Steel wire for machine structural parts and method for producing the same

Abstract

To provide a steel wire for machine structural components excellent in cold workability and excellent in hardening, and a manufacturing method of the steel wire for machine structural components.SOLUTION: A steel wire for machine structural components comprises: C, Si, Mn, P, S, Al, Cr and N each contain a predetermined amount, and the balance made of iron and inevitable impurities, and when a total content (mass%) of Cr and Mn in cementite in a metal structure is set to {Cr+Mn}, a total content (mass%) of Cr and Mn in the steel is set to [Cr+Mn], and an amount of C (mass%) in the steel is set to [C], a concentration ratio {Cr+Mn} / [Cr+Mn] is (0.5 [C]+0.040) or more, and an average equivalent circle diameter of all cementite is (1.668-2.13 [C]) μm or more and (1.863- 2.13 [C]) μm.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a steel wire for machine structure parts and a method for manufacturing the same.

Background Art

[0002] When manufacturing various machine structure parts such as parts for automobiles and parts for construction machinery, spheroidizing annealing is usually performed on a bar steel including a hot-rolled wire rod for the purpose of imparting cold workability. Then, cold working is performed on the steel wire obtained by spheroidizing annealing, and thereafter, machining such as cutting is performed to form a predetermined part shape. Further, quenching and tempering are performed to finally adjust the strength, and a machine structure part is manufactured.

[0003] In recent years, in the cold working process, in order to prevent cracking of steel materials and improve the die life, a steel wire that is further softened than before has been desired.

[0004] As a method for obtaining a softened steel wire, for example, Patent Document 1 shows that, as a method for manufacturing a medium carbon steel excellent in cold forging properties, heating in the austenitizing temperature range two or more times is performed in the spheroidizing annealing treatment. According to the manufacturing method of Patent Document 1, it is shown that a steel for cold forging can be obtained in which the hardness after spheroidizing annealing is 83 HRB or less and the ratio of spherical carbides in the structure is 70% or more.

[0005] Patent Document 2 discloses a steel material having characteristics of low deformation resistance after spheroidizing annealing and excellent cold forging properties, and a manufacturing method thereof. As the manufacturing method, after hot working a steel satisfying a predetermined component composition and cooling it to room temperature, then heating it to a temperature range of A1 point to A1 point + 50 °C, holding it for 0 to 1 hour in the temperature range of A1 point to A1 point + 50 °C after heating, and then performing annealing treatment of cooling from the temperature range of A1 point to A1 point + 50 °C to a temperature range of A1 point - 100 °C to A1 point - 30 °C at an average cooling rate of 10 to 200 °C / hr two or more times. Then, when heating to a temperature range of A1 point to A1 point + 30 °C, holding it in the temperature range of A1 point to A1 point + 30 °C and then cooling, when heating, after reaching the A1 point and holding it in the temperature range of A1 point to A1 point + 30 °C and then cooling, the residence time in the temperature range of A1 point to A1 point + 30 °C until reaching the A1 point is set to 10 minutes to 2 hours, and after cooling the cooling temperature range from the temperature range of A1 point to A1 point + 30 °C to A1 point - 100 °C to A1 point - 20 °C at an average cooling rate of 10 to 100 °C / hr, holding it for 10 minutes to 5 hours in the cooling temperature range and then further cooling is shown.

[0006] Patent Document 3 discloses a steel wire for mechanical structural parts having a predetermined component composition, which can reduce the deformation resistance during cold working and improve the crack resistance, and exhibit excellent cold workability. The metal structure of the steel is composed of ferrite and cementite, and the proportion of the number of cementite existing at the ferrite grain boundaries is 40% or more based on the total number of cementite. In Patent Document 3, the manufacturing conditions of the rolled wire rod for spheroidizing annealing are finish rolling at 800 °C or higher and 1050 °C or lower, and perform the first cooling with an average cooling rate of 7 °C / sec or higher, the second cooling with an average cooling rate of 1 °C / sec or higher and 5 °C / sec or lower, and the third cooling with an average cooling rate faster than the second cooling and 5 °C / sec or higher in this order. It is shown that it is preferable to perform the end of the first cooling and the start of the second cooling within the range of 700 to 750 °C, perform the end of the second cooling and the start of the third cooling within the range of 600 to 650 °C, and make the end of the third cooling 400 °C or lower.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, in the conventional technologies disclosed in Patent Documents 1 to 3, the hardness after spheroidizing annealing cannot be sufficiently reduced, and the workability in cold working performed after spheroidizing annealing is inferior, or the hardness cannot be sufficiently increased by the quenching treatment performed after cold working, that is, the hardenability may be inferior. That is, conventionally, there has been no technology focusing on improving both cold workability and hardenability.

[0009] The present invention has been made in view of such circumstances, and its object is to provide a steel wire for mechanical structure parts that has a sufficiently low hardness, excellent cold workability, and can obtain a high hardness by quenching treatment, that is, has excellent hardenability, and a method for manufacturing a steel wire for mechanical structure parts that can manufacture the steel wire for mechanical structure parts in a relatively short time.

[0010] In this specification, "wire rod" and "bar steel" each refer to a linear or bar-shaped steel material obtained by hot rolling, and a steel material that has not been subjected to any heat treatment such as spheroidizing annealing or wire drawing. Further, "steel wire" refers to a wire rod or bar steel that has been subjected to at least one of heat treatment such as spheroidizing annealing and wire drawing. In this specification, the above wire rod, bar steel, and steel wire are collectively referred to as "long steel products".

Means for Solving the Problems

[0011] Aspect 1 of the present invention is C: 0.05% by mass to 0.60% by mass, Si: 0.005% by mass to 0.50% by mass, Mn: 0.30 mass% to 1.20 mass%, P: More than 0 mass%, 0.050 mass% or less, S: More than 0 mass%, 0.050 mass% or less, Al: 0.001 mass% to 0.10 mass%, Cr: More than 0 mass%, 1.5 mass% or less, and N: More than 0 mass%, 0.02 mass% or less containing, the balance being iron and unavoidable impurities, When the total content (mass%) of Cr and Mn in cementite in the metallographic structure is {Cr+Mn}, the total content (mass%) of Cr and Mn in the steel is [Cr+Mn], and the C content (mass%) in the steel is represented by [C], the concentration ratio {Cr+Mn} / [Cr+Mn] is (0.5[C]+0.040) or more, and further, The average equivalent circle diameter of all cementite is (1.668 - 2.13[C]) μm or more and (1.863 - 2.13[C]) μm or less when the C content (mass%) in the steel is represented by [C], and it is a steel wire for mechanical structure parts.

[0012] Aspect 2 of the present invention is Furthermore, Cu: More than 0 mass%, 0.25 mass% or less, Ni: More than 0 mass%, 0.25 mass% or less, Mo: More than 0 mass%, 0.50 mass% or less and B: More than 0 mass%, 0.01 mass% or less, and it is a steel wire for mechanical structure parts according to Aspect 1, containing one or more selected from the group consisting of.

[0013] Aspect 3 of the present invention is Furthermore, Ti: More than 0 mass%, 0.2 mass% or less, Nb: More than 0 mass%, 0.2 mass% or less, and V: More than 0 mass%, 0.5 mass% or less, and it is a steel wire for mechanical structure parts according to Aspect 1 or 2, containing one or more selected from the group consisting of.

[0014] Aspect 4 of the present invention is Furthermore, Mg: More than 0 mass %, 0.02 mass % or less, Ca: More than 0 mass %, 0.05 mass % or less, Li: More than 0 mass %, 0.02 mass % or less, and REM: It contains one or more selected from the group consisting of more than 0 mass %, 0.05 mass % or less, and is the steel wire for mechanical structure parts according to any one of Aspects 1 to 3.

