TWI806526B - Steel wire for mechanical structural parts and manufacturing method thereof - Google Patents

Abstract

A steel wire for mechanical structural parts, which contains predetermined amounts of C, Si, Mn, P, S, Al, Cr, and N respectively, and the rest is composed of iron and unavoidable impurities. The total content (mass%) of Cr and Mn in carbon iron is represented by {Cr+Mn}, the total content (mass%) of Cr and Mn in steel is represented by [Cr+Mn], and the C in steel When the amount (mass%) is represented by [C], the concentration ratio {Cr+Mn}/[Cr+Mn] is (0.5[C]+0.040) or more, and the average circle equivalent diameter of all Xueming carbon iron is ( 1.668-2.13[C])μm~(1.863-2.13[C])μm.

Description

Steel wire for mechanical structural parts and manufacturing method thereof

The present invention relates to a steel wire for mechanical structural parts and a manufacturing method thereof.

When manufacturing various machine structural parts such as automobile parts and construction machinery parts, spheroidizing annealing is generally performed in order to impart cold workability to steel strips including hot-rolled wire rods. Then, the steel wire obtained by the spheroidizing annealing is subjected to cold working, and then to machining such as cutting, thereby shaping it into a predetermined part shape. Further quenching and tempering are performed to adjust the final strength to manufacture mechanical structural parts.

In recent years, in order to prevent cracking of the steel material during the cold working process and to prolong the life of the die, there is a demand for a steel wire that is softer than before.

As a method of obtaining softened steel wire, for example, Patent Document 1 discloses a method for producing medium-carbon steel excellent in cold forgeability, in which heating to the Wostian ironization temperature range is performed twice or more during the spheroidizing annealing treatment. According to the production method of Patent Document 1, a steel for cold forging having a hardness of 83 HRB or less after spheroidizing annealing and a ratio of spherical carbides in the structure of 70% or more can be obtained.

Patent Document 2 discloses a steel material having low deformation resistance after spheroidizing annealing and excellent cold forgeability, and a manufacturing method thereof. As this manufacturing method, the steel satisfying the predetermined composition is subjected to thermal processing, cooled to room temperature, and then heated to a temperature range from point A1 to point A1 + 50°C. Keep the temperature range of 50°C for 0~1hr, and then go from the aforementioned temperature range of A1 point to point A1+50°C to point A1 Cool at an average cooling rate of 10 to 200°C/hr in the temperature range of -100°C to A1 -30°C and perform annealing treatment. After performing this annealing treatment more than two times, heat up to A1 point to A1 point +30°C Temperature range and cooling after holding in the temperature range from point A1 to point A1 + 30°C, and cooling after heating up to point A1 and holding in the temperature range from point A1 to point A1 + 30°C, it will reach point A1 So far, the residence time in the temperature range from point A1 to point A1 + 30°C is set to 10 minutes to 2 hours, and from the temperature range from point A1 to point A1 + 30°C to point A1 -100°C ~ point A1 -20°C After cooling in the cooling temperature range at an average cooling rate of 10-100°C/hr, keep in the cooling temperature range for 10 minutes to 5 hours and then cool further.

Patent Document 3 discloses a steel wire for mechanical structural parts. In order to reduce the deformation resistance during cold working and improve the crack resistance to exert excellent cold workability, it has a predetermined composition. The metal structure of the steel is composed of ferrite Composed of and Xueming carbon iron, and relative to the total amount of Xueming carbon iron, the proportion of Xueming carbon iron existing in the grain boundary of fertilized iron is more than 40%. In Patent Document 3, as the production conditions of the rolled wire rod for spheroidizing annealing, it is preferable to implement finish rolling at 800°C to 1050°C, and to perform sequentially: the first cooling with an average cooling rate of 7°C/s or more , the second cooling with an average cooling rate of 1°C/s~5°C/s, the third cooling with an average cooling rate faster than the aforementioned second cooling and 5°C/s or more; the end of the aforementioned first cooling and the aforementioned second cooling The start of cooling is carried out in the range of 700~750°C, the end of the second cooling and the start of the third cooling are carried out in the range of 600~650°C, and the end of the third cooling is set below 400°C . [Prior Art Literature] [Patent Document]

[Patent Document 1] Japanese Unexamined Patent Publication No. 2011-256456 [Patent Document 2] Japanese Unexamined Patent Publication No. 2012-140674 [Patent Document 3] Japanese Patent Laid-Open No. 2016-194100

[Problem to be solved by the invention]

However, according to the conventional technologies disclosed in Patent Documents 1 to 3, the hardness after spheroidizing annealing cannot be sufficiently lowered, resulting in poor workability of cold working after spheroidizing annealing, or after cold working Quenching treatment cannot sufficiently increase the hardness, that is, the hardenability is poor. In other words, there is no technology that improves both cold workability and hardenability.

The present invention has been developed in view of such a situation, and its object is to provide a steel wire for mechanical structural parts that has sufficiently reduced hardness to provide excellent cold workability and high hardness obtained by quenching treatment, that is, excellent hardenability, and Provided is a method of manufacturing a steel wire for machine structural parts which can be manufactured in a relatively short period of time.

In this specification, "wire rod" and "bar steel" are wire-shaped and rod-shaped steel materials obtained by hot rolling, respectively, and refer to steel materials that have not been subjected to heat treatment such as spheroidizing annealing and wire drawing. Also, "steel wire" refers to at least one of heat treatment such as spheroidizing annealing and wire drawing processing performed on wire rod or steel bar. In this specification, the above-mentioned wire rod, bar steel and steel wire are collectively referred to as "bar steel". [Technical means to solve the problem]

The steel wire for mechanical structural parts of aspect 1 of the present invention contains: 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 not more than 0.050% by mass, S: More than 0% by mass and not more than 0.050% by mass, Al: 0.001% by mass to 0.10% by mass, Cr: more than 0% by mass and not more than 1.5% by mass, and N: More than 0% by mass and not more than 0.02% by mass, The remainder is composed of iron and unavoidable impurities, The total content (mass %) of Cr and Mn in the metal structure of snow clear carbon iron is represented by {Cr+Mn}, and the total content (mass %) of Cr and Mn in steel is represented by [Cr+Mn] , and when the amount of C (mass %) in the steel is represented by [C], the concentration ratio {Cr+Mn}/[Cr+Mn] is (0.5[C]+0.040) or more, And when the amount of C (mass %) in steel is represented by [C], the average circle equivalent diameter of all Xueming carbon iron is (1.668-2.13[C]) μm~(1.863-2.13[C]) μm.

Aspect 2 of the present invention is the steel wire for mechanical structural parts described in Aspect 1, further containing: selected from Cu: More than 0% by mass and not more than 0.25% by mass, Ni: More than 0% by mass and not more than 0.25% by mass, Mo: More than 0% by mass and not more than 0.50% by mass, and B: More than 0% by mass and not more than 0.01% by mass One or more of the groups formed.

Aspect 3 of the present invention is the steel wire for machine structural parts described in Aspect 1 or 2, further containing: selected from Ti: More than 0% by mass and not more than 0.2% by mass, Nb: more than 0% by mass and not more than 0.2% by mass, and V: More than 0% by mass and not more than 0.5% by mass One or more of the groups formed.

