one of them.

[0056] <5> tensile strength of 1200 MPa or less <1> inside <4> Wire material listed in any one of them.

[0057] <6> After drawing the above wire rod with a reduction rate in the range of 30% to 50%, the annealed material obtained by heating at a heating rate of 150℃ / hour as an annealing process, maintaining at 720℃ for 12 hours, and slow cooling has a tensile strength of 500±15MPa and a cross-sectional shrinkage rate of 80.0±5.0%. <1> inside <5> Wire material listed in any one of them. Effects of the invention

[0058] According to the present disclosure, when a steel wire is manufactured by performing drawing and annealing treatments on a low-alloy steel wire containing Mn and Cr, a wire is provided that can be manufactured having strength equivalent to that of a wire that has been annealed twice, even if the number of annealing cycles is one, and also has good workability. Brief explanation of the drawing

[0059] Figure 1a is an SEM image showing an example of a microstructure etched with a 1 / 4D portion of a wire using picrol. Figure 1b is a drawing showing martensite marked in the SEM image shown in Figure 1a. FIG. 2 is a schematic diagram illustrating an example of the full width at half maximum of a diffraction peak in an X-ray diffraction pattern measured by an X-ray stress diffraction device in the 1 / 4D portion of a wire. Figure 3 is a schematic diagram illustrating the area for measuring ferrite particles in a steel wire. Specific details for implementing the invention

[0060] An exemplary embodiment of the present disclosure will be described.

[0061] The wire material according to the present disclosure is a wire material that can be made into a steel wire with high strength and high ductility (workability) after drawing and annealing, and is suitable for use as a material for machine parts such as bolts, screws, and nuts formed by, for example, cold forging or rolling. In addition, the wire material targeted in the present disclosure includes a "bar coil" in which a hot-rolled wire material is wound into a coil.

[0062] In the present disclosure, a numerical range expressed using “to” means a range that includes the values ​​described before and after “to” as lower and upper limits.

[0063] In cases where "greater than" or "less than" is attached to the numerical values ​​listed before or after "or," the numerical range refers to a range that does not include these values ​​as lower or upper limits.

[0064] The elemental content of a chemical composition is sometimes expressed in terms of the amount of each element (e.g., amount of C, amount of Si, etc.).

[0065] Regarding the elemental content of a chemical composition, "%" means "mass%".

[0066] When the elemental content of a chemical composition is stated as "0 to," it means that that element may not be included.

[0067] The term "process" includes not only independent processes but also cases where a process cannot be clearly distinguished from other processes, provided that its intended purpose is achieved.

[0068] The "surface" of the wire refers to the "outer surface."

[0069] "C-section" refers to a cross-section perpendicular to the longitudinal direction of the wire.

[0070] "L-section" refers to a cross-section parallel to the longitudinal direction of a wire or steel wire.

[0071] The notation "1 / 4D" means a position at a depth of 1 / 4 times the diameter D from the surface toward the central axis (in the diameter direction), where D is the diameter of the wire, and is synonymous with "D / 4".

[0072] To improve machinability after spheroidizing annealing, it is effective to improve ductility (crack suppression, cross-sectional shrinkage) and reduce deformation resistance (strength). To improve ductility, it is necessary to suppress coarse spheroidal cementite, which serves as the fracture initiation point during plastic deformation, and this can be achieved by maintaining the microstructure as bainite prior to spheroidizing annealing. By using bainite, in which carbides are uniformly dispersed compared to ferrite and pearlite, the dispersion state of cementite after annealing can be made uniform and fine. However, if the cementite is excessively fine, the deformation resistance becomes high.

[0073] The inventors of the present disclosure focused on coarsening the ferrite crystal grain size to reduce deformation resistance. Coarsening the ferrite crystal grain size is effective for reducing deformation resistance. The inventors of the present disclosure discovered that deformation resistance is further reduced by performing drawing and spheroidizing annealing at an A1 point or lower on a wire rod mainly composed of a bainite structure obtained by isothermal transformation treatment at a temperature of less than 500°C in a molten salt bath (first holding process / second holding process). It is thought that the ferrite grains became coarse because the bainite structure obtained by isothermal transformation treatment at a temperature of less than 500°C had a high full-width of the X-ray diffraction peak of the (211) plane as a function of dislocation density, which became the driving force for recrystallization. It is estimated that when the isothermal transformation temperature is high (500°C or higher) or the tempering temperature is high (500°C or higher), alloying elements such as Mo become concentrated at the grain boundaries, which delays recrystallization during spheroidizing annealing and hinders the coarsening of the ferrite grain size.

[0074] Chemical Composition

[0075] The chemical composition (steel component) of the wire material according to the present disclosure is in mass%,

[0076] C: 0.10 to 0.60%,

[0077] Si: 0.01 to 0.45%,

[0078] Mn: 0.30 to 1.00%,

[0079] P: 0.030% or less,

[0080] S: 0.001 to 0.050%,

[0081] Al: 0.001 to 0.080%,

[0082] Cr: 0.85 to 1.50%,

[0083] N: 0.0010 to 0.0200%,

[0084] O: 0.004% or less, and the remainder consists of Fe and impurities.

[0085] In addition, the wire material according to the present disclosure may further contain one or more types selected from the group consisting of Group A, Group B and Group C below, instead of some of Fe.