[0015] Aspect 5 of the present invention is the steel wire for mechanical structure parts according to any one of Aspects 1 to 4, wherein the average value of the ferrite crystal grain size is 30 μm or less.

[0016] Aspect 6 of the present invention is The bar steel satisfying the chemical component composition according to any one of Aspects 1 to 4 is It is a method for manufacturing a steel wire for mechanical structure parts according to any one of Aspects 1 to 5, including a step of performing spheroidizing annealing including the following steps (1) to (3). (1) After heating to a temperature T1 of (A1 + 8 °C) to (A1 + 31 °C), it is heated and held at the temperature T1 for more than 1 hour and 6 hours or less. (2) Cooling to a temperature T2 of more than 650 °C and (A1 - 17 °C) or less, and then heating to a temperature T3 of (A1 + 8 °C) to (A1 + 31 °C) at an average heating rate of 75 °C / hour to 160 °C / hour. The cooling-heating process is carried out a total of 2 to 6 times. (3) Cooling from the temperature T3 of the final cooling-heating process. Here, A1 is calculated by the following formula (1). A1 (°C) = 723 + 29.1 × [Si] - 10.7 × [Mn] + 16.9 × [Cr] - 16.9 × [Ni] ··· (1) However, [element] represents the content (mass %) of each element, and the content of an element not contained is taken as zero.

[0017] Aspect 7 of the present invention is The method for manufacturing a steel wire for mechanical structure parts according to Aspect 6, wherein the bar steel is a steel wire obtained by performing wire drawing on a wire rod with a reduction ratio of more than 5%.

Effects of the Invention

[0018] According to the present invention, it is possible to provide a steel wire for mechanical structural parts that is excellent in cold workability and hardenability, and a method for manufacturing the steel wire for mechanical structural parts.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0020] The inventors of the present invention have conducted intensive research to realize a steel wire for mechanical structural parts that is excellent in cold workability and hardenability. As a result, it has been found that the total content of Mn and Cr in cementite should be a certain ratio or more with respect to the total content of Mn and Cr in the steel, and the average size of all cementite should be within a certain range according to the C content in the steel. Furthermore, in order to realize the above metal structure, it is effective to have a metal structure with a chemical composition within a certain range and to perform spheroidizing annealing under specific conditions in the method for manufacturing a steel wire for mechanical structural parts. Hereinafter, first, the steel wire for mechanical structural parts according to the present embodiment will be described in terms of the metal structure of the steel wire for mechanical structural parts.

[0021] 1. Metal Structure Conventionally, the steel material has been subjected to spheroidizing annealing to obtain a metal structure composed of ferrite and cementite, thereby ensuring cold workability. However, in order to obtain both excellent cold workability and hardenability, the above-mentioned metal structure alone cannot be used. For this reason, the present inventors have conducted intensive research from various angles in order to realize a steel wire for machine structural parts that has both excellent cold workability and excellent hardenability. First, the present inventors focused on the Mn and Cr amounts in the cementite. For example, by performing spheroidizing annealing under the manufacturing conditions described below, the average size of all cementite is set to a certain level or more, and the Mn and Cr amounts in the cementite are increased, and it has been found that the Mn and Cr amounts in the ferrite can be relatively reduced, and hardening due to solid solution strengthening can be suppressed, low hardness can be achieved, and cold workability can be improved. In addition, it has been found that by suppressing the average size of all cementite to a certain level or less, it is possible to suppress the indissolution of cementite during high temperature holding in the quenching process, and hardenability can be improved. So far, no study has focused on both the Mn and Cr contents in cementite and the average size of the entire cementite.

[0022] [When the total content (mass%) of Cr and Mn in cementite is {Cr+Mn}, the total content (mass%) of Cr and Mn in steel is [Cr+Mn], and the amount of C (mass%) in steel is [C], the concentration ratio {Cr+Mn} / [Cr+Mn] is (0.5[C]+0.040) or more] Cr and Mn are typical elements that easily dissolve in cementite. However, a part of them dissolves in ferrite, and the greater the amount of solid solution, the more the ferrite matrix is strengthened and the hardness increases. Therefore, the ratio of the total content of Cr and Mn in cementite {Cr+Mn} to the total content of Cr and Mn in steel [Cr+Mn], that is, the concentration ratio {Cr+Mn} / [Cr+Mn], the greater it is, the lower the total content of Cr and Mn in ferrite occupying the phase other than cementite can be. As a result, the solid solution strengthening amount of ferrite by Cr and Mn decreases, and accordingly, the hardness is reduced and the cold workability is improved. The lower limit of the concentration ratio {Cr+Mn} / [Cr+Mn] is set to (0.5[C]+0.040) or more, where [C] is the C content (mass%) in the steel, because the C content in the steel has an impact. The concentration ratio {Cr+Mn} / [Cr+Mn] is preferably (0.5[C]+0.042) or more. On the other hand, considering possible manufacturing conditions and the like, the upper limit of the concentration ratio {Cr+Mn} / [Cr+Mn] is approximately 0.5[C]+0.500.

[0023] Regarding the above cementite, the form is not particularly limited, and in addition to spherical cementite, rod-shaped cementite with a large aspect ratio is included. The above aspect ratio is the ratio (major axis / minor axis) of the major axis, which is the longest length of the cementite particle, to the minor axis, which is the longest length in the direction perpendicular to the major axis. Incidentally, the standard for the size of the cementite to be measured is not limited, but as shown in the examples described later, the size of the cementite at which the total content of Cr and Mn can be measured is the minimum size. Specifically, when measuring the electrolytic extraction residue by the method shown in the examples described later, the cementite remaining on the filter with a pore size of 0.10 μm is the measurement object. Also, the total content of Cr and Mn in the steel is the total of the average Cr content and the average Mn content in the steel, as shown in the examples described later. For example, when the metal structure is formed of ferrite and cementite, it refers to the total content of Cr and Mn in mass% in ferrite and cementite.

[0024] When the average equivalent circle diameter of all cementite is (1.668 - 2.13[C]) μm or more and (1.863 - 2.13[C]) μm or less, where [C] represents the C content (mass%) in the steel. When the amount of cementite in the steel is constant, the larger the size of the cementite, the lower the number density of the cementite and the longer the distance between the cementites. The longer the distance between the cementites in the steel, the more difficult it is to achieve precipitation strengthening, and as a result, the hardness can be reduced. Also, by making the size of the cementite equal to or larger than a certain value, the hardness reduction effect due to increasing the total content of Cr and Mn in the cementite can be easily exerted. From these viewpoints, in the present invention, the average equivalent circle diameter of all cementite is set to be (1.668 - 2.13[C]) μm or more when the C content (mass%) in the steel is represented by [C]. The average equivalent circle diameter of all cementite is preferably (1.669 - 2.13[C]) μm or more. On the other hand, if the cementite becomes too coarse, during the high-temperature holding in the quenching process after cold working, the cementite cannot be sufficiently dissolved, and a sufficiently high hardness cannot be obtained by quenching. Therefore, in the present invention, the average equivalent circle diameter of all cementite is set to be (1.863 - 2.13[C]) μm or less. Preferably, it is (1.858 - 2.13[C]) μm or less.

[0025] Patent Document 3 shows that cementite existing at the ferrite grain boundaries has a smaller amount of strain during cold working compared to cementite existing within the ferrite grains, and thus reduces the deformation resistance. However, in Patent Document 3, the average size of all cementite is not controlled, and as a result, the cementite cannot be sufficiently dissolved during the high-temperature holding in the quenching process, resulting in poor hardenability. The present invention is a technology that focuses on both the ratio of the total content of Cr and Mn in the cementite and the average size of all cementite in order to realize a steel wire for mechanical structural parts having excellent cold workability and excellent hardenability.