Aspect 4 of the present invention is the steel wire for mechanical structural parts described in any one of aspects 1 to 3, further containing: selected from Mg: More than 0% by mass and not more than 0.02% by mass, Ca: More than 0% by mass and not more than 0.05% by mass, Li: More than 0% by mass and not more than 0.02% by mass, and REM: More than 0% by mass and not more than 0.05% by mass One or more of the groups formed.

Aspect 5 of the present invention is the steel wire for mechanical structural parts described in any one of aspects 1 to 4, The average grain size of ferrite ferrite is 30μm or less.

其中，[元素]表示各元素的含量(質量%)，不包含的元素之含量為零。 Aspect 6 of the present invention is a method for manufacturing a steel wire for mechanical structural parts described in any one of Aspects 1 to 5, which includes: satisfying any one of Aspects 1 to 4. The bar steel of the composition is subjected to the spheroidizing annealing process including the following steps (1)~(3). (1) After heating to the temperature T1 of (A1+8℃)~(A1+31℃), at this temperature T1 is heated and maintained for more than 1 hour and less than 6 hours, (2) After cooling to a temperature T2 exceeding 650°C and below (A1-17°C), heat it to (A1 +8°C) to (A1+31°C) temperature T3 cooling-heating process, and the cooling-heating process is implemented for a total of 2 to 6 times, (3) starting from the last temperature T3 of the cooling-heating process Cooling, here, A1 is calculated according to the following formula (1):

Here, [element] represents the content (mass %) of each element, and the content of elements not included is zero.

Aspect 7 of the present invention is the manufacturing method of steel wire for machine structural parts described in aspect 6, The aforementioned bar steel is a steel wire obtained by drawing a wire rod with a reduction of area exceeding 5%. [Effect of Invention]

According to the present invention, it is possible to provide a steel wire for machine structural parts excellent in cold workability and excellent hardenability, and a method of manufacturing the steel wire for machine structural parts.

The inventors of the present invention conducted painstaking research in order to realize a steel wire for mechanical structural parts having both excellent cold workability and hardenability, and found that the amount of snow carbon iron compared to the total content of Mn and Cr in the steel The total content of Mn and Cr in the steel should be set at a ratio above a certain level, and the average size of all snow-white carbon irons should be set within a certain range according to the amount of C in the steel. It has also been found that in order to realize the above-mentioned metal structure, a metal structure whose chemical composition is set within a certain range is used, and in a method of manufacturing a steel wire for mechanical structural parts, in particular, spheroidizing annealing is performed under predetermined conditions, which is Effective. First, the steel wire for machine structural parts of this embodiment will be described from the metal structure of the steel wire for machine structural parts.

1. Metal structure In the past, steel was subjected to spheroidizing annealing to form a metal structure composed of ferrite and snow-white carbon iron, thereby ensuring cold workability. However, in order to achieve better cold workability and hardenability, the above metal structure cannot be adopted. achieved. The inventors of the present invention have conducted painstaking research from various angles in order to realize a steel wire for machine structural parts having both excellent cold workability and hardenability. First, the inventors of the present invention focused on the amount of Mn and Cr in Xueming carbon iron. For example, spheroidizing annealing is carried out under the following manufacturing conditions, so that the average size of all snow-bright carbon irons becomes more than a certain level, and if the amount of Mn and Cr in the snow-bright carbon irons are increased, the amount of Mn and Cr in the fat grain iron can be increased. The relatively reduced amount of Cr suppresses hardening due to solid solution strengthening, thereby achieving low hardness and improving cold workability. In addition, it has been found that by suppressing the average size of all the snow-white carbon irons to a certain level or less, it is possible to suppress the undissolution of the snow-white carbon irons during high-temperature holding in the quenching treatment process, thereby improving the hardenability. So far, there has been no technology focusing on both the amount of Mn and the amount of Cr in iron carbon, and the average size of all iron carbon.

[The total content (mass%) of Cr and Mn in Xueming carbon iron is represented by {Cr+Mn}, the total content (mass%) of Cr and Mn in steel is represented by [Cr+Mn], and When the amount of C (mass %) in steel is represented by [C], the concentration ratio {Cr+Mn}/[Cr+Mn] is (0.5[C]+0.040) or more] Cr and Mn are representative elements that are easy to dissolve in Xueming carbon iron. However, a part of it will be solid-dissolved in ferrite, and as the amount of solid solution increases, the matrix of ferrite will be strengthened to increase the hardness. Therefore, the ratio of the total content of Cr and Mn in steel [Cr+Mn] to the total content of Cr and Mn {Cr+Mn} in Xueming carbon iron, that is, the concentration ratio {Cr+Mn}/ The larger the [Cr+Mn], the less the total content of Cr and Mn in ferrite which accounts for phases other than Xueming carbon iron. As a result, the solid solution strengthening of ferrite based on Cr and Mn decreases, accompanied by This lowers the hardness and improves cold workability. The lower limit of the concentration ratio {Cr+ Mn}/[Cr+Mn] is set to (0.5[C]+ 0.040) or more. The concentration ratio {Cr+Mn}/[Cr+Mn] is preferably (0.5[C]+0.042) or more. On the other hand, the upper limit of the concentration ratio {Cr+Mn}/[Cr+Mn] is set to about 0.5[C]+0.500 in consideration of possible manufacturing conditions and the like.

There is no particular limitation on the shape of the above-mentioned snow bright iron, and rod-shaped snow bright iron with a large aspect ratio (aspect ratio) is also included in addition to the spherical snow bright iron. The above-mentioned aspect ratio refers to the ratio (major axis/short axis) of the longest long axis of the Xueming carbon iron particle to the longest short axis in the direction perpendicular to the long axis. Also, although the size standard of the snowy carbon iron as the measurement object is not limited, as shown in the examples described later, the size of the snowy carbon iron that can measure the total content of Cr and Mn is taken as the minimum size. Specifically, when the electrolytic extraction residue is measured according to the method shown in the examples described later, the snow-white carbon iron remaining on the filter with a pore diameter of 0.10 μm is used as the measurement object. In addition, the total content of Cr and Mn in the steel is the sum of the average Cr content and the average Mn content in the steel, as shown in the examples below, for example, when the metal structure is formed of ferrite and snow-white carbon iron , refers to the total content of Cr and Mn in mass % in Feili iron and Xueming carbon iron.

[When the amount of C (mass %) in steel is represented by [C], the average circle equivalent diameter of all Xueming carbon iron is (1.668-2.13[C])μm~(1.863-2.13[C])μm] When the amount of carbon in the steel is constant, the larger the size of the carbon, the smaller the number density of the carbon, and the longer the distance between the carbon. The longer the distance between the snow-white carbon and iron in the steel, the harder it is for precipitation strengthening, and as a result, the hardness can be reduced. Also, by making the size of the snow-white carbon iron more than a certain level, the effect of reducing the hardness by increasing the total content of Cr and Mn in the snow-white carbon iron can be easily exhibited. Based on these points of view, in the present invention, when the amount of C (mass %) in the steel is represented by [C], the average circle-equivalent diameter of all snowflake carbon irons is set to be (1.668-2.13[C]) μm or more. The average circle equivalent diameter of all Xueming carbon iron is preferably (1.669-2.13[C]) μm or more. On the other hand, if the iron carbon is excessively coarsened, the iron carbon cannot be fully dissolved when the high temperature is maintained in the quenching process after cold working, and a sufficiently high hardness cannot be obtained during quenching. Therefore, in the present invention, the average circle-equivalent diameter of all snowflake carbon irons is set to be (1.863-2.13[C]) μm or less, preferably (1.858-2.13[C]) μm or less.