[0086] [Group A]

[0087] Mo: 0 to 1.00%,

[0088] V: 0 to 0.50%,

[0089] Nb: 0 to 0.050%, and

[0090] Ti: 0 to 0.100%

[0091] One or more types selected from the group consisting of

[0092] [Group B]

[0093] Cu: 0 to 0.40%,

[0094] Ni: 0 to 0.30%,

[0095] Sn: 0 to 0.10%, and

[0096] B: 0 to 0.0200%

[0097] One or more types selected from the group consisting of

[0098] [Group C]

[0099] Ca: 0 to 0.0050%, and

[0100] Mg: 0 to 0.0050%

[0101] That is, the elements of groups A to C are optional elements, and these elements do not need to be contained in the wire material relating to the present disclosure, but if they are contained, they shall be within the above range.

[0102] Hereinafter, each element in the wire material relating to the present disclosure will be described.

[0103] C: 0.10 to 0.60%

[0104] Carbon is added to ensure strength as a machine part. If the amount of carbon is less than 0.10%, it is difficult to ensure the necessary strength as a machine part. On the other hand, if the amount of carbon exceeds 0.60%, ductility, toughness, and cold forgeability deteriorate. Therefore, the amount of carbon is set to 0.10 to 0.60%. The preferred range for the amount of carbon that achieves high strength, ductility, toughness, and cold workability is 0.15 to 0.50%.

[0105] Si: 0.01 to 0.45%

[0106] Si functions as a deoxidizing element and is an effective element for imparting the necessary strength to machine parts by imparting quenchability and improving resistance to tempering softening. If the Si content is less than 0.01%, these effects are insufficient. If the Si content exceeds 0.45%, the ductility and toughness of the machine parts deteriorate, and the deformation resistance of the steel wire increases, thereby deteriorating cold forging properties. Therefore, the Si content is set to 0.01 to 0.45%. The preferred range for the Si content is 0.03 to 0.35%. A more preferred range for the Si content is 0.05 to 0.30%.

[0107] Mn: 0.30 to 1.00%

[0108] Mn is an element necessary to impart quenchability and provide the required strength to machine parts. If the amount of Mn is less than 0.30%, the effect is insufficient. If the amount of Mn exceeds 1.00%, the toughness of the machine parts deteriorates, and the deformation resistance of the steel wire increases, thereby deteriorating cold forgeability. Therefore, the amount of Mn is set to 0.30 to 1.00%. The preferred range for the amount of Mn is 0.35 to 0.90%. A more preferred range for Mn is 0.40 to 0.85%.

[0109] P: 0.030% or less

[0110] P is contained in the wire rod as an impurity. Since P segregates at the grain boundaries of machine parts after quenching and tempering, degrading toughness, it is desirable to reduce it. For this reason, the upper limit of the P amount is set to 0.030%. The preferred upper limit of the P amount is 0.020%. A more preferred upper limit of the P amount is 0.015% or less. In addition, the lower limit of the P amount is preferably 0% (i.e., not included), but it may exceed 0% (or 0.0001% or more) from the perspective of reducing the cost of removing P.

[0111] S: 0.001 to 0.050%

[0112] S is contained in the wire rod as sulfides such as MnS. These sulfides improve the machinability of the steel wire. If the S content exceeds 0.050%, it degrades the cold forgeability of the steel wire and also degrades the toughness of machine parts after quenching and tempering. For this reason, the upper limit of the S content is set to 0.050%. The preferred upper limit of the S content is 0.040%. A more preferred upper limit of the S content is 0.030%. In addition, the lower limit of the S content is 0.001% or higher from the perspective of reducing the cost of removing S.

[0113] Al: 0.001 to 0.080%

[0114] Al functions as a deoxidizing element and forms AlN to finer the austenite grains, thereby improving the toughness of machine parts. Additionally, by fixing dissolved N, it suppresses dynamic strain aging and reduces strain resistance. If the amount of Al is less than 0.001%, these effects are insufficient. If the amount of Al exceeds 0.080%, the effects become saturated and may reduce manufacturability. Therefore, the amount of Al is set to 0.001 to 0.080%. The preferred range for the amount of Al is 0.010 to 0.060%. A more preferred range for the amount of Al is 0.020 to 0.050%.

[0115] Cr: 0.85 to 1.50%

[0116] Cr is an element necessary to impart the required strength to machine parts by improving quenchability. In addition, the inclusion of Cr causes the shape of carbides after annealing to become spherical, thereby improving cold workability. If the Cr content is less than 0.85%, the effect is insufficient. If the Cr content exceeds 1.50%, the spheroidization time of carbides becomes prolonged, increasing manufacturing costs, and the deformation resistance of the steel wire increases, deteriorating cold forgeability. Therefore, the Cr content is set to 0.85 to 1.50%. The preferred range for the Cr content is 0.87 to 1.40%. A more preferred range for the Cr content is 0.90 to 1.30%.

[0117] N: 0.0010 to 0.0200%

[0118] N forms nitrides with Al, Ti, Nb, V, etc., refines the austenite grains, and has the effect of improving the toughness of machine parts. If the amount of N is less than 0.0010%, the amount of nitride precipitation is insufficient, so the effect is not obtained. If the amount of N exceeds 0.0200%, the deformation resistance of the steel wire increases due to dynamic strain aging caused by dissolved N, thereby deteriorating workability. Therefore, the amount of N is set to 0.0010 to 0.0200%. The preferred range for the amount of N is 0.0020 to 0.0080%. A more preferred range for the amount of N is 0.0030 to less than 0.0050%.