[0026] The metallographic structure of the steel wire for mechanical structural parts according to the present embodiment is a spheroidized structure having spheroidized cementite, and can be obtained, for example, by subjecting a bar steel satisfying the chemical composition described below to spheroidizing annealing described below.

[0027] The metallographic structure of the steel wire for machine structure parts of the present invention is substantially composed of ferrite and cementite. The above "substantially" means that the ferrite in the metallographic structure of the steel wire for machine structure parts of the present invention has an area ratio of 90% or more, and the rod-shaped cementite with an aspect ratio of 3 or more has an area ratio of 5% or less. If the adverse effect on cold workability is small, nitrides such as AlN and inclusions other than nitrides are allowed to have an area ratio of less than 3%. The area ratio of the ferrite may further be 95% or more.

[0028] In this specification, "ferrite" refers to a portion having a bcc crystal structure, and also includes ferrite in pearlite which is a lamellar structure of ferrite and cementite. In addition, the "ferrite crystal grains" which are the measurement object of the "ferrite crystal grain size" include crystal grains containing rod-shaped cementite generated during spheroidizing annealing with insufficient spheroidization, but crystal grains (pearlite crystal grains) containing rod-shaped cementite that can remain before spheroidizing annealing are excluded. Specifically, after etching using nital (2% by volume of nitric acid, 98% by volume of ethanol), it refers to "crystal grains in which cementite does not exist inside the grains" and "crystal grains in which cementite exists inside the grains and the shape of the cementite can be observed (that is, the boundary between cementite and ferrite can be clearly observed)" that can be confirmed when observed at 1000 times magnification using an optical microscope. Crystal grains in which the shape of cementite cannot be observed (that is, the boundary between cementite and ferrite cannot be clearly observed) at 1000 times magnification using the above optical microscope are not the object of judgment in this embodiment and are not included in the "ferrite crystal grains".

[0029] [Average value of ferrite crystal grain size: 30 μm or less] The steel wire for mechanical structure parts according to this embodiment preferably has an average value of the ferrite crystal grain size in the metal structure of 30 μm or less. If the average value of the ferrite crystal grain size is 30 μm or less, the ductility of the steel wire for mechanical structure parts can be improved, and the occurrence of cracks during cold working can be further suppressed. The average value of the ferrite crystal grain size is more preferably 25 μm or less, and still more preferably 20 μm or less. The smaller the average value of the ferrite crystal grain size, the more preferable it is. However, considering possible manufacturing conditions and the like, the lower limit can be approximately 2 μm.

[0030] (Characteristic) The steel wire for mechanical structure parts according to this embodiment that satisfies the following chemical composition and has the above-described metal structure can achieve both a low hardness that enables good cold working and a high hardness after quenching treatment. In this embodiment, when the C content (mass %), Cr content (mass %), and Mo content (mass %) in the steel are represented by [C], [Cr], and [Mo], respectively (elements not included are considered to have a zero mass %), when the hardness, specifically the hardness after spheroidizing annealing in the examples described below, satisfies the following formula (2), and the hardness after quenching treatment satisfies the following formula (3), it is determined that the hardness is sufficiently low and the cold workability is excellent, and at the same time, a high hardness after quenching treatment is achieved, that is, the hardenability is excellent. Hardness (HV) after spheroidizing annealing < 91([C] + [Cr] / 9 + [Mo] / 2) + 91 ···(2) Hardness (HV) after quenching treatment > 380ln([C]) + 1010 ···(3)

[0031] 2. Chemical composition The chemical composition of the steel wire for mechanical structure parts according to this embodiment will be described.

[0032] [C: 0.05 mass% to 0.60 mass%] C is an element that controls the strength of steel, and the higher the content, the higher the strength after quenching and tempering. To effectively exert the above effects, the lower limit of the C content was set at 0.05% by mass. The C content is preferably 0.10% by mass or more, more preferably 0.15% by mass or more, and still more preferably 0.20% by mass or more. However, if the C content is excessive, the number of spherical cementite in the structure after spheroidizing annealing becomes excessive, the hardness increases, and the cold workability deteriorates. Therefore, the upper limit of the C content was set at 0.60% by mass. The C content is preferably 0.55% by mass or less, more preferably 0.50% by mass or less.

[0033] [Si: 0.005% by mass to 0.50% by mass] Si is used as a deoxidizer during melting and also contributes to the improvement of strength. To effectively exert the effect, the lower limit of the Si content was set at 0.005% by mass. The Si content is preferably 0.010% by mass or more, more preferably 0.050% by mass or more. However, Si contributes to the solid solution strengthening of ferrite and has the effect of significantly increasing the strength after spheroidizing annealing. If the Si content is excessive, the cold workability deteriorates due to the above effect. Therefore, the upper limit of the Si content was set at 0.50% by mass. The Si content is preferably 0.40% by mass or less, more preferably 0.35% by mass or less.

[0034] [Mn: 0.30% by mass to 1.20% by mass] Mn effectively acts as a deoxidizer and is an element that contributes to the improvement of hardenability. To fully exert the effect, the lower limit of the Mn content was set at 0.30% by mass. The Mn content is preferably 0.35% by mass or more, more preferably 0.40% by mass or more. However, if the Mn content is excessive, segregation is likely to occur and the toughness decreases. Therefore, the upper limit of the Mn content was set at 1.20% by mass. The Mn content is preferably 1.10% by mass or less, more preferably 1.00% by mass or less.

[0035] [P: More than 0% by mass and 0.050% by mass or less] P (phosphorus) is an inevitable impurity and a harmful element that causes grain boundary segregation in steel, adversely affecting forgeability and toughness. Therefore, the P content was set at 0.050 mass% or less. The P content is preferably 0.030 mass% or less, more preferably 0.020 mass% or less. The lower the P content, the more preferable, but it is usually contained at 0.001 mass% or more.

[0036] [S: More than 0 mass%, 0.050 mass% or less] S (sulfur) is an inevitable impurity and a harmful element for cold workability because it forms MnS in steel and deteriorates ductility. Therefore, the S content was set at 0.050 mass% or less. The S content is preferably 0.030 mass% or less, more preferably 0.020 mass% or less. The lower the S content, the more preferable, but it is usually contained at 0.001 mass% or more.

[0037] [Al: 0.001 mass% - 0.10 mass%] Al is an element contained as a deoxidizer and has the effect of reducing impurities during deoxidation. To exert this effect, the lower limit of the Al content was set at 0.001 mass%. The Al content is preferably 0.005 mass% or more, more preferably 0.010 mass% or more. However, if the Al content is excessive, non-metallic inclusions increase and toughness decreases. Therefore, the upper limit of the Al content was set at 0.10 mass%. The Al content is preferably 0.08 mass% or less, more preferably 0.05 mass% or less.

[0038] [Cr: More than 0 mass%, 1.5 mass% or less] Cr is an element that has the effect of improving the hardenability of steel and increasing its strength, and also has the effect of promoting the spheroidization of cementite. Specifically, Cr dissolves in cementite and delays the dissolution of cementite during the heating of spheroidizing annealing. By having a part of the cementite remaining without dissolving during heating, it becomes difficult to form rod-shaped cementite with a large aspect ratio during cooling, and a spheroidized structure is more easily obtained. Therefore, the amount of Cr is more than 0 mass%, preferably 0.01 mass% or more. It may be further 0.05 mass% or more, and even more preferably 0.10 mass% or more. From the viewpoint of further promoting the spheroidization of cementite, it can be more than 0.30 mass%, and can also be more than 0.50 mass%. If the amount of Cr is excessive, the diffusion of elements containing carbon is delayed, and the dissolution of cementite is delayed more than necessary, making it difficult to obtain a spheroidized structure. As a result, the effect of reducing hardness according to the present invention may be reduced. Therefore, the amount of Cr is 1.50 mass% or less, preferably 1.40 mass% or less, and more preferably 1.25 mass% or less. From the viewpoint of accelerating the diffusion of elements more, the amount of Cr can be further 1.00 mass% or less, further 0.80 mass% or less, and further 0.30 mass% or less.