It is disclosed in Patent Document 3 that compared with the Xueming carbon iron existing in the ferrite crystal grain boundaries, the amount of strain received during cold working is smaller than that of the ferrite carbon existing in the ferrite crystal grains, which can make the deformation resistance decrease. However, in Patent Document 3, the average size of all the iron carbides is not controlled. As a result, the iron carbides cannot be sufficiently dissolved during the high-temperature maintenance in the quenching treatment process, and the hardenability is poor. The technology of the present invention focuses on the ratio of the total content of Cr and Mn and the total content of the snowy carbon iron in order to realize a steel wire for machine structural parts having excellent cold workability and excellent hardenability. The average size of both sides.

The metal structure of the steel wire for mechanical structural parts of this embodiment is a spheroidized structure of snow-bright carbon iron, which can be obtained by, for example, performing spheroidizing annealing on a bar steel satisfying the chemical composition described below.

The metal structure of the steel wire for mechanical structural parts in this embodiment is substantially composed of ferrite and snow-white carbon iron. The above "substantially" means that if the area ratio of ferrite in the metal structure of the steel wire for mechanical structural parts of this embodiment is 90% or more, and the area ratio of rod-shaped snowy carbon iron with an aspect ratio of 3 or more is 5 % or less, as long as the adverse effect on cold workability is small enough, nitrides such as AlN, and inclusions other than nitrides can be allowed to be less than 3% in terms of area ratio. The area ratio of the above-mentioned fertilized iron can be more than 95%.

In this specification, "ferric iron" refers to the part whose crystal structure is bcc structure, and includes ferric iron in the layered structure of ferric iron and snow-bright carbon iron, that is, ferric iron. The "fertilized iron crystal grains" as the measurement object of "fertilized iron crystal grain size", including the crystal grains of rod-shaped snow bright carbon iron produced in the spheroidizing annealing due to incomplete spheroidization, are also subject to evaluation, However, crystal grains containing rod-shaped snow bright iron that may remain before the spheroidizing annealing (pulley iron crystal grains) do not belong to the evaluation object. Specifically, after etching with nitric acid (2% by volume of nitric acid, 98% by volume of ethanol), it can be confirmed by observing with an optical microscope at 1000 times that "the crystal grains of snow-bright carbon iron do not exist in the grains." and "Crystal grains in which snowy carbon iron exists in the grains and the shape of snowy carbon iron can be observed (that is, the boundary between snowy carbon iron and fat grain iron can be clearly observed)". Utilize above-mentioned optical microscope to be unable to observe the crystal grain of the shape of Xueming carbon iron (that is, the boundary of Xueming carbon iron and fat particle iron cannot be clearly observed) under 1000 times, be outside the judging object of this embodiment, and Not included in "Fertilized Iron Crystals".

[Average grain size of ferrite grains: 30 μm or less] In the steel wire for mechanical structural parts of this embodiment, the average grain size of ferrite grains in the metal structure is preferably 30 μm or less. If the average grain size of ferrite grains is 30 μm or less, the ductility of the steel wire for mechanical structural parts can be improved, and the occurrence of cracks during cold working can be further suppressed. The average value of ferrite crystal grain size is more preferably at most 25 μm, particularly preferably at most 20 μm. Although the average value of ferrite grain size is as small as possible, the lower limit may be about 2 μm if possible manufacturing conditions are taken into consideration.

(characteristic) The steel wire for machine structural parts of this embodiment satisfying the following chemical composition and having the above-mentioned metal structure can have both low hardness that can be cold-worked well and high hardness after quenching. In this embodiment, when the amount of C (mass %), Cr (mass %), and Mo (mass %) in the steel are represented by [C], [Cr], and [Mo] respectively (elements not included are zero mass %), when the hardness, that is, the hardness after the spheroidizing annealing of the examples described later satisfies the following formula (2) and the hardness after quenching satisfies the following formula (3), the hardness can be sufficiently reduced, that is, the cold workability is excellent , and achieve high hardness after quenching treatment, that is, excellent hardenability. (After spheroidizing annealing) hardness (HV) <91([C]+[Cr]/9+[Mo]/2)+91 ...(2) Hardness (HV) after quenching treatment > 380ln ([C]) + 1010 ... (3)

2. Chemical composition The chemical composition of the steel wire for machine structural parts of this embodiment will be described.

[C: 0.05% by mass to 0.60% by mass] C is an element controlling the strength of steel materials, and as the content increases, the strength after quenching and tempering becomes higher. In order to effectively exhibit the above effects, the lower limit of the amount of C is set to 0.05% by mass. The amount of C is preferably at least 0.10% by mass, more preferably at least 0.15% by mass, and most preferably at least 0.20% by mass. However, if the amount of C is too large, the amount of spherical snow-white carbon iron in the structure after spheroidizing annealing becomes too large, the hardness increases, and the cold workability decreases. Therefore, the upper limit of the amount of C is set to 0.60% by mass. The amount of C is preferably at most 0.55% by mass, more preferably at most 0.50% by mass.

[Si: 0.005% by mass to 0.50% by mass] Si not only serves as a deoxidizing material during smelting, but also contributes to an increase in strength. In order to effectively exhibit this effect, the lower limit of the amount of Si is set to 0.005% by mass. The amount of Si is preferably at least 0.010 mass %, more preferably at least 0.050 mass %. However, Si contributes to the solid-solution strengthening of ferrite, and has the effect of significantly improving the strength after spheroidizing annealing. If the Si content is too large, the cold workability will be deteriorated due to the above-mentioned action, so the upper limit of the Si amount is made 0.50% by mass. The amount of Si is preferably at most 0.40 mass %, more preferably at most 0.35 mass %.

[Mn: 0.30% by mass to 1.20% by mass] Mn is an element that functions effectively as a deoxidizing material and contributes to the improvement of hardenability. In order to fully exhibit this effect, the lower limit of the amount of Mn is set to 0.30% by mass. The amount of Mn is preferably at least 0.35 mass %, more preferably at least 0.40 mass %. However, if the amount of Mn is too large, segregation is likely to occur and the toughness will decrease. Therefore, the upper limit of the amount of Mn is set to 1.20% by mass. The amount of Mn is preferably at most 1.10 mass %, more preferably at most 1.00 mass %.

[P: More than 0% by mass and 0.050% by mass or less] P (phosphorus) is an unavoidable impurity, and is a harmful element that causes grain boundary segregation in steel and adversely affects forgeability and toughness. Therefore, the amount of P is made 0.050 mass % or less. The amount of P is preferably at most 0.030 mass %, more preferably at most 0.020 mass %. The less the amount of P, the better, and usually 0.001% by mass or more.

[S: more than 0% by mass and less than 0.050% by mass] S (sulfur) is an unavoidable impurity that forms MnS in steel to deteriorate ductility, and therefore is an element detrimental to cold workability. Therefore, the amount of S is set to be 0.050% by mass or less. The amount of S is preferably at most 0.030 mass %, more preferably at most 0.020 mass %. The smaller the amount of S, the better, and usually 0.001% by mass or more.