[0119] O: 0.004% or less

[0120] O is inevitably contained in wire rods and exists as oxides of Al, Ti, etc. If the amount of O is high, coarse oxides are formed, which causes a decrease in the fatigue strength of machine parts. Therefore, the amount of O is limited to 0.004% or less.

[0121] The wire material according to the present disclosure may contain one or more types selected from the group consisting of Group A, Group B, and Group C below for the purpose of improving the characteristics described below.

[0122] [Group A]

[0123] Mo: 0.01 to 1.00%,

[0124] V: 0.01 to 0.50%,

[0125] Nb: 0.001 to 0.050%, and

[0126] Ti: 0.005 to 0.100%

[0127] One or more types selected from the group consisting of

[0128] [Group B]

[0129] Cu: 0.02 to 0.40%,

[0130] Ni: 0.02 to 0.30%,

[0131] Sn: 0.01 to 0.10%, and

[0132] B: 0.0003 to 0.0200%

[0133] One or more types selected from the group consisting of

[0134] [Group C]

[0135] Ca: 0.0001 to 0.0050%, and

[0136] Mg: 0.0001 to 0.0050%

[0137] Type 1 or Type 2 selected from the group consisting of

[0138] Mo: 0 to 1.00%

[0139] Mo has the effect of improving quenchability and imparting the necessary strength to machine parts. If the amount of Mo exceeds 1.00%, the alloy cost increases, and the deformation resistance of the steel wire rises, deteriorating cold forgeability. Therefore, when Mo is included, it is preferable to have the amount of Mo greater than 0 and 1.00%. The preferred range for the amount of Mo is 0.01 to 0.90% and 0.05 to 0.80%.

[0140] V: 0 to 0.50%

[0141] V has the effect of increasing the strength of machine parts by precipitating VC carbides. If the amount of V exceeds 0.50%, the alloy cost increases. Therefore, when V is included, it is preferable to have the amount of V greater than 0 and 0.50%. The preferred range for the amount of V is 0.01 to 0.45%, and more preferably 0.05 to 0.40%.

[0142] Nb: 0 to 0.050%

[0143] Nb has effects such as increasing the strength of machine parts by precipitating carbides or nitrides, improving toughness by fine-graining austenite grains, and reducing deformation resistance by reducing dissolved N. If the amount of Nb exceeds 0.050%, the effects may become saturated and cold forgeability may deteriorate. Therefore, when Nb is included, it is preferable to keep the amount of Nb greater than 0 and 0.050%. The preferred range for the amount of Nb is 0.001 to 0.030%. More preferably, it is 0.005 to 0.020%.

[0144] Ti: 0 to 0.100%

[0145] In addition to functioning as a deoxidizing element, Ti has the effect of improving the toughness of machine parts by forming nitrides or carbides to finer the austenite grains, the effect of increasing quenchability by promoting the formation of solid solution B, and the effect of reducing deformation resistance by fixing solid solution N to suppress dynamic strain aging. If the amount of Ti exceeds 0.100%, these effects become saturated and coarse oxides or nitrides are formed, which may degrade the fatigue strength of machine parts. Therefore, when Ti is included, it is preferable to keep the amount of Ti greater than 0 and 0.100%. The preferred range for the amount of Ti is 0.005 to 0.050%. A more preferred range for the amount of Ti is 0.008 to 0.030%, and even more preferably 0.010 to 0.025%.

[0146] Cu: 0 to 0.40%

[0147] Cu has the effect of improving quenchability or precipitating finely to impart the necessary strength to machine parts, while also improving corrosion resistance. If the amount of Cu exceeds 0.40%, hot ductility deteriorates, making the surface prone to scratches. Therefore, when Cu is included, it is preferable to have the amount of Cu greater than 0 and 0.40%. The preferred range for the amount of Cu is 0.02 to 0.30%, and more preferably 0.05 to 0.25%.

[0148] Ni: 0 to 0.30%

[0149] Ni has the effect of improving quenchability and providing the necessary strength to machine parts. If the amount of Ni exceeds 0.30%, it increases the alloy cost. Therefore, when including Ni, it is preferable to have the amount of Ni greater than 0 and 0.30%. The preferred range for the amount of Ni is 0.02 to 0.25%, and more preferably 0.05 to 0.20%.

[0150] Sn: 0 to 0.10%

[0151] Sn has the effect of improving corrosion resistance. If the amount of Sn exceeds 0.10%, ductility decreases and cold workability deteriorates, so it is limited to 0.10% or less. When Sn is included, it is preferable that the amount of Sn be greater than 0 and 0.10%. The preferred range for the amount of Sn is 0.01 to 0.08%.

[0152] B: 0 to 0.0200%

[0153] B has the effect of segregating at grain boundaries as a solid solution B to improve quenchability and impart the necessary strength to machine parts. If the amount of B exceeds 0.0200%, carbides may form at grain boundaries, which may degrade the cold processability. Therefore, when B is included, it is preferable to have the amount of B greater than 0 and 0.0200%. The preferred range for the amount of B is 0.0003 to 0.0100%. A more preferred range for the amount of B is 0.0005 to 0.0040%, and even more preferably 0.0010 to 0.0030%.