[0039] [N: More than 0 mass%, 0.02 mass% or less], N is an impurity inevitably contained in steel. If a large amount of N is dissolved in the steel, it causes an increase in hardness and a decrease in ductility due to strain aging, and the cold workability deteriorates. Therefore, the amount of N is 0.02 mass% or less, preferably 0.015 mass% or less, and more preferably 0.010 mass% or less.

[0040] [Remainder] The balance consists of iron and inevitable impurities. As inevitable impurities, the inclusion of trace elements (such as As, Sb, Sn, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed. For example, elements such as P and S, usually, the lower the content, the more preferable. Therefore, although they are inevitable impurities, there are elements whose composition ranges are separately defined as above. For this reason, in this specification, when referring to "inevitable impurities" constituting the balance, it is a concept excluding elements whose composition ranges are separately defined.

[0041] The steel wire for mechanical structure parts according to this embodiment only needs to contain the above elements in its chemical composition. The selected elements described below may not be contained, but by containing them as needed together with the above elements, it is possible to more easily achieve ensuring hardenability and the like. The selected elements will be described below.

[0042] [One or more selected from the group consisting of: Cu: more than 0 mass%, 0.25 mass% or less, Ni: more than 0 mass%, 0.25 mass% or less, Mo: more than 0 mass%, 0.50 mass% or less, and B: more than 0 mass%, 0.01 mass% or less] Cu, Ni, Mo, and B are all elements effective in increasing the strength of the final product by improving the hardenability of the steel material, and are contained alone or in two or more kinds as needed. The effects of these elements increase as their contents increase. The preferable lower limit for effectively exerting the above effects is more than 0 mass% for each of Cu, Ni, and Mo, more preferably 0.02 mass% or more, and still more preferably 0.05 mass% or more, and for B it is more than 0 mass%, more preferably 0.0003 mass% or more, and still more preferably 0.0005 mass% or more.

[0043] On the other hand, if the content of these elements becomes excessive, the strength may become too high and the cold workability may deteriorate. Therefore, the preferable upper limits of each were determined as described above. More preferably, the content of each of Cu and Ni is 0.22% by mass or less, still more preferably 0.20% by mass or less. The content of Mo is more preferably 0.40% by mass or less, still more preferably 0.35% by mass or less. The B content is more preferably 0.007% by mass or less, still more preferably 0.005% by mass or less.

[0044] [One or more selected from the group consisting of Ti: more than 0% by mass and 0.2% by mass or less, Nb: more than 0% by mass and 0.2% by mass or less, and V: more than 0% by mass and 0.5% by mass or less] Ti, Nb, and V can be contained alone or in combination of two or more as needed because they form compounds with N and reduce the solid-solution N, thereby exerting the effect of reducing the deformation resistance. The effect of these elements increases as their content increases. For any of these elements, the preferable lower limit for effectively exerting the above effect is more than 0% by mass, more preferably 0.03% by mass or more, still more preferably 0.05% by mass or more. However, if the content of these elements becomes excessive, the formed compounds may cause an increase in the deformation resistance, and conversely, the cold workability may decrease. Therefore, the content of each of Ti and Nb is preferably 0.2% by mass or less, and the content of V is preferably 0.5% by mass or less. The content of each of Ti and Nb is more preferably 0.18% by mass or less, still more preferably 0.15% by mass or less, and the V content is more preferably 0.45% by mass or less, still more preferably 0.40% by mass or less.

[0045] [One or more selected from the group consisting of Mg: more than 0% by mass and 0.02% by mass or less, Ca: more than 0% by mass and 0.05% by mass or less, Li: more than 0% by mass and 0.02% by mass or less, and rare earth metal (REM): more than 0% by mass and 0.05% by mass or less] Mg, Ca, Li, and REM are elements effective in spheroidizing sulfide-based inclusions such as MnS and improving the deformability of steel. Such effects increase as their contents increase. To effectively exhibit the above effects, the contents of Mg, Ca, Li, and REM are preferably each more than 0% by mass, more preferably 0.0001% by mass or more, and still more preferably 0.0005% by mass or more. However, even if they are contained in excess, the effects saturate and effects commensurate with the contents cannot be expected. Therefore, the contents of Mg and Li are preferably each 0.02% by mass or less, more preferably 0.018% by mass or less, and still more preferably 0.015% by mass or less, and the contents of Ca and REM are preferably each 0.05% by mass or less, more preferably 0.045% by mass or less, and still more preferably 0.040% by mass or less. Note that Mg, Ca, Li, and REM may each be contained alone or in combination of two or more, and the contents in the case of containing two or more may be any contents within the above ranges. The REM means including lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium).

[0046] The shape etc. of the steel wire for mechanical structure parts according to the present embodiment is not particularly limited. For example, those having a diameter of 5.5 mm to 60 mm can be mentioned.

[0047] 3. Manufacturing method In order to obtain the metallographic structure of the steel wire for mechanical structure parts according to the present invention embodiment, when manufacturing the steel wire for mechanical structure parts, it is preferable to appropriately control the spheroidizing annealing conditions as described below. The hot rolling process for manufacturing the wire rod or bar to be subjected to spheroidizing annealing is not particularly limited, and may follow a conventional method. As described later, wire drawing may be applied before spheroidizing annealing. The diameter of the wire rod, steel wire, or bar which is the bar steel to be subjected to spheroidizing annealing is not particularly limited. In the case of wire rod and steel wire, for example, it is 5.5 mm to 55 mm, and in the case of bar steel, for example, it is 18 mm to 105 mm.

[0048] While referring to FIG. 1, the spheroidizing annealing conditions in the method for manufacturing a steel wire for mechanical structure parts according to an embodiment of the present invention will be described. FIG. 1 shows an example of a diagram for explaining the spheroidizing annealing conditions in the manufacturing method according to the embodiment of the present invention, and the number of repetitions of the cooling-heating process and the like are not limited to FIG. 1.

[0049] The method for manufacturing a steel wire for mechanical structure parts according to an embodiment of the present invention includes a spheroidizing annealing process including the following steps (1) to (3). (1) After heating to a temperature T1 of (A1 + 8°C) to (A1 + 31°C), heat and hold at the temperature T1 for more than 1 hour and 6 hours or less. (2) Cool to a temperature T2 above 650°C and below (A1 - 17°C), and then perform a total of 2 to 6 cooling-heating processes of heating to a temperature T3 of (A1 + 8°C) to (A1 + 31°C) at an average heating rate of 75°C / hour to 160°C / hour. (3) Cool from the temperature T3 of the final cooling-heating process. Here, A1 is calculated by the following formula (1). A1 (°C) = 723 + 29.1×[Si] - 10.7×[Mn] + 16.9×[Cr] - 16.9×[Ni] ··· (1) However, [element] represents the content (% by mass) of each element, and the content of an element not contained is set to zero.

[0050] [After heating to a temperature T1 of (A1 + 8°C) to (A1 + 31°C), heat and hold at the temperature T1 for more than 1 hour and 6 hours or less ([2] in FIG. 1)] By heating to a temperature T1 of (A1 + 8°C) to (A1 + 31°C), the dissolution of rod-shaped cementite with a large aspect ratio generated in the rolling stage is promoted. If the temperature T1 is too low, the rod-shaped cementite will not dissolve during the heat retention, and will remain in the ferrite crystal grains, increasing the hardness. To obtain a sufficiently softened steel wire, the temperature T1 needs to be (A1 + 8°C) or higher. The temperature T1 is preferably (A1 + 15°C) or higher, and more preferably (A1 + 20°C) or higher. On the other hand, if the temperature T1 is too high, the crystal grains will become too coarse, and it will be difficult for spherical cementite to precipitate at the ferrite crystal grain boundaries during the cooling process of the next step, the rod-shaped cementite will increase, and the hardness will increase. Therefore, the temperature T1 is set to (A1 + 31°C) or lower. The temperature T1 is preferably (A1 + 30°C) or lower, and more preferably (A1 + 29°C) or lower.