[Al: 0.001% by mass to 0.10% by mass] [Al: 0.001% by mass to 0.10% by mass] Al is an element serving as a deoxidizing material, and has an effect of reducing impurities along with deoxidation. In order to exhibit this effect, the lower limit of the amount of Al is set to 0.001% by mass. The amount of Al is preferably at least 0.005% by mass, more preferably at least 0.010% by mass. However, if the amount of Al is too large, non-metallic inclusions increase to lower the toughness. Therefore, the upper limit of the amount of Al is set to 0.10% by mass. The amount of Al is preferably at most 0.08% by mass, more preferably at most 0.05% by mass.

[Cr: more than 0% by mass and less than 1.5% by mass] Cr has the effect of improving the hardenability of the steel to increase the strength, and the effect of promoting the spheroidization of Xueming carbon iron. Specifically, Cr will solid-dissolve in the Xueming iron and delay the dissolution of the Xueming iron during the heating of the spheroidizing annealing. When heated, the snow-bright carbon iron cannot be dissolved and a part remains, so that it is not easy to form a rod-shaped snow-bright carbon iron with a large aspect ratio during cooling, and it is easy to obtain a spheroidized structure. Therefore, the amount of Cr is more than 0% by mass, preferably 0.01% by mass or more. More preferably, it is at least 0.05% by mass, and most preferably, it is at least 0.10% by mass. From the viewpoint of further promoting spheroidization of Xueming iron carbon, it may be more than 0.30 mass %, and may be more than 0.50 mass %. If the amount of Cr is too much, the diffusion of elements including carbon will be delayed, and the dissolution of Xueming carbon iron will be delayed too much, making it difficult to obtain a spheroidized structure. As a result, the hardness reduction effect of the present embodiment may decrease. Therefore, the amount of Cr is 1.50 mass % or less, Preferably it is 1.40 mass % or less, More preferably, it is 1.25 mass % or less. The amount of Cr may be 1.00 mass % or less, further 0.80 mass % or less, and further 0.30 mass % or less from the viewpoint of accelerating the diffusion of elements.

[N: more than 0% by mass and less than 0.02% by mass], N is an impurity unavoidably contained in steel, and if a large amount of solid-solution N is contained in steel, hardness increases due to strain aging, ductility decreases, and cold workability deteriorates. Therefore, the amount of N is set to be 0.02 mass % or less, preferably 0.015 mass % or less, more preferably 0.010 mass % or less.

[The remaining part] The remainder is iron and unavoidable impurities. As unavoidable impurities, the incorporation of trace elements (such as As, Sb, Sn, etc.) brought in according to the conditions of raw materials, materials, manufacturing equipment, etc. is allowed. Also, for example, like P and S, since the content is generally as small as possible, it is an unavoidable impurity, but its composition range is an element specified separately as above. Therefore, in this specification, when referring to "inevitable impurities" constituting the remainder, it is a concept excluding elements whose composition ranges are separately specified.

The steel wire for machine structural parts according to the present embodiment may contain the above-mentioned elements in the chemical composition. The optional elements described below may not be contained, but by containing them as necessary together with the above-mentioned elements, it is possible to more easily ensure hardenability and the like. Hereinafter, the selection element will be described.

[Selected from Cu: more than 0 mass % and less than 0.25 mass %, Ni: more than 0 mass % and less than 0.25 mass %, Mo: more than 0 mass % and less than 0.50 mass %, and B: more than 0 mass % and 0.01 mass % One or more of the following groups] Cu, Ni, Mo, and B are all elements effective in increasing the strength of the final product by improving the hardenability of steel materials, and may be contained alone or in combination of two or more types as necessary. The effects based on these elements become larger as their content increases. In order for the above-mentioned effects to be effectively exerted, the preferable lower limit is more than 0 mass % for Cu, Ni, and Mo, more preferably 0.02 mass % or more, particularly preferably 0.05 mass % or more, and B is more than 0 mass %, more preferably 0.0003% by mass or more, and preferably 0.0005% by mass or more.

On the other hand, if the content of these elements is too large, the strength may become too high and cold workability may be deteriorated, so the preferable upper limit of each element is set as described above. More preferably, the respective contents of Cu and Ni are at most 0.22% by mass, most preferably at most 0.20% by mass, more preferably at most 0.40% by mass, most preferably at most 0.35% by mass, and more preferably at most 0.007% by mass. , preferably 0.005% by mass or less.

[One or more species selected from the group consisting of Ti: more than 0% by mass and not more than 0.2% by mass, Nb: more than 0% by mass and not more than 0.2% by mass, and V: more than 0% by mass and not more than 0.5% by mass] Ti, Nb, and V form a compound with N to reduce solid-solution N and exert an effect of reducing deformation resistance, so they may be contained alone or in combination of two or more as necessary. The effects based on these elements become larger as their content increases. For any of the elements, the lower limit is preferably more than 0% by mass, more preferably at least 0.03% by mass, and most preferably at least 0.05% by mass in order to effectively exert the above-mentioned effects. However, if the content of these elements is too large, the deformation resistance of the formed compound will increase, which may worsen the cold workability. Therefore, the respective content of Ti and Nb is preferably 0.2% by mass or less, and the V content is preferably 0.5% by mass. Mass% or less. The content of Ti and Nb is more preferably at most 0.18 mass %, most preferably at most 0.15 mass %, and the V content is more preferably at most 0.45 mass %, most preferably at most 0.40 mass %.

[Selected from Mg: more than 0 mass% and less than 0.02 mass%, Ca: more than 0 mass% and less than 0.05 mass%, Li: more than 0 mass% and less than 0.02 mass%, and rare earth elements (Rare Earth Metal: REM) : more than 0 mass % and 0.05 mass % or less of one or more species] Mg, Ca, Li, and REM are elements effective in improving the deformability of steel by spheroidizing sulfide compound-based inclusions such as MnS. This effect increases with the increase of its content. In order to effectively exert the above effects, the contents of Mg, Ca, Li, and REM are preferably more than 0% by mass, more preferably at least 0.0001% by mass, and most preferably at least 0.0005% by mass. However, even if it is excessively contained, the effect will be saturated and the corresponding effect cannot be expected. Therefore, the contents of Mg and Li are preferably 0.02% by mass or less, more preferably 0.018% by mass or less, and most preferably 0.015% by mass. Below, the content of Ca and REM is preferably at most 0.05 mass %, more preferably at most 0.045 mass %, and most preferably at most 0.040 mass %. In addition, Mg, Ca, Li, and REM may be contained individually or in two or more kinds, and when two or more kinds are contained, the content may be any content within the above-mentioned range. The aforementioned REM means that it includes lanthanide elements (15 elements from La to Lu), Sc (scandium) and Y (yttrium).

The shape and the like of the steel wire for machine structural parts of this embodiment are not particularly limited. Examples thereof include those with a diameter of 5.5 mm to 60 mm.