[0154] Ca: 0 to 0.0050%

[0155] Ca is added for the purpose of deoxidizing elements. These elements have the effect of improving fatigue strength by refining oxides. If the amount added exceeds 0.0050%, the effect becomes saturated and coarse oxides are generated, which may degrade fatigue properties. Therefore, when Ca is included, it is preferable to set the amount of Ca to greater than 0 and 0.0050%. The preferred amount of Ca is 0.0001 to 0.0030%. More preferably, it is 0.0005 to 0.0020%, and even more preferably, 0.0008 to 0.0015%.

[0156] Mg: 0 to 0.0050%

[0157] Mg is added for the purpose of deoxidizing elements. These elements have the effect of improving fatigue strength by refining oxides. If the amount added exceeds 0.0050%, the effect becomes saturated and coarse oxides are formed, which may degrade fatigue properties. Therefore, when Mg is included, it is preferable to keep the amount of Mg greater than 0 and 0.0050%. The preferred amount of Mg is 0.0001 to 0.0040%. More preferably, it is 0.0005 to 0.0035%.

[0158] Remainder: Fe and impurities

[0159] In the chemical composition of the wire material according to the present disclosure, the remainder is Fe and impurities.

[0160] Here, impurities refer to components contained in raw materials or incorporated during the manufacturing process, which are not intentionally included. Furthermore, even if intentionally included, impurities also include components present in amounts that do not affect the performance of the steel wire.

[0161] [Metal structure]

[0162] Next, the metal structure of the wire material according to the present disclosure will be described.

[0163] The wire rod according to the present disclosure, when the diameter of the wire rod is D, contains bainite in an area percentage of 96% or more in the 1 / 4D portion at a depth of 1 / 4D from the surface of the wire rod in a cross section perpendicular to the longitudinal direction of the wire rod (C cross section). The area percentage of the metal rod in the 1 / 4D portion of the C cross section may not be 100% bainite, i.e., may not contain a structure other than bainite, and may contain martensite in a total of 4% or less. Furthermore, since the inclusion of pearlite in the metal rod lowers the ductility of the steel wire after annealing and deteriorates cold forgingability, it is preferable that pearlite is not included.

[0164] Bainite

[0165] The metal structure in the 1 / 4D portion of the wire according to the present disclosure is mainly bainite. The area percentage of bainite in the 1 / 4D portion is 96% or more, the preferred area percentage of bainite is 97% or more, and more preferably 98% or more.

[0166] In addition, the bainite in the present disclosure contains a ferrite phase (α) and a cementite phase (Fe3C), similar to pearlite, but pearlite is a structure in which the ferrite phase and the cementite phase are alternately and continuously stacked in layers, and bainite is a structure in which lath (acicular lower structure) is contained within the grain and granular or acicular carbides are dispersed.

[0167] <Martensite>

[0168] In the wire rod according to the present disclosure, if the area ratio of martensite in the 1 / 4D portion exceeds 4%, the strength of the wire rod increases, the cross-sectional shrinkage rate decreases, and wire breakage is likely to occur during drawing. Therefore, it is preferable that the area ratio of martensite in the 1 / 4D portion be 4% or less, more preferably 3% or less, and even more preferably 2% or less. Furthermore, the metal structure of the wire rod according to the present disclosure does not need to contain martensite. That is, the area ratio of martensite may be 0%.

[0169] (Organizational measurement methods)

[0170] The measurement of the metal structure of the wire material according to the present disclosure is performed by the following method.

[0171] The area percentage (area %) of bainite and martensite is determined by the following procedure.

[0172] First, cross-section C of the wire to be measured (hereinafter referred to as the “object”) is mirror-polished, and then the microstructure is revealed by etching with Picral (5% picric acid + 95% ethanol solution).

[0173] Next, when the diameter of the object is D, at a depth position of 1 / 4D from the surface of the object, areas of 80㎛ in the depth direction and 120㎛ in the circumference direction are defined at four locations every 90° in the circumference direction of the object. Then, for these four locations, a field emission scanning electron microscope (SEM) manufactured by Hitachi High Technologies Co., Ltd. is used to take microstructure images at a magnification of 2000x under conditions of 2560×1920 pixels, an acceleration voltage of 15kV, and a secondary electron image. FIG. 1a is an SEM image showing an example of a microstructure etched with picrol at the 1 / 4D portion of the wire. FIG. 1b is a drawing showing martensite marked in the SEM image shown in FIG. 1a.

[0174] In the present disclosure, in the photograph of the microstructure, the region with weak corrosion compared to Nital and Picrel is determined to be ferrite, and the microstructure in which cementite with a (long axis length) / (short axis length) of 5.0 or more is continuously stacked in layers alternating with ferrite and does not contain granular or acicular cementite between layers is defined as pearlite. Bainite is defined as a microstructure in which granular or acicular carbides are dispersed between or within the laths, or in which fragmented cementite is arranged in a thermal pattern, containing laths within the grains.

[0175] In addition, each structure of bainite and / or martensite in the captured tissue photograph is visually marked (see FIG. 1b), and the area of ​​each structure is determined by image analysis (software name: Nireco Small General-Purpose Image Processing Analysis System LUZEX_AP). In addition, this operation is measured and calculated for at least two samples, and the average value is calculated and said average value is set as the area percentage of each structure in the present disclosure.