[0051] Also, if the heat retention time (t1) at the temperature T1 is too short, the rod-shaped cementite will remain in the ferrite crystal grains, increasing the hardness. To obtain a sufficiently softened steel wire, the heat retention time (t1) needs to be more than 1 hour and 6 hours or less. The preferred heat retention time (t1) is 1.5 hours or more, and more preferably 2.0 hours or more. If the heat retention time (t1) is too long, the heat treatment time will be long and the productivity will decrease. Therefore, the heat retention time (t1) is 6 hours or less, preferably 5 hours or less, and more preferably 4 hours or less. Note that since the average heating rate during heating to the temperature T1 of (A1 + 8°C) to (A1 + 31°C) ([1] in Fig. 1) does not affect the steel material properties, the temperature can be raised at any rate. For example, the temperature can be raised at 30°C / hour to 100°C / hour.

[0052] Note that the temperature of the above A1 point is calculated by the following formula (1) described on page 273 of Leslie's Steel Materials Science (Maruzen). A1(°C) = 723 + 29.1×[Si] - 10.7×[Mn] + 16.9×[Cr] - 16.9×[Ni] ··· (1) However, [element] represents the content (mass%) of each element, and the content of elements not contained is set to zero.

[0053] [(2) Cool to a temperature T2 above 650 °C and below (A1 - 17 °C), and then heat to a temperature T3 of (A1 + 8 °C) to (A1 + 31 °C) at an average heating rate of 75 °C / hour to 160 °C / hour. Perform the cooling-heating process a total of 2 to 6 times ([3] to [7] in Fig. 1)]

[0054] (2-i) Cool to a temperature T2 above 650 °C and below (A1 - 17 °C) (Fig. 1's [3]) After the heat retention in (1) above, in order to promote the precipitation of cementite with high concentrations of Mn and Cr, cool to a temperature T2 above 650 °C and below (A1 - 17 °C). If the temperature T2 is too low, it will lead to a longer annealing time. Also, if the temperature T2 is too low, the cementite will be overly refined, and it will be easier to generate cementite with low contents of Cr and Mn. Therefore, the cooling temperature T2 needs to be above 650 °C. According to the manufacturing method according to this embodiment, even if the cooling temperature T2 is above 650 °C, the desired cementite can be obtained without performing long-time annealing. The temperature T2 is preferably 670 °C or higher. On the other hand, if the temperature T2 is too high, the cementite will not precipitate sufficiently. As a result, Cr and Mn will not be sufficiently concentrated in the cementite, the total content of Cr and Mn in the cementite will be low, the hardness will increase, and the cold workability will decrease. Therefore, the upper limit of the temperature T2 is set to A1 - 17 °C. The temperature T2 is preferably A1 - 18 °C or lower. Also, if it is held after reaching the temperature T2, it will cause a longer heat treatment time. Therefore, it is better not to hold it from these viewpoints. However, in order to make the temperature variation in the furnace uniform, it may be held for a short time. The holding time (t2) at the cooling temperature T2 is preferably within 1 hour.

[0055] Incidentally, the average cooling rate during cooling in the cooling-heating process (step [3] in FIG. 1) is not particularly limited. From the viewpoint of further promoting the penetration of Mn and Cr in the parent phase into cementite, it is preferable that the average cooling rate during cooling from temperature T1 or temperature T3 to temperature T2 is 100 ° C / hour or less. Further, from the viewpoints of further suppressing excessive coarsening of the cementite generated in the step (2) to enhance hardenability and further enhancing productivity, it is preferable that the average cooling rate is 5 ° C / hour or more.

[0056] (2-ii) Heating to a temperature T3 of (A1 + 8 ° C) to (A1 + 31 ° C) at an average temperature increase rate of 75 ° C / hour to 160 ° C / hour (steps [5] and [6] in FIG. 1) In order to increase the contents of Cr and Mn in the cementite precipitated by the cooling in the above (2-i), heating is performed from temperature T2 to a temperature T3 of (A1 + 8 ° C) to (A1 + 31 ° C) at an average temperature increase rate R of 75 ° C / hour to 160 ° C / hour. If the average temperature increase rate R is too fast, the diffusion of Cr and Mn becomes insufficient, and the contents of Cr and Mn in the cementite generated by the above heat retention are insufficient, the hardness increases, and the cold workability deteriorates. Therefore, the average temperature increase rate R is 160 ° C / hour or less. The average temperature increase rate R is preferably 155 ° C / hour or less, more preferably 150 ° C / hour or less. Even more preferably, it is 120 ° C / hour or less, and particularly preferably 100 ° C / hour or less. On the other hand, if the average temperature increase rate R is too slow, the cementite dissolves more than necessary, and as a result, the total content of Cr and Mn in the cementite decreases. Further, if the average temperature increase rate R is too slow, the cementite generated during cooling from temperature T1 coarsens excessively, and as a result, the cementite is not sufficiently dissolved during the high-temperature retention in the quenching treatment step, and the hardness after the quenching treatment decreases, that is, the hardenability deteriorates. Further, it leads to an increase in the annealing time and a decrease in productivity. Therefore, the average temperature increase rate R is 75 ° C / hour or more, preferably 80 ° C / hour or more.

[0057] In addition, in the cooling-heating process, if the temperature T3, which is the temperature reached during heating, is too low, the diffusion of Cr and Mn becomes insufficient, and the contents of Cr and Mn in the cementite formed in the above heating and holding process are insufficient, resulting in an increase in hardness and a decrease in cold workability. Therefore, the temperature T3 needs to be (A1 + 8°C) or higher. The temperature T3 is preferably (A1 + 15°C) or higher, and more preferably (A1 + 20°C) or higher. On the other hand, if the temperature T3, which is the temperature reached during heating, is too high, the cementite dissolves more than necessary, and as a result, the total content of Cr and Mn in the cementite decreases. Therefore, the temperature reached during heating (T3) is set to (A1 + 31°C) or lower. The temperature T3 is preferably (A1 + 30°C) or lower, and more preferably (A1 + 29°C) or lower.

[0058] After reaching the temperature T3, which is the temperature reached during heating, it may be held at that temperature T3. However, if the holding time (t3) at that temperature T3 is too long, the spherical cementite formed in the heating and holding process at temperature T1 is likely to redissolve, and the hardness may increase. Also, if the holding time (t3) at the above temperature T3 is too long, it may lead to an increase in the annealing time and a decrease in productivity. Therefore, the holding time (t3) at the above temperature T3 is preferably within 1 hour.

[0059] In the manufacturing method according to this embodiment, as will be described later, the cooling-heating process of the cooling in (2-i) and the heating in (2-ii) is repeated a plurality of times. However, in each cycle, the temperature T2, which is the temperature reached during cooling, the average heating rate R, and the temperature T3 need to satisfy the above ranges.

[0060] Note that the magnitude relationship between the above temperature T3 and the above temperature T1 is not particularly limited. For example, the above temperature T3 may be set to the same temperature as the above temperature T1, or the above temperature T3 may be set higher than the above temperature T1. Or, from the perspective of sufficiently dissolving the rod-shaped cementite in austenite, the above temperature T1 may be set higher than the above temperature T3.