3. Manufacturing method In order to obtain the metal structure of the steel wire for machine structural parts of this embodiment, when manufacturing the steel wire for machine structural parts, it is preferable to appropriately control the spheroidizing annealing conditions as described below. There is no particular limitation on the hot rolling process for producing the wire rod or steel bar for spheroidizing annealing, and the usual method may be followed. As will be described later, wire drawing may be applied before the spheroidizing annealing. The diameter of the rod steel for spheroidizing annealing, that is, wire rod, steel wire, and bar steel, is not particularly limited, and the diameter of the wire rod and steel wire is, for example, 5.5 mm to 55 mm, and the case of bar steel is, for example, 18 mm to 105 mm.

Referring to FIG. 1 , the spheroidizing annealing conditions in the manufacturing method of the steel wire for machine structural parts according to the embodiment of the present invention will be described. FIG. 1 shows an example of an explanatory diagram of 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 is not limited to this FIG. 1 .

其中，[元素]表示各元素的含量(質量%)，不包含的元素之含量為零。 The manufacturing method of the steel wire for machine structural parts according to the embodiment of the present invention includes a spheroidizing annealing step including the following steps (1) to (3). (1) After heating to a temperature T1 of (A1+8°C)~(A1+31°C), heating and maintaining at this temperature T1 for more than 1 hour and less than 6 hours, (2) cooling to more than 650°C and (A1- The temperature T2 below 17°C) is followed by a cooling-heating process of heating to a temperature T3 of (A1+8°C)~(A1+31°C) at an average temperature rise rate of 75°C/hour~160°C/hour, and the cooling - The heating process is performed 2 to 6 times in total, (3) Cooling is performed from the temperature T3 of the last cooling-heating process. Here, A1 is calculated according to the following formula (1):

Here, [element] represents the content (mass %) of each element, and the content of elements not included is zero.

[(1) After heating to a temperature T1 of (A1+8°C)~(A1+31°C), heat and hold at this temperature T1 for more than 1 hour and less than 6 hours ([2] in Figure 1)] By heating to the temperature T1 of (A1+8°C)~(A1+31°C), the dissolution of the rod-shaped snow-white carbon iron with a large aspect ratio generated in the rolling stage is promoted. If the temperature T1 is too low, the rod-shaped snow-bright carbon iron cannot be dissolved during heating and holding, and will remain in the ferrite grains to increase the hardness. In order to obtain a sufficiently softened steel wire, the temperature T1 must be set to (A1+8°C) or higher. The temperature T1 is preferably above (A1+15°C), more preferably above (A1+20°C). On the other hand, if the temperature T1 is too high, the crystal grains become excessively coarse, and it is not easy to precipitate spherical snow-bright carbon iron at the grain boundary of fertilized iron in the cooling process of the next process, and the rod-shaped snow-bright carbon iron will increase. increase the hardness. Therefore, temperature T1 is set to (A1+31 degreeC) or less. The temperature T1 is preferably below (A1+30°C), more preferably below (A1+29°C).

And if the heating time (t1) at temperature T1 is too short, the rod-shaped snow-bright carbon iron will remain in the ferrite grains and increase the hardness. In order to obtain a sufficiently softened steel wire, the heating retention time (t1) must be more than 1 hour and 6 hours or less. A preferable heating retention time (t1) is 1.5 hours or more, more preferably 2.0 hours or more. If the heating retention time (t1) is too long, the heat treatment time will become long and productivity will fall. Therefore, the heating retention time (t1) is 6 hours or less, preferably 5 hours or less, more preferably 4 hours or less. The average heating rate ([1] in Fig. 1) until the temperature T1 of (A1+8°C)~(A1+31°C) does not affect the properties of the steel, so the temperature can be raised at any speed. For example, the temperature is raised at 30° C./hour to 100° C./hour.

其中，[元素]表示各元素的含量(質量%)，不包含的元素之含量為零。 The temperature at the above-mentioned point A1 is calculated according to the following formula (1) described on page 273 of Leslie Iron and Steel Materials Science (Maruzen).

Here, [element] represents the content (mass %) of each element, and the content of elements not included is zero.

[(2) Cool to a temperature T2 exceeding 650°C and below (A1-17°C) and then heat to (A1+8°C)~(A1+31°C) at an average heating rate of 75°C/hour~160°C/hour The cooling-heating process of the temperature T3, and the cooling-heating process is implemented for a total of 2~6 times ([3]~[7] of Figure 1)]

(2-i) Cooling to temperature T2 ([3] of Fig. 1 ) exceeding 650°C and not exceeding (A1-17°C) After the heating and holding in (1) above, in order to promote the precipitation of the high concentration of Mn and Cr, it is cooled to a temperature T2 below (A1-17°C) exceeding 650°C. If the temperature T2 is too low, the annealing time will be prolonged. And if the temperature T2 is too low, the iron carbon will be excessively micronized, and the iron carbon with less Cr and Mn content will be easily produced. Therefore, the cooling reaching temperature T2 must exceed 650°C. According to the production method of this embodiment, even if the cooling temperature T2 exceeds 650° C., desired snow-white carbon iron can be obtained without long-term annealing. The temperature T2 is preferably 670° C. or higher. On the other hand, if the temperature T2 is too high, the Xueming carbon iron cannot be fully separated, and as a result, the Xueming carbon iron cannot fully concentrate Cr and Mn, and the total content of Cr and Mn in the Xueming carbon iron decreases, and the hardness increases. It will increase and cause cold workability to decrease. Therefore, the upper limit of the temperature T2 is set to A1-17°C. The temperature T2 is preferably below A1-18°C. After reaching the temperature T2, if the temperature is maintained, the heat treatment time will be prolonged. Therefore, based on these points, it is advisable not to carry out the maintenance. However, in order to make the temperature change in a furnace uniform, holding|maintenance may be performed for a short time. The holding time (t2) at the cooling reaching temperature T2 is preferably within 1 hour.

Also, the average cooling rate during cooling in the cooling-heating step ([3] in FIG. 1 ) is not particularly limited. The average cooling rate during cooling from temperature T1 or temperature T3 to temperature T2 is preferably 100° C./hour or less from the viewpoint of promoting the intrusion of Mn and Cr in the parent phase into snow-white carbon iron. From the viewpoint of further suppressing excessive coarsening of the snow-white carbon iron formed in the step (2), further improving hardenability, and further improving productivity, the above-mentioned average cooling rate is preferably 5° C./hour or more.

(2-ii) Heating to a temperature T3 of (A1+8°C)~(A1+31°C) at an average heating rate of 75°C/hour~160°C/hour ([5] and [6] in Figure 1) In order to increase the content of Cr and Mn in the snowy carbon iron precipitated during the cooling of (2-i) above, heat from temperature T2 to (A1+8 ℃)~(A1+31℃) temperature T3. If the average heating rate R is too fast, the diffusion of Cr and Mn will not be complete, and the content of Cr and Mn in the Xueming carbon iron produced by the above-mentioned heating will be insufficient, and the hardness will increase and the cold workability will decrease. Therefore, the average temperature increase rate R is set to be 160° C./hour or less. The average temperature increase rate R is preferably at most 155°C/hour, more preferably at most 150°C/hour, still more preferably at most 120°C/hour, particularly preferably at most 100°C/hour. On the other hand, if the average heating rate R is too slow, the Xueming Iron Carbon will be excessively dissolved, resulting in a decrease in the total content of Cr and Mn in the Xueming Iron Carbon. If the average heating rate R is too slow, the snow-white carbon iron generated when cooling from the temperature T1 becomes excessively coarsened. As a result, the snow-white carbon iron cannot be fully dissolved in the high temperature maintenance of the quenching treatment process, resulting in The hardness after the quenching treatment decreases, that is, the hardenability deteriorates. Furthermore, the annealing time is lengthened and the productivity is lowered. Therefore, the average temperature increase rate R is set to be 75° C./hour or more, preferably 80° C./hour or more.