[0176] In addition, if it is difficult to distinguish between bainite and martensite, the observation location is identified by an indentation, the microstructure is photographed after etching with picrel, then retilled and the microstructure is revealed by etching with Nital (5% nitric acid + 95% ethanol solution). A microstructure photograph of the same location is taken using an SEM at a magnification of 2000x. Areas that are etched with Nital but have weak etching with picrel are determined to be martensite, and the areas of each microstructure are visually marked using the method described above, and the area percentage is calculated by image analysis.

[0177] (Full width of the diffraction peak of the (211) plane of the X-ray diffraction pattern)

[0178] The wire according to the present disclosure includes a central axis C of the wire, and the half-width of the diffraction peak of the (211) plane of the X-ray diffraction pattern measured in the 1 / 4D portion (a portion at a depth of 0.25 times the diameter D of the wire in the direction from the surface of the wire toward the center of the wire) from the surface of a cross section (L cross section) parallel to the central axis C (length direction) is 2.20° or greater, preferably 2.60° or greater. By making the half-width of the high 2.20° or greater in the 1 / 4D portion, the recrystallization rate of ferrite grains during annealing after drawing the wire can be increased. As a result, coarse ferrite grains can be formed, and the tensile strength of the steel wire can be lowered.

[0179] (211) The upper limit of the half-width of the plane diffraction peak is not specifically limited, but if the half-width is too high, it is easy to break the wire during fresh processing, so it is desirable to have it be 3.20° or less.

[0180] (211) Plane diffraction peak full width measurement is performed using an Auto Mate manufactured by Rigaku as an X-ray stress diffraction device, under the conditions of X-ray: Cr-Kα line, tube flux voltage: 40 kV, tube current: 40 mA, and diffraction plane: α (211) plane. For the measurement, the Kα1 and Kα2 lines of the Cr-Kα line were separated by the measurement device, and the Kα1 line was used for the measurement. FIG. 2 schematically illustrates an example of the full width of a diffraction peak in an X-ray spectrum (X-ray diffraction pattern) measured by an X-ray stress diffraction device.

[0181] (Sectional shrinkage rate)

[0182] The wire rod according to the present disclosure has a cross-sectional shrinkage rate of 30.0% or more. Since the cross-sectional shrinkage rate is 30.0% or more, it is possible to draw the wire without breakage during the subsequent drawing process. The cross-sectional shrinkage rate of the wire rod according to the present disclosure is preferably 35.0% or more, and more preferably 40.0% or more.

[0183] The cross-sectional shrinkage rate of the wire is determined by cutting the wire to a length of 340 mm, straightening it into a straight rod, and performing a tensile test by chucking the upper and lower 70 mm sections, with a chuck length (test length) of 200 mm.

[0184] In the above tensile test, the cross-sectional shrinkage rate (%) of the wire is calculated by the following formula.

[0185] Cross-sectional shrinkage rate (%) = (Cross-sectional area before tensile test - Cross-sectional area of ​​fractured portion during tensile test) / Cross-sectional area before tensile test × 100

[0186] (tensile strength)

[0187] The tensile strength of the wire according to the present disclosure is not particularly limited, but it is preferably 1200 MPa or less. Since the wire is prone to breakage during drawing before annealing if the tensile strength of the wire exceeds 1200 MPa, the tensile strength of the wire is preferably 1200 MPa or less, and more preferably 1150 MPa or less.

[0188] If the tensile strength of the wire rod according to the present disclosure is excessively low, the softening after annealing is insufficient, and the cold forgingability deteriorates. Therefore, it is preferable that the tensile strength of the wire rod according to the present disclosure be 900 MPa or higher, and more preferable that it be 950 MPa or higher.

[0189] The tensile strength of the wire rod is a value measured by using a 9A test specimen of JIS Z2241:2011 and performing a tensile test according to the test method of JIS Z2241:2011.

[0190] (Average grain size of ferrite after fresh and annealing)

[0191] In the wire rod according to the present disclosure, it is preferable that the average grain size of the ferrite in the steel wire after drawing and annealing be 20 μm or more. This is because if the average grain size of the ferrite is less than 20 μm, the strength is high and the ductility (processability) is insufficient. In addition, the average grain size of the ferrite in the steel wire can be measured by the following method.

[0192] (Method for measuring the average grain size of ferrite)

[0193] The grain size of the ferrite after drawing and annealing the wire rod according to the present disclosure is measured by the following method.

[0194] -Method for measuring ferrite particles-

[0195] The average particle size of ferrite particles can be measured by Electron Back Scattering Diffraction (EBSD). For the analysis, a Field Emission Scanning Electron Microscope (SEM) manufactured by Hitachi High Technologies Co., Ltd., a CCD camera manufactured by TSL, and OIM Analysis manufactured by TSL were used as the analysis software. Specifically, as shown in FIG. 3, the crystal orientation of bcc-Fe at each measurement point within the region is measured with an acceleration voltage of 20 kV, an irradiation current of 20 nA, and a measurement step of 1.0 μm in each of the regions, each having a diameter of 500 μm, as shown in FIG. 3, centered on a cross section (L cross section) parallel to the central axis C (length direction) and including the central axis C of the steel wire, a depth of 250 μm (the central portion of the surface layer in the depth direction), a 1 / 4 D portion (a portion at a depth of 0.25 times the diameter D of the steel wire in the direction from the surface of the steel wire toward the center of the steel wire), and a 1 / 2 D portion (the central portion of the steel wire). Here, a boundary with an orientation difference of 15 degrees or more is defined as a ferrite grain boundary. In addition, an area of ​​5 pixels or more surrounded by these ferrite grain boundaries is defined as a ferrite particle. The ferrite particle size was calculated using the Johnson-Saltikov method (see *Morphological Mechanics* by Rogakuho Uchida, published July 30, 1972; original work: RT DeHoff, FN Rhiness, p. 189). This was performed on two samples, and the average value of the average particle size measured in a total of six measurement areas was defined as the average particle size of the ferrite particle.