[0061] (2-iii) Perform the cooling-heating process a total of 2 to 6 times ([7] in FIG. 1) While increasing the concentrations of Mn and Cr in the cementite and promoting the coarsening of the cementite, it is necessary to perform the above cooling-heating process a total of 2 to 6 times. If the cooling-heating process is not repeated, the concentrations of Mn and Cr in the cementite will be insufficient, or the coarsening of the cementite will be insufficient. As a result, the hardness after spheroidizing annealing increases. Therefore, the cooling-heating process is performed 2 or more times. Preferably, it is 3 or more times. The hardness decreases as the number of times of implementation increases, but the effect saturates even if the number of times of implementation is too large. Also, it leads to an increase in the annealing time and a decrease in productivity. Therefore, the number of times of performing the cooling-heating process is set to 6 or less. In the case of FIG. 1, the number of times of performing the cooling-heating process is 4 times. The temperature T2 which is the temperature reached during each cooling, the average heating rate R, and the temperature T3 which is the temperature reached during heating may be different within the respectively specified ranges.

[0062] [(3) Cooling from the temperature T3 of the final time of the cooling-heating process ( [8] in FIG. 1)] Cooling is performed from the temperature T3 of the final time of the cooling-heating process. The average cooling rate and the temperature reached during cooling are not particularly limited. From the viewpoint of further suppressing the reprecipitation of rod-shaped cementite, the average cooling rate may be, for example, 100 ° C / hour or less. Also, from the viewpoint of further suppressing excessive coarsening of the cementite, the average cooling rate may be 5 ° C / hour or more. Also, the temperature reached during cooling can be, for example, (A1 - 30 ° C) or less. For example, cooling at the above average cooling rate to a temperature range of (A1 - 30 ° C) or less and (A1 - 100 ° C) or more, and then air-cooling can be mentioned. Alternatively, by setting the temperature reached during cooling to, for example, less than (A1 - 100 ° C), the reprecipitation of rod-shaped cementite can be further suppressed and the cold workability can be further improved. In this case, from the viewpoint of shortening the annealing time, the temperature reached during cooling may be (A1 - 250 ° C) or more, further (A1 - 200 ° C) or more, and further (A1 - 150 ° C) or more.

[0063] The spheroidizing annealing (steps (1) to (3)) as described above may be repeated once or a plurality of times. From the viewpoints of suppressing excessive coarsening of cementite and ensuring productivity, for example, it is preferably 4 times or less, more preferably 3 times or less. When the spheroidizing annealing is repeated a plurality of times, within the above-specified range, it may be repeated under the same conditions or under different conditions. Further, when the spheroidizing annealing is repeated a plurality of times, wire drawing may be applied between the spheroidizing annealings. For example, it can be carried out in the order of wire drawing before the spheroidizing annealing to be described later → the first spheroidizing annealing → wire drawing → the second spheroidizing annealing.

[0064] In the method for manufacturing a steel wire for mechanical structural parts according to the present embodiment, the steps other than the spheroidizing annealing step are not particularly limited. For example, after the spheroidizing annealing, a wire drawing step with a reduction ratio preferably of 15% or less may be included for the purpose of adjusting the dimensions. By setting the reduction ratio to 15% or less, an increase in hardness before cold working can be suppressed. The reduction ratio is more preferably 10% or less, still more preferably 8% or less, and even more preferably 5% or less.

[0065] In order to promote the formation of the tissue form of the present invention, it is preferable to provide a step of performing wire drawing on the wire rod at a reduction ratio of more than 5% before the spheroidizing annealing. By performing wire drawing at the above reduction ratio, cementite in the steel is broken, and aggregation of cementite can be promoted in the subsequent spheroidizing annealing, so that cementite can be coarsened appropriately, which is effective for softening. Further, by performing wire drawing at the above reduction ratio, the movement of the interface and the diffusion of elements become active, and the contents of Cr and Mn in cementite increase. The reduction ratio is more preferably 10% or more, still more preferably 15% or more, and even more preferably 20% or more. On the other hand, if the reduction ratio is made excessively large, there is a possibility of causing a risk of wire breakage. Therefore, the reduction ratio is preferably 50% or less. When wire drawing is performed a plurality of times, the number of wire drawing operations is not particularly limited, and for example, it can be 2 times. In addition, when a plurality of wire drawing operations are performed, the "reduction ratio during wire drawing" means the reduction ratio from the steel material before wire drawing to the steel material after a plurality of wire drawing operations are performed.

Examples

[0066] Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and it is of course possible to appropriately modify and implement it within the scope that conforms to the above-mentioned and following gists, and all of them are included in the technical scope of the present invention.

[0067] After melting a test material with the chemical composition shown in Table 1 in a converter and then casting it, hot rolling was performed on the obtained steel slab to produce wire rods with a diameter of 12 to 16 mm. In Table 2 described later, in the case of "yes" for wire drawing before spheroidizing annealing, that is, for Sample No. 2 in Table 3 manufactured under Manufacturing Condition B, the steel wire obtained by performing wire drawing on the above wire rod with a reduction ratio of 25% was subjected to spheroidizing annealing.

[0068] Using the above wire rod or steel wire, annealing was carried out using a laboratory furnace. In the annealing, the wire rod or steel wire was heated to T1 shown in Table 2 and held for t1 hours. Then, it was cooled to the temperature T2 in Table 2 at an average cooling rate of 5 to 100 °C / hour, and then heated to the temperature T3 at the average heating rate R shown in Table 2. This cooling and heating process was carried out for the number of cooling-heating implementation times shown in Table 2. And it was cooled from the heating temperature in the final cycle of the cooling-heating process to obtain a sample.

[0069] As a comparative example, in Sample No. 12 shown in Table 3, as Manufacturing Condition H1, the heat treatment process shown in Fig. 2, that is, the cooling-heating process with 0 cycles of heat treatment was carried out. Incidentally, under this Manufacturing Condition H1, wire drawing was not performed with a reduction ratio of 25% before annealing. Also, in Sample No. 13 shown in Table 3, as Manufacturing Condition H2, using the steel wire obtained by performing wire drawing with a reduction ratio of 25% before annealing, the heat treatment process shown in Fig. 2, that is, the cooling-heating process with 0 cycles of heat treatment was carried out.

[0070] Furthermore, as comparative examples, in Sample No. 14 shown in Table 3, as manufacturing condition I, heat treatment conditions satisfying the manufacturing conditions of Patent Document 3 were used. Specifically, the conditions shown as SA2 in the examples of Patent Document 3 were implemented, that is, the heat treatment process shown in FIG. 3 was repeated 5 times. In Sample No. 18 shown in Table 3, as manufacturing condition M, heat treatment conditions satisfying the manufacturing conditions of Patent Document 1 were used. Specifically, the fifth spheroidizing annealing condition in No. 1 of Table 2 of Patent Document 1 was implemented, that is, the heat treatment process shown in FIG. 4 was repeated 3 times. Also, in Sample No. 19 shown in Table 3, as manufacturing condition N, heat treatment conditions satisfying the manufacturing conditions of Patent Document 2 were used. Specifically, condition c in Table 2 of Patent Document 2 was used, that is, heat treatment of the pattern shown in FIG. 5 was performed.

[0071] T1, T2, and T3, which are the annealing parameters described in Table 2, are the set temperatures of the heat treatment furnace. When a thermocouple was attached to the steel material and the deviation between the actual temperature of the steel material and the set temperature was tested, it was confirmed that the temperature of the steel material and the set temperature were about the same.

[0072] Using the samples obtained by the above annealing, as an evaluation of the metallographic structure, the average value of the ferrite crystal grain size, the average size of all cementite, and the total content of Cr and Mn in the cementite were determined as follows. Also, as characteristics, the hardness after spheroidizing annealing and the hardness after quenching treatment were measured and evaluated by the following methods.