And if the heating temperature T3 in the cooling-heating process is too low, the diffusion of Cr and Mn is not complete, and the content of Cr and Mn in the Xueming carbon iron generated in the above-mentioned heating and holding process is insufficient, and the hardness will decrease. The increase reduces the cold workability. Therefore, the temperature T3 needs to be (A1+8°C) or higher. The temperature T3 is preferably above (A1+15°C), more preferably above (A1+20°C). On the other hand, if the heating temperature T3 is too high, the Xueming iron carbon will be excessively dissolved, and as a result, the total content of Cr and Mn in the Xueming iron carbon will decrease. Therefore, the reaching temperature (T3) of heating is set to (A1+31 degreeC) or less. The temperature T3 is preferably below (A1+30°C), more preferably below (A1+29°C).

After reaching the temperature T3, which is the reaching temperature for heating, it can be kept at the temperature T3. If the holding time (t3) at the temperature T3 is too long, it is easy to make the spherical snow formed in the heating and holding process at the temperature T1 bright. Carbon iron redissolves, and hardness may increase. Also, if the holding time (t3) at the above-mentioned temperature T3 is too long, the annealing time will be prolonged, and productivity may be lowered. Therefore, the retention time (t3) at the temperature T3 is preferably within 1 hour.

In the manufacturing method of this embodiment, as described later, the cooling-heating process including the cooling of the above-mentioned (2-i) and the heating of the above-mentioned (2-ii) is repeated several times, and in each time, the attained temperature of the cooling is The temperature T2, the average temperature rise rate R, and the temperature T3 must satisfy the above-mentioned ranges.

The magnitude relationship between the temperature T3 and the temperature T1 is not particularly limited. For example, the temperature T3 can be set to be the same temperature as the temperature T1, or the temperature T3 can be set to be higher than the temperature T1. Alternatively, the above-mentioned temperature T1 may be set higher than the above-mentioned temperature T3 from the viewpoint of sufficiently dissolving the rod-shaped snow-bright carbon iron in the Worth field iron.

(2-iii) Perform the cooling-heating process 2 to 6 times in total ([7] in Fig. 1) In order to increase the concentration of Mn and Cr in Xueming iron carbon and promote the coarsening of Xueming iron carbon, the above cooling-heating process must be carried out for a total of 2~6 times. If the above-mentioned cooling-heating process is not repeated, the concentrations of Mn and Cr in the Xueming iron carbon will be insufficient, or the coarsening of the Xueming iron carbon will not be complete. As a result, the hardness after spheroidizing annealing increases. Therefore, the above-mentioned cooling-heating process is performed two or more times. Preferably it is 3 times or more. As the number of implementations increases, the hardness will decrease, but even if the number of implementations is too high, the effect will still be saturated. In addition, the annealing time is lengthened, and the productivity is lowered. Therefore, the number of implementations of the cooling-heating process is set to 6 or less. Also in the case of FIG. 1 , the number of implementations of the above-mentioned cooling-heating process is four. The temperature T2 which is the attained temperature of each cooling, the average temperature increase rate R, and the temperature T3 which is the attained temperature of the heating may be different from each other within a predetermined range.

[(3) Start cooling from the last temperature T3 in the cooling-heating process ([8] in Fig. 1)] Cooling is performed from the last temperature T3 in the cooling-heating process. The average cooling rate and the cooling attained temperature during this cooling are not particularly limited. From the viewpoint of further suppressing the re-precipitation of rod-shaped snow bright iron, the average cooling rate can be set to, for example, 100° C./hour or less. Also, from the viewpoint of further suppressing the excessive coarsening of Xueming Carbon, the average cooling rate is set to be 5° C./hour or more. Further, the cooling temperature can be set to be lower than (A1-30° C.), for example. For example, it cools at the said average cooling rate to the temperature range of (A1-30 degreeC) or less and (A1-100 degreeC) or more, and performs air cooling after that. Or set the cooling attained temperature lower than (A1-100° C.), for example, to further suppress the re-precipitation of rod-shaped snow-white carbon iron, and further improve the cold workability. In this case, from the viewpoint of shortening the annealing time, the cooling attained temperature may be set to (A1-250°C) or higher, further set to (A1-200°C) or higher, and further set to (A1-150°C) or higher.

The above general spheroidizing annealing (steps (1) to (3)) can be performed once or repeated multiple times. From the viewpoint of suppressing excessive coarsening of iron carbon and securing productivity, for example, it is preferably 4 times or less, and more preferably 3 times or less. When the above-mentioned spheroidizing annealing is repeated a plurality of times, it may be repeated under the same conditions or under different conditions within the range specified above. In addition, when the above-mentioned spheroidizing annealing is repeated several times, wire drawing processing may be added between spheroidizing annealing. For example, wire drawing process before spheroidizing annealing described later → first spheroidizing annealing → wire drawing process → second spheroidizing annealing may be performed in order.

In the manufacturing method of the steel wire for machine structural parts of this embodiment, the steps other than the above-mentioned spheroidizing annealing step are not particularly limited. For example, after the spheroidizing annealing, it may include a process of performing wire drawing with an area reduction ratio of preferably 15% or less for size adjustment. By setting the area reduction ratio to 15% or less, an increase in hardness before cold working can be suppressed. The area shrinkage rate is more preferably less than 10%, particularly preferably less than 8%, and more preferably less than 5%.

In order to promote the formation of the microstructure of the present invention, before the spheroidizing annealing, it is preferable to set up a process of drawing the wire at a reduction rate of more than 5%. By carrying out the wire drawing process at the above surface reduction ratio, the Xueming carbon iron in the steel is destroyed, and the subsequent spheroidizing annealing can promote the aggregation of the Xueming carbon iron, and the Xueming carbon iron can be moderately coarsened, and the soft The chemical aspect is effective. Furthermore, by performing wire drawing processing at the above-mentioned area reduction ratio, the movement of the interface and the diffusion of elements become active, thereby increasing the content of Cr and Mn in Xueming carbon iron. The surface reduction rate is more preferably 10% or more, particularly preferably 15% or more, and more preferably 20% or more. On the other hand, if the surface reduction ratio is too large, there is a possibility of causing a risk of wire breakage. Therefore, the surface reduction ratio is preferably 50% or less. When the wire drawing is performed a plurality of times, the number of times of the wire drawing is not particularly limited, and may be, for example, two times. In addition, when multiple times of wire drawing are performed, the above-mentioned "area reduction rate during wire drawing" refers to the area reduction rate from the steel material before wire drawing to the steel material after multiple times of wire drawing. [Example]

Hereinafter, the present invention will be described in more detail by way of examples. The present invention is not limited to the following examples, and it is of course possible to implement the present invention by adding appropriate changes within the range that can meet the above-mentioned and hereinafter-described purposes, and all of these are included in the technical scope of the present invention.