[0196] [Method for manufacturing wire rod]

[0197] An example of a method for manufacturing a wire rod according to the present disclosure is described. Although the method for manufacturing a wire rod according to the present disclosure is not particularly limited, it can be suitably manufactured by a method comprising a heating process, a hot rolling process, a coiling process, a cooling process, a primary holding process (cooling bath), and a secondary holding process (isotropic heat treatment or tempering: isothermal bath).

[0198] Specifically, a steel billet made of a predetermined component satisfying the chemical composition described above is heated to 1050 to 1250°C and maintained for at least 90 minutes, then hot-rolled at a finishing rolling temperature of 750 to 1000°C and wound into a ring shape at 780 to 820°C.

[0199] After winding, it is cooled to 400 to 495℃ at an average cooling rate of 30 to 250℃ / s.

[0200] After that, T1: holding at 400 to 495℃ for 20 to 100 seconds (first holding process) is performed.

[0201] In addition, T2: holding for 35 to 100 seconds at 400 to 495°C (second holding process) is performed. Specifically, T1: holding for 20 to 100 seconds at 400 to 495°C (first holding process) is performed, and T2: holding for 35 to 100 seconds at 495°C (second holding process) is performed, so that the cumulative holding time (t1+t2) of the first holding process and the second holding process is 60 to 200 seconds.

[0202] Afterwards, the wire may be obtained by water cooling. Each process is explained in detail below.

[0203] (Heating process)

[0204] In the heating process, a steel billet having the compositional composition of the wire rod according to the present disclosure is heated to 1050 to 1250°C and maintained for 90 minutes or more. If the heating temperature is less than 1050°C or the holding time is less than 90 minutes, the solid solution of carbides becomes insufficient, the ductility of the wire rod becomes insufficient, and wire breakage is likely to occur. On the other hand, if the heating temperature exceeds 1250°C, the crystal grains become coarse, the workability of the wire rod deteriorates, and wire breakage is likely to occur during the wire drawing process.

[0205] (Hot rolling process)

[0206] In the hot rolling process, a heated steel billet is hot-rolled at a finish rolling temperature of 750 to 1000°C. If the finish rolling temperature is below 750°C, the crystal grain size becomes non-uniform, the ductility of the wire rod decreases, and it becomes prone to breakage during the wire drawing process. If the finish rolling temperature exceeds 1000°C, the crystal grains become coarse, the workability of the wire rod deteriorates, and it becomes prone to breakage during the wire drawing process. Furthermore, the finish rolling temperature refers to the temperature at the entry side of the finish rolling and the surface temperature of the wire rod immediately after rolling. That is, hot rolling should be performed such that both the temperature at the entry side of the finish rolling and the surface temperature of the wire rod immediately after rolling are within the range of 750 to 1000°C.

[0207] (Winding process)

[0208] After hot rolling, the wire rod is wound into a ring shape at a winding temperature of 770 to 820°C. If the winding temperature is below 770°C, the full width at half maximum of the (211) plane diffraction peak of the wire rod decreases, and the ductility after drawing and annealing decreases. If the winding temperature exceeds 820°C, the grain size becomes coarser, and the workability of the wire rod deteriorates. In addition, the winding temperature refers to the surface temperature of the wire rod immediately after being loaded onto a conveyor when winding the wire rod after hot rolling.

[0209] In the cooling process, a wire rod at 770 to 820°C, wound into a ring after hot rolling, is cooled to 400 to 495°C at an average cooling rate of 30 to 250°C / s. If the average cooling rate is less than 30°C / s, a ferrite and pearlite structure is formed, making it impossible to obtain the microstructure of the present disclosure, and the cross-sectional shrinkage rate after annealing decreases. In order to exceed an average cooling rate of 250°C / s, manufacturing costs increase. Furthermore, the average cooling rate refers to the average cooling rate on the surface of the wire rod. For example, a wire rod after hot rolling can be wound into a ring and cooled by air cooling or air cooling to achieve the above average cooling rate.

[0210] (1st maintenance process)

[0211] After that, it is held in a molten salt bath at 400 to 495°C for 20 to 100 seconds. If the holding temperature is below 400°C, the martensite increases, the strength of the wire is high, and the cross-sectional shrinkage rate decreases, making it prone to breakage during the wire drawing process.

[0212] In addition, if the holding temperature exceeds 495℃, the transformation completion time in the secondary holding process becomes significantly longer, and an untransformed portion remains after the primary and secondary holding processes. The untransformed portion causes wire breakage in the fresh processing process.

[0213] (Secondary maintenance process)

[0214] It is held in a molten salt bath at 400 to 495°C for 35 to 100 seconds. If the holding temperature is below 400°C, the martensite increases, the strength of the wire is high, and the cross-sectional shrinkage rate decreases, making it prone to breakage during the wire drawing process.

[0215] In addition, if the holding temperature exceeds 495℃, the transformation completion time in the secondary holding process becomes significantly longer, and an untransformed portion remains after the primary and secondary holding processes. The untransformed portion causes wire breakage in the fresh processing process.