[0073] 〔Evaluation of Metallographic Structure〕 [Average Value of Ferrite Crystal Grain Size] First, the measurement of the ferrite crystal grain size was carried out as follows. The test piece was resin-embedded so that the cross-section of the steel wire after spheroidizing annealing, that is, the D / 4 position (D: diameter of the steel wire) of the cross-section perpendicular to the axial direction of the steel wire, could be observed. Using nital (2% by volume of nitric acid, 98% by volume of ethanol) as the etching solution, the above test piece was etched to reveal the structure. Then, the structure of the test piece with the revealed structure was observed under an optical microscope at a magnification of 400 times, and one field of view where ferrite crystal grains with an average size representative of the entire structure of the steel wire could be observed within the evaluation plane was selected, and a micrograph was obtained. Next, the value of the ferrite crystal grain size (G) was calculated from the taken micrograph based on the comparison method of JIS G0551 (2020). Then, using the calculated value of the ferrite crystal grain size (G), in the relationship between various quantities regarding crystal grain size and particle diameter described in Table 1 on page 32 of "Introduction Course - Technical Terms - Steel Materials Edition - 3 Crystal Grain Number and Crystal Grain Diameter", Minoru Umemoto, Ferramu Vol.2 (1997) No.10, pp29 - 34, the average value dn of the ferrite crystal grain diameter was obtained from the following formula (4), which shows the relationship between the ferrite crystal grain size G (or N) and the average value dn of the ferrite crystal grain diameter. The results are shown in Table 3. In this example, for all of Sample Nos. 1 to 11 in Table 3, the area ratio of ferrite was 90% or more. dn = 0.254 / (2 (G-1) / 2 ) ···(4)

[0074] [Average size of all cementite] The measurement of the average size of all cementite in the steel wire after spheroidizing annealing was carried out by resin-embedding the test piece so that the cross-section could be observed, and the cut surface was mirror-polished with emery paper and a diamond buff. Then, the cut surface was etched for 30 seconds to 1 minute using nital (2% by volume of nitric acid, 98% by volume of ethanol) as the etching solution to reveal the ferrite crystal grain boundaries and cementite at the D / 4 position (D: diameter of the steel wire). Then, using a FE-SEM (Field-Emission Scanning Electron Microscope), the structure of the test piece with the revealed cementite etc. was observed, and three fields of view were photographed at a magnification of 2500 times.

[0075] An OHP film was overlaid on the photographed micrograph, and the entire cementite in the micrograph was painted over from above the OHP film to obtain a projected image for analysis. The projected image was binarized to obtain a black-and-white photograph, and the equivalent circle diameter of the entire cementite was calculated using image package software "Particle Analysis ver3.5" (Nippon Steel Technology Co., Ltd.). The average size of the entire cementite described in Table 3 is the average value of the values calculated from 3 fields of view. Also, the minimum size (equivalent circle diameter) of the cementite to be measured was set to 0.3 μm.

[0076] [Measurement of the total content of Cr and Mn in cementite (electrolytic extraction residue measurement)] A sample of about 9 g was cut or polished from a part excluding the surface layer (less than 1 mm) of the steel wire so that it could be electrolyzed to prepare a test material. The test material was immersed in an electrolytic solution (10% acetylacetone - 1% tetramethylammonium chloride - methanol), and about 9 g of the test material was electrolyzed at a constant current by applying an electric current. Then, the electrolytic solution after electrolysis was filtered through a filter with a pore size of 0.10 μm (polycarbonate type membrane filter manufactured by Advantec Toyo Co., Ltd.), and the residue remaining on the filter was obtained as cementite in the steel. Next, the above residue was dissolved in an acid solution and analyzed by ICP emission spectrometry to determine the amounts of Cr and Mn in the cementite, and the total value was obtained as the total content {Cr + Mn} of Cr and Mn in the cementite in mass%.

[0077] Also, the total content of Cr and Mn in the steel in mass% was measured as follows. About 4 g of a sample was taken from the above sample, dissolved in an acid solution, and then analyzed by ICP emission spectrometry to determine the amounts of Cr and Mn in the steel, and the total value [Cr + Mn] was obtained. Then, the total content {Cr + Mn} of Cr and Mn in the cementite in mass% was divided by the total content [Cr + Mn] of Cr and Mn in the steel in mass% to obtain the value of the concentration ratio {Cr + Mn} / [Cr + Mn].

[0078] [Evaluation of properties] [Measurement of hardness after spheroidizing annealing] To evaluate the cold workability, the hardness of each sample after spheroidizing annealing was measured as follows. At the D / 4 position (D: diameter of the steel wire) of the cross-section of the test piece, a Vickers hardness test was carried out in accordance with JIS Z2244 (2009). The Vickers hardness obtained by calculating the average of three or more points was taken as the hardness after spheroidizing annealing. The measurement results are shown in Table 3. In Table 3, the hardness after spheroidizing annealing is indicated as "spheroidized hardness". In this example, when the hardness after spheroidizing annealing is expressed as [C], [Cr], and [Mo] for the C content (mass%), Cr content (mass%), and Mo content (mass%) in the steel, respectively (elements not included are taken as 0 mass%), the case where the following formula (2) is satisfied is evaluated as "OK" as having excellent cold workability, and the case where the following formula (2) is not satisfied is evaluated as "NG" as having poor cold workability. Hardness after spheroidizing annealing (HV) < 91([C] + [Cr] / 9 + [Mo] / 2) + 91 ···(2)

[0079] [Measurement of hardness after quenching treatment] To evaluate the hardenability, the hardness of each sample after quenching treatment was measured as follows. First, as a quenching treatment sample, each sample after spheroidizing annealing was processed into a sample with a thickness (t) of 5 mm, which is the length in the rolling direction, so that sufficient hardening could occur during the quenching treatment. For this sample, as the quenching treatment, high-temperature holding at A3+(30~50°C) for 5 minutes was carried out, and after the high-temperature holding, water cooling was performed. The A3 is a value derived from the following formula (5). Also, the time of high-temperature holding here was the time after the furnace temperature reached the set temperature. A3 (°C) = 910 - 203×√([C]) - 14.2×[Ni] + 44.7×[Si] + 104×[V] + 31.5×[Mo] + 13.1×[W] - 30×[Mn] - 11×[Cr] - 20×[Cu] + 700×[P] + 400×[Al] + 120×[As] + 400×[Ti]···(5) However, [element] represents the content (mass%) of each element, and elements not included are calculated as 0%.

[0080] Then, a Vickers hardness test was carried out at the t / 2 position and the D / 4 position (D: diameter of the steel wire, t: thickness of the sample) of the sample after the quenching treatment. The Vickers hardness obtained by calculating the average of three or more points was taken as the hardness after the quenching treatment. The measurement results are shown in Table 3. In Table 3, the hardness after the quenching treatment is indicated as "quenching hardness". In this example, when the hardness after the quenching treatment satisfies the following formula (3) when the C content (mass%) in the steel is represented by [C], it is evaluated as "OK" as having excellent hardenability, and when it does not satisfy the following formula (3), it is evaluated as "NG" as having poor hardenability. Hardness after quenching treatment (HV) > 380ln([C]) + 1010 ···(3)

[0081] In Table 3, when both the hardness after the spheroidizing annealing and the hardness after the quenching treatment are OK, the comprehensive judgment is "OK" as having both excellent cold workability and excellent hardenability, and when at least one of the hardness after the spheroidizing annealing and the hardness after the quenching treatment is NG, the comprehensive judgment is "NG" as not being able to have both excellent cold workability and excellent hardenability. In Tables 2 and 3, the underlined numerical values indicate that they are outside the scope of the present invention or do not satisfy the desired characteristics.

[0082]

Table 1

[0083]

Table 2

[0084]

Table 3

[0085] Consider the results in the table. The following No. indicates the sample No. in Table 3. Nos. 1 to 11 are invention examples that satisfy all of the component compositions, metal structures, and spheroidizing annealing conditions defined in the embodiments of the present invention.