The test material with the chemical composition shown in Table 1 was melted in a converter, then cast to obtain a steel sheet, and the steel sheet was hot-rolled to produce a wire rod with a diameter of 12 to 16 mm. In the following Table 2, when the wire drawing process before the spheroidizing annealing is "yes", that is, the sample No. The steel wire obtained by wire drawing is subjected to spheroidizing annealing.

Annealing is performed using the above-mentioned wire rod or steel wire in a laboratory furnace. In annealing, the temperature of the wire rod or steel wire is raised to T1 shown in Table 2 and kept for t1 hours. Next, after cooling to the temperature T2 in Table 2 at an average cooling rate of 5 to 100° C./hour, it was heated to a temperature T3 at an average heating rate R shown in Table 2. The cooling and heating steps were performed for the number of times of cooling and heating shown in Table 2. Next, cooling is performed from the last heating temperature in the cooling-heating process to obtain a sample.

As a comparative example, in sample No. 12 shown in Table 3, as the manufacturing condition H1, the heat treatment process shown in FIG. 2 was implemented, that is, the heat treatment process in which the cooling-heating process was zero. Also, in this manufacturing condition H1, wire drawing was not performed at a reduction in area of 25% before annealing. Also in sample No.13 shown in Table 3, as the manufacturing condition H2, the steel wire obtained by wire drawing with a reduction in area of 25% was used before annealing, and the heat treatment process shown in FIG. 2 was implemented, that is, The cooling-heating process is a heat treatment process of 0 times.

Also as a comparative example, in sample No.14 shown in Table 3, as the manufacturing condition I, the heat treatment conditions satisfying the manufacturing conditions of Patent Document 3 are implemented, and in detail, it is expressed by SA2 in the examples of Patent Document 3. Conditions, that is, the heat treatment process shown in Figure 3 was repeated 5 times. In sample No. 18 shown in Table 3, as the manufacturing condition M, heat treatment conditions satisfying the manufacturing conditions of Patent Document 1 are implemented, and in detail, No. 5 in Table 2 No. 1 of Patent Document 1 is implemented. The spheroidizing annealing conditions, that is, the heat treatment process shown in FIG. 4 was repeated three times. In Sample No. 19 shown in Table 3, as the manufacturing condition N, the heat treatment conditions satisfying the manufacturing conditions of Patent Document 2 are implemented. The patterns shown are heat treated.

The annealing parameters T1, T2 and T3 recorded in Table 2 are the set temperatures of the heat treatment furnace. As a result of installing a thermocouple on the steel material and testing the deviation between the actual steel material temperature and the set temperature, it was confirmed that the steel material temperature is approximately the same as the set temperature.

Using the sample obtained by the above-mentioned annealing, as the evaluation of the metal structure, the average value of the grain size of ferrite grains, the average size of all the snowy carbon iron, and the total content of Cr and Mn in the snowy carbon iron were respectively It can be obtained as follows. Moreover, as characteristics, the hardness after spheroidizing annealing and the hardness after quenching treatment were measured and evaluated by the following methods.

[Evaluation of metal structure] [Average value of ferrite crystal grain size] First, the grain size of ferrite ferrite was measured as follows. Embed the test piece with resin in such a way 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 can be observed, and use nitric acid An etchant (2% by volume of nitric acid, 98% by volume of ethanol) was used as an etching solution to etch the above-mentioned test piece to reveal the structure. Next, observe the structure of the test piece showing the above-mentioned structure with an optical microscope at a magnification of 400 times. In the evaluation plane, select a field of view where ferrite crystal grains of the average size representing the structure of the entire steel wire can be observed, and obtain microscope photo. Next, the numerical value of ferric ferrite crystal grain size (G) was calculated from the photographed micrograph according to the comparison method of JIS G0551 (2020). Next, using the calculated numerical value of the ferrite grain size (G), the average value dn of the grain size of ferrite crystals was obtained according to the following formula (4). This formula (4) is described in "Introductory Lecture Terminology - Iron and Steel Materials - 3 Crystal Grain Size Number and Crystal Grain Size" Umemoto Mimi, Journal of the Japan Iron and Steel Association Vol.2(1997) No.10, p29~34 In Table 1 of p32, the relationship between the crystal grain size and the particle diameter is shown as the relationship between the ferrite crystal grain size G(orN) and the average value dn of the ferrite crystal grain size. The results are shown in Table 3. Also in this embodiment, the sample Nos. 1 to 13 in Table 3 are all, and the area ratio of fertilized iron is 90% or more.

[Average size of all Xueming carbon iron] The measurement of the average size of all snow-clear carbon and iron in the steel wire after spheroidizing annealing is to embed the test piece with resin in such a way that the cross-section can be observed, and use emery paper and diamond polishing wheel to mirror-polish the cut surface. Next, use nitric etchant (2 vol% nitric acid, 98 vol% ethanol) as the etchant for 30 seconds to 1 minute on the cut surface, so that the fat at the D/4 position (D: steel wire diameter) Granular iron grain boundaries and snowy carbon iron appear. Next, using FE-SEM (Field-Emission Scanning Electron Microscope, Field-Emission Scanning Electron Microscope, Field-Emission Scanning Electron Microscope), the structure observation of the test piece after the above-mentioned Xueming carbon iron, etc. were exposed was performed, and 3 fields of view were photographed at a magnification of 2500 times.

The OHP film was superimposed on the above-mentioned microscopic photograph, and all the snow-bright carbon iron in the microscopic photograph was coated on the OHP film to obtain a projected image for analysis. The projected image was binarized into a black and white photo, and the circle-equivalent diameters of all Xueming carbon irons were calculated using the image analysis software "Particle Analysis ver. In addition, the average size of all snow bright carbon irons recorded in Table 3 is the average value of values calculated from 3 fields of view. The minimum size (circle equivalent diameter) of Xueming carbon iron measured is 0.3μm.

[Determination of the total content of Cr and Mn in Xueming carbon iron (determination of electrolytic extraction residue)] The parts except the surface layer (less than 1mm) of the steel wire were cut or ground in such a way that about 9g of the sample could be electrolyzed to prepare the test material. This test material was immersed in an electrolytic solution (10% acetylacetone-1% tetramethylammonium chloride-methanol), and about 9 g of the above test material was subjected to constant current electrolysis by energizing. Then, the electrolyzed electrolyte solution was filtered through a filter with a pore size of 0.10 μm (manufactured by Advantec Toyo Co., Ltd., a polycarbonate-type membrane filter), and the residue remaining on the filter was used as snow bright carbon iron in steel. . Next, the above residue is dissolved in an acid solution, and analyzed by ICP emission spectrometry to obtain the amount of Cr and Mn in Xueming carbon iron, and use the total value as Cr in Xueming carbon iron in mass % and the total content of Mn {Cr+Mn}.

Moreover, the total content of Cr and Mn in steel was measured as follows in mass %. About 4 g of a sample was collected from the above sample, dissolved in an acid solution, and then analyzed by ICP emission spectrometry to determine the amount of Cr and Mn in the steel and obtain the total value [Cr+Mn]. Next, the total content {Cr+Mn} of Cr and Mn in mass % in the above Xueming carbon iron is divided by the total content of Cr and Mn in steel [Cr+Mn] in mass % to obtain the concentration The value of ratio {Cr+Mn}/[Cr+Mn].