[0216] -Cumulative holding time of the 1st and 2nd holding processes-

[0217] The cumulative holding time (t1+t2) of the first holding time t1 and the second holding time t2 is set to 60 to 200 seconds. If the cumulative holding time is less than 60 seconds, the bainite transformation is incomplete, and an untransformed portion is generated. The untransformed portion causes wire breakage during the fresh processing process.

[0218] If the cumulative holding time at 400 to 495°C exceeds 200 seconds, the bainite transformation is complete, and the cost increases even if heat treatment is performed for a longer period.

[0219] A wire rod according to the present disclosure can be suitably manufactured through the above process. In addition, the wire diameter D of the wire rod according to the present disclosure is not particularly limited, but is preferably 5.5 mm to 21.0 mm from the viewpoint of manufacturability, and more preferably 9.5 to 16.0 mm.

[0220] [Method for manufacturing steel wire]

[0221] When manufacturing steel wire using the wire rod according to the present disclosure, a method including a drawing process and an annealing (spheroidizing annealing) process may be cited.

[0222] (Fresh processing process)

[0223] After the second holding process, the wire rod, cooled to room temperature after tempering as needed, is drawn with a total reduction rate of, for example, 30 to 50%. By performing the drawing process, the spheroidization of carbides is promoted during the annealing process, and the growth of ferrite grains is promoted. If the total reduction rate during the drawing process is less than 20%, these effects are insufficient and cold workability deteriorates. On the other hand, if the total reduction rate during the drawing process exceeds 50%, the effects become saturated, and the wire diameter decreases, limiting its applications.

[0224] In addition, the wire diameter of the steel wire manufactured by drawing the wire material according to the present disclosure is not particularly limited, but is preferably 2.5 to 18.0 mm, and more preferably 4.5 to 16.0 mm.

[0225] (Annealing process)

[0226] After fresh processing, as an annealing process, it is desirable to maintain the temperature at 650°C or higher and less than 750°C for at least 3 hours and then cool it. If the annealing temperature is below 650°C, it takes time for ferrite grain growth, which increases manufacturing costs. On the other hand, if the annealing temperature is 750°C or higher, the ferrite grains become finer and the strength increases.

[0227] In addition, if the holding time in the annealing process is less than 3 hours, the cementite particles become fine and the cold workability deteriorates.

[0228] By using the wire material according to the present disclosure in this way, a steel wire with good workability can be obtained even if the number of spheroidizing annealing cycles is reduced to one after drawing.

[0229] For example, when the wire rod according to the present disclosure is subjected to drawing processing in the range of a reduction rate of 30% to 50%, and then heated at a heating rate of 150°C / hour as an annealing process, and then held at 720°C for 12 hours and slowly cooled, it is possible to obtain an annealed material having a tensile strength of 500±15MPa and a cross-sectional shrinkage rate of 80.0±5.0%.

[0230] [Example]

[0231] Hereinafter, the wire material according to the present disclosure will be described in more detail with reference to examples. However, each of these examples is not intended to limit the wire material according to the present disclosure.

[0232] Steel billets of steel grades A to M having the chemical compositions shown in Table 1 were manufactured. In addition, in Table 1, blank spaces indicate that the component (element) was not intentionally added, and the remainder is Fe and impurities. Also, underlined parts in each table indicate that they are outside the scope of the present disclosure.

[0233]

[0234] Using each manufactured steel billet, wire rods were manufactured by performing heating, hot rolling, coiling, cooling, a first holding process, and a second holding process under the conditions shown in Table 2, and some of the wire rods were tempered. By doing so, wire rods (wire diameter: 10 mm) were manufactured.

[0235]

[0236] For each wire manufactured, the area ratio of the metal structure in the 1 / 4D section and the full width at half maximum of the (211) plane diffraction peak were measured by the method described above. In addition, the tensile strength and cross-sectional shrinkage rate were measured by the tensile test described above. The results are shown in Table 3.

[0237]

[0238] A steel wire was manufactured by performing drawing and two rounds of spheroidizing annealing on a wire rod of test number 24, with a diameter of 10.0 mm, under the conditions shown in Table 4 (conventional conditions).

[0239] Steel wires were manufactured by performing drawing and one spheroidizing annealing on wire rods with diameters of 10.0 mm, test numbers 1, 3 to 23, under the conditions shown in Tables 5 and 6. In addition, the heating rate to the annealing temperature was set to 150°C / hour.

[0240]

[0241]

[0242] For the steel wire manufactured as described above, the average grain size of the ferrite was measured by the method described above.

[0243] In addition, tensile tests were performed on these steel wires to evaluate their mechanical properties. Similar to the tensile test of wire rods, the tensile test was performed using specimen 9A of JIS Z2241:2011 in accordance with the test method of JIS Z2241:2011. Specifically, the test was conducted in an ambient atmosphere at room temperature using a steel wire cut to a length of 390 mm, with a gauge spacing of 100 mm and a crosshead speed of 10 mm / min, and the tensile strength and cross-sectional shrinkage values ​​were measured. The tensile test was performed on three steel wires for each test number, and the average value was used. The evaluation results are shown in Table 6. When the tensile strength was 430 to 560 MPa and the cross-sectional shrinkage was 75.0% or higher, it was judged to have high strength and "good" workability, and when either the tensile strength or the cross-sectional shrinkage was outside the above range, it was judged to be "poor." Table 6 also shows the number of spheroidization annealing cycles, etc.