[0086] Nos. 12, 20, 22, and 23 had a low total content of Cr and Mn in cementite or insufficient coarsening of cementite due to insufficient numbers of cooling-heating processes, resulting in a hardness after spheroidizing annealing higher than the reference value and poor cold workability.

[0087] No. 13 is an example where annealing was performed after wire drawing at a reduction ratio of 25%. By wire drawing, the total content of Cr and Mn in cementite could be increased. However, since the cooling-heating process was 0 times, the average size of all cementite could not be made constant or more, resulting in a hardness after spheroidizing annealing higher than the reference value and poor cold workability.

[0088] No. 14 is an example where annealing was performed under annealing condition SA2, which is manufacturing condition I satisfying the manufacturing conditions shown in Patent Document 3. Under this manufacturing condition, cementite was excessively coarsened by annealing, resulting in a hardness after quenching lower than the reference value and poor hardenability.

[0089] No. 15 had insufficient coarsening of cementite during cooling from temperature T1 and a low total content of Cr and Mn in cementite because temperature T2 was 710°C, which is higher than A1 - 17°C, resulting in a hardness after spheroidizing annealing higher than the reference value and poor cold workability.

[0090] Nos. 16 and 17 had a low total content of Cr and Mn in cementite because the average heating rate R from temperature T2 was low, resulting in a hardness after spheroidizing annealing not less than the reference value and poor cold workability, or a hardness after quenching lower than the reference value and poor hardenability.

[0091] No. 18 is an example where annealing was performed under manufacturing condition M, which satisfies the manufacturing conditions shown in Patent Document 1. Under this manufacturing condition, since the holding time for heating at temperature T1 was particularly short at 0.5 hours, many small-sized rod-shaped cementite remained in the crystal grains, the average size of all cementite did not become constant or more, resulting in a hardness after spheroidizing annealing higher than the reference value and poor cold workability.

[0092] No. 19 is an example of annealing under condition c of Patent Document 2 as the production condition N that satisfies the production conditions shown in Patent Document 2. Under this production condition, due to the lack of holding at temperature T1, etc., a large amount of small-sized rod-shaped cementite remains in the crystal grains, and the average size of all cementite does not reach a certain level or more. Also, since the average heating rate R from temperature T2 is low, the total content of Cr and Mn in the cementite becomes low, the hardness after spheroidizing annealing does not fall below the reference value, resulting in poor cold workability.

[0093] No. 21 has a temperature T3 of 730°C, which is below (A1 + 8°C). Therefore, the total content of Cr and Mn in the cementite becomes low, the hardness after spheroidizing annealing does not fall below the reference value, resulting in poor cold workability.

[0094] For No. 24 to 27, since the cooling-heating process is not carried out or not repeated, the coarsening of the cementite is insufficient, the average size of all cementite does not reach a certain level or more, the hardness after spheroidizing annealing does not fall below the reference value, resulting in poor cold workability.

Industrial Applicability

[0095] The steel wire for mechanical structure parts according to this embodiment has low deformation resistance at room temperature when manufacturing various mechanical structure parts, can suppress wear and breakage of plastic working tools such as dies, and can also suppress the occurrence of cracks during forging, for example, thus exhibiting excellent cold workability. Furthermore, since it has excellent hardenability, high hardness can also be ensured by the hardening treatment after cold working. From these facts, the steel wire for mechanical structure parts according to this embodiment is useful as a steel wire for cold working mechanical structure parts. For example, the steel wire for mechanical structure parts according to this embodiment is used in the manufacture of various mechanical structure parts such as automobile parts and construction machinery parts by being subjected to cold working such as cold forging, cold rolling, and cold swaging. As such mechanical structure parts, specifically, mechanical parts such as bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, hubs, covers, cases, bearing washers, tappets, saddles, bulges, inner cases, clutches, sleeves, outer races, sprockets, cores, stators, anvils, spiders, rocker arms, bodies, flanges, drums, joints, connectors, pulleys, fittings, yokes, bases, valve lifters, spark plugs, pinion gears, steering shafts, common rails, etc., and electrical parts, etc. can be mentioned.

Claims

1. C: 0.05% by mass to 0.60% by mass, Si: 0.005% by mass to 0.50% by mass, Mn: 0.30% by mass to 1.20% by mass, P: More than 0% by mass and 0.050% by mass or less, S: More than 0% by mass and 0.050% by mass or less, Al: 0.001% by mass to 0.10% by mass, Cr: More than 0% by mass and 1.5% by mass or less, and N: More than 0% by mass and 0.02% by mass or less are contained, the balance being composed of iron and inevitable impurities, when the total content (mass%) of Cr and Mn in cementite in the metallographic structure is denoted as {Cr + Mn}, the total content (mass%) of Cr and Mn in the steel is denoted as [Cr + Mn], and the C content (mass%) in the steel is denoted as [C], the concentration ratio {Cr + Mn} / [Cr + Mn] is (0.5[C] + 0.040) or more, and further, when the minimum size (equivalent circle diameter) of the measured cementite is 0.3 μm, the average equivalent circle diameter of all the cementite is (1.668 - 2.13[C]) μm or more and (1.863 - 2.13[C]) μm or less when the C content (mass%) in the steel is denoted as [C], a steel wire for mechanical structure parts having an average value of ferrite crystal grain size of 30 μm or less.

2. Furthermore, Cu: More than 0% by mass and 0.25% by mass or less, Ni: More than 0% by mass and 0.25% by mass or less, Mo: More than 0% by mass and 0.50% by mass or less and B: More than 0% by mass and 0.01% by mass or less, and contains one or more selected from the group consisting of, the steel wire for mechanical structure parts according to claim 1.

3. Furthermore, Ti: More than 0% by mass and 0.2% by mass or less, Nb: More than 0% by mass and 0.2% by mass or less, and V: More than 0% by mass and 0.5% by mass or less, and contains one or more selected from the group consisting of, the steel wire for mechanical structure parts according to claim 1 or 2.

4. Furthermore, Mg: More than 0% by mass and 0.02% by mass or less, Ca: More than 0% by mass and 0.05% by mass or less, Li: More than 0% by mass and 0.02% by mass or less, and REM: More than 0% by mass and 0.05% by mass or less, and contains one or more selected from the group consisting of, the steel wire for mechanical structure parts according to any one of claims 1 to 3.

5. A method for manufacturing a steel wire for mechanical structure parts according to any one of claims 1 to 4, comprising a step of subjecting a bar steel satisfying the chemical composition according to any one of claims 1 to 4 to spheroidizing annealing including the following steps (1) to (3). The method for manufacturing a steel wire for mechanical structure parts according to any one of claims 1 to 4, comprising a step of subjecting the bar steel to spheroidizing annealing including the following steps (1) to (3). (1) After heating to a temperature T1 of (A1 + 8°C) to (A1 + 31°C), heat and hold at the temperature T1 for more than 1 hour and 6 hours or less. (2) Cool to a temperature T2 above 650°C and below (A1 - 17°C), and then heat to a temperature T3 of (A1 + 8°C) to (A1 + 31°C) at an average heating rate of 75°C / hour to 160°C / hour. Perform the cooling-heating process a total of 2 to 6 times. (3) Cool from the temperature T3 in the final cooling-heating process. Here, A1 is calculated by the following formula (1). A1 (°C) = 723 + 29.1×[Si] - 10.7×[Mn] + 16.9×[Cr] - 16.9×[Ni]... (1) However, [element] represents the content (mass %) of each element, and the content of elements not contained is taken as zero.

6. The method for manufacturing a steel wire for a mechanical structure part according to claim 5, wherein the bar steel is a steel wire obtained by subjecting a wire rod to wire drawing with a reduction ratio of more than 5%.

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