[0244]

[0245] From the above results, the wire rod satisfying the requirements of the present disclosure had good workability even when the number of spheroidizing annealing cycles was one after drawing. In test numbers 1, 7 to 12, and 16, the wire rod was drawn in the range of a reduction rate of 30% to 50%, and then, as an annealing process, the temperature was raised at a heating rate of 150°C / hour, maintained at 720°C for 12 hours, and then slowly cooled. The tensile strength of the annealed material obtained was 500±15MPa and the cross-sectional shrinkage rate was 80.0±5.0%.

[0246] Test numbers 3 and 6 had a tempering heat treatment temperature after the secondary holding process (isothermal transformation) that was too high, resulting in a small half-width (dislocation density) of the (211) plane diffraction peak and insufficient cross-sectional shrinkage.

[0247] Test No. 17 had a heating temperature that was too high before rolling, resulting in insufficient bainite area ratio and cross-sectional shrinkage, which caused a break in the wire during drawing.

[0248] Test No. 18 had a heating temperature that was too low before rolling, resulting in insufficient bainite area ratio and cross-sectional shrinkage, which caused a break in the wire during drawing.

[0249] Test No. 19 had a finish rolling temperature that was too high, and the cross-sectional shrinkage rate was insufficient, resulting in a breakage during drawing.

[0250] Test No. 20 had a finish rolling temperature that was too low, so the (211) plane diffraction peak half-width was low and the cross-sectional shrinkage rate after annealing was insufficient.

[0251] Test No. 21 had a temperature in the primary holding process that was too low, resulting in insufficient bainite area ratio and cross-sectional shrinkage, which caused a break in the wire during fresh processing.

[0252] Test No. 22 had a low bainite area ratio because the temperature of the secondary holding process was too high, resulting in a low cross-sectional shrinkage rate after freshing and annealing.

[0253] Test No. 23 had an excessively short cumulative time for the first and second holding processes, resulting in insufficient bainite area ratio and insufficient cross-sectional shrinkage after freshing and annealing.

[0254] Test No. 24 is a conventional manufacturing method. Slow cooling was performed after coiling without undergoing a holding process in a molten salt bath. As a result, a two-phase structure of ferrite and pearlite was formed, and spheroidization was insufficient in a short period of time. Therefore, even if annealing was performed twice, the cross-sectional shrinkage rate after spheroidizing annealing is insufficient. Industrial applicability

[0255] The uses of the wire material according to the present disclosure are not limited, but it can be suitably used for machine structural steel used as a material for machine parts such as bolts, screws, and nuts. The wire material according to the present disclosure is extremely useful industrially because it can produce a steel wire with excellent workability through a single annealing process after drawing.

[0256] By using the wire material according to the present disclosure, it is possible to shorten the spheroidizing annealing time and achieve excellent cold forging properties after the spheroidizing treatment.

Claims

Claim 1 In mass%, C: 0.10 to 0.60%, Si: 0.01 to 0.45%, Mn: 0.30 to 1.00%, P: 0.030% or less, S: 0.001 to 0.050%, Al: 0.001 to 0.080%, Cr: 0.85 to 1.50%, N: 0.0010 to 0.0200%, O: 0.004% or less, Mo: 0 to 1.00%, V: 0 to 0.50%, Nb: 0 to 0.050%, Ti: 0 to 0.100%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, Sn: 0 to 0.10%, B: 0 to A wire having a chemical composition of 0.0200%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, remainder: Fe and impurities, wherein, when the diameter of the wire is D, the metal structure in the 1 / 4D portion at a depth of 1 / 4D from the surface of the wire in a cross-section perpendicular to the longitudinal direction of the wire comprises bainite with an area percentage of 96% or more and 100% or less, and the remainder when the area percentage of the bainite is less than 100% is martensite, wherein in the 1 / 4D portion of the cross-section parallel to the longitudinal direction including the central axis of the wire, the half-width of the diffraction peak of the (211) plane, measured by Cr-Kα rays using an X-ray stress diffraction device, is 2.20° or more, and the cross-sectional shrinkage rate is 30.0% or more. Claim 2 The wire rod according to claim 1, comprising one or more types selected from the group consisting of, in mass%, Mo: 0.01 to 1.00%, V: 0.01 to 0.50%, Nb: 0.001 to 0.050%, and Ti: 0.005 to 0.100%. Claim 3 A wire rod according to claim 1 or 2, comprising one or more types selected from the group consisting of, in mass%, Cu: 0.02 to 0.40%, Ni: 0.02 to 0.30%, Sn: 0.01 to 0.10%, and B: 0.0003 to 0.0200%. Claim 4 A wire rod comprising, in any one of claims 1 to 3, one or two types selected from the group consisting of, in mass%, Ca: 0.0001 to 0.0050% and Mg: 0.0001 to 0.0050%. Claim 5 A wire rod having a tensile strength of 1200 MPa or less, in any one of paragraphs 1 to 4. Claim 6 A wire rod according to any one of claims 1 to 5, wherein the wire rod is subjected to drawing processing in the range of a reduction rate of 30% to 50%, then heated at a heating rate of 150℃ / hour as an annealing process, maintained at 720℃ for 12 hours, and then slowly cooled, such that the annealed material has a tensile strength of 500±15MPa and a cross-sectional shrinkage rate of 80.0±5.0%.

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