[Characteristic evaluation] [Measurement of hardness after spheroidizing annealing] In order to evaluate cold workability, the hardness of each sample after spheroidizing annealing was measured as follows. At the D/4 position (D: steel wire diameter) of the cross-section of the test piece, a Vickers hardness test was performed in accordance with JIS Z2244 (2009). The Vickers hardness obtained by calculating the average of 3 or more points was used as the hardness after spheroidizing annealing. The measurement results are shown in Table 3. In Table 3, the hardness after spheroidizing annealing is represented by "spheroidizing hardness". In this embodiment, the hardness after spheroidizing annealing, when the amount of C (mass %), Cr (mass %), and Mo (mass %) in the steel are represented by [C], [Cr], [Mo] respectively (elements not included are zero mass %), the case where the following formula (2) is satisfied is evaluated as excellent in cold workability "OK", and the case where the following formula (2) is not satisfied is evaluated as poor cold workability "NG". Hardness (HV) after spheroidizing annealing <91([C]+[Cr]/9+[Mo]/2)+91 …(2)

其中，[元素]表示各元素的含量(質量%)，不包含的元素為0%。 [Hardness measurement after quenching treatment] In order to evaluate hardenability, the hardness of each sample after quenching treatment was measured as follows. First, as a sample for quenching treatment, each sample after spheroidizing annealing was processed into a sample having a length in the rolling direction, that is, a thickness (t) of 5 mm so that quenching could be sufficiently performed in the quenching treatment. As the quenching treatment of this sample, high-temperature holding was performed at A3+ (30 to 50° C.) for 5 minutes, and water cooling was performed after the high-temperature holding. The aforementioned A3 is a value derived from the following formula (5). The time for maintaining the high temperature here is the time counted from when the furnace temperature reaches the set temperature.

Here, [element] represents the content (mass %) of each element, and the element not contained is 0%.

Next, a Vickers hardness test was implemented at the t/2 position and the D/4 position (D: steel wire diameter, t: sample thickness) of the sample after the quenching treatment. The Vickers hardness obtained by calculating the average of three or more points was used as the hardness after the quenching treatment. The measurement results are shown in Table 3. In Table 3, the hardness after quenching treatment is represented by "quenching hardness". In this example, when the hardness after quenching treatment is represented by [C], when the amount of C (mass %) in the steel is represented by [C], the case where the following formula (3) is satisfied is evaluated as being excellent in hardenability "OK", and the case where it is not satisfied is evaluated as "OK". The case of the following formula (3) was evaluated as poor hardenability "NG". Hardness (HV) after quenching treatment > 380ln ([C]) + 1010 ... (3)

In Table 3, the case where both the hardness after spheroidizing annealing and the hardness after quenching treatment are both OK is judged to have both excellent cold workability and excellent hardenability "OK", and the hardness after spheroidizing annealing and When at least one of the hardnesses after the quenching treatment is NG, it is comprehensively judged that it is "NG" that both excellent cold workability and excellent hardenability cannot be achieved at the same time. In Table 2 and Table 3, the underlined numerical value indicates that it exceeds the specified range of the present invention or cannot satisfy desired characteristics.

Examine the results of the table. The following No. represents the sample No. in Table 3. Nos. 1 to 11 are invention examples satisfying the composition, metal structure, and spheroidizing annealing conditions specified in the embodiments of the present invention.

No.12, 20, 22 and 23 are due to the insufficient number of cooling-heating processes, the total content of Cr and Mn in Xueming carbon iron is low, or the coarsening of Xueming carbon iron is not complete, and the hardness ratio after spheroidizing annealing A high reference value results in poor cold workability.

No.13 is an example of performing annealing after wire drawing at a reduction in area of 25%. By wire drawing, the total content of Cr and Mn in Xueming carbon iron can be increased, but because the cooling-heating process is 0 times, the average size of all snow-bright carbon irons cannot be made more than a certain level, and the hardness after spheroidizing annealing is higher than the reference value, resulting in poor cold workability.

No. 14 is annealed under the annealing condition SA2 of Patent Document 3 as the production condition I satisfying the production conditions shown in Patent Document 3. Under these manufacturing conditions, the snow bright carbon iron is excessively coarsened by annealing, and the hardness after the quenching treatment is lower than the reference value, resulting in poor hardenability.

No.15 is higher than A1-17℃ because the temperature T2 is 710°C, the coarsening of Xueming carbon iron is not complete when cooling from temperature T1, and the total content of Cr and Mn in Xueming carbon iron becomes low, The hardness after the spheroidizing annealing was higher than the reference value, which resulted in poor cold workability.

No. 16 and 17 have low average heating rate R from temperature T2, the total content of Cr and Mn in Xueming carbon iron decreases, and the hardness after spheroidizing annealing does not reach the standard value, resulting in poor cold workability. Or the hardness after the quenching treatment is lower than the reference value, resulting in poor hardenability.

No. 18 is an example in which annealing is performed under the manufacturing conditions M satisfying the manufacturing conditions shown in Patent Document 1. In this manufacturing condition, especially because the heating and holding time at temperature T1 is only 0.5 hours, a large amount of small-sized rod-shaped snow-bright carbon iron remains in the crystal grains, and the average size of all snow-bright carbon iron cannot exceed a certain level. The hardness after the spheroidizing annealing was higher than the reference value, which resulted in poor cold workability.

No. 19 is annealed according to the condition c of Patent Document 2 as the production condition N satisfying the production conditions shown in Patent Document 2. In this manufacturing condition, because there is no maintenance at temperature T1, etc., a large amount of rod-shaped snow-bright carbon iron with a small size remains in the crystal grains, and the average size of all snow-bright carbon iron cannot become more than a certain level, and because the temperature T2 As a result, the average heating rate R is low, the total content of Cr and Mn in Xueming carbon iron becomes low, and the hardness after spheroidizing annealing does not reach the standard value, resulting in poor cold workability.

Because the temperature T3 of No.21 is 730°C, which is lower than (A1+8°C), the total content of Cr and Mn in Xueming carbon iron becomes lower, and the hardness after spheroidizing annealing does not reach the standard value, and it becomes poor in cold workability result.

Because No. 24~27 did not carry out the cooling-heating process or did not repeat the implementation, the coarsening of Xueming carbon iron was not complete, and the average size of all Xueming carbon iron could not be made above a certain level, and the hardness after spheroidizing annealing did not reach As a result of poor cold workability.

This application claims the priority of Japanese Patent Application No. 2021-061575 and Japanese Patent Application No. 2021-211501. Japanese Patent Application No. 2021-061575 and Japanese Patent Application No. 2021-211501 are incorporated herein by reference. [Industrial Utilization]

The steel wire for mechanical structural parts of this embodiment has low deformation resistance at room temperature when manufacturing various mechanical structural parts, and can suppress wear and tear of jigs and tools for plastic processing such as molds, and can also suppress, for example, Cracks occur during (heading) processing and exhibit excellent cold workability. Furthermore, since it is excellent in hardenability, high hardness can be ensured by quenching treatment after cold working. Therefore, the steel wire for machine structural parts of this embodiment is useful as a steel wire for machine structural parts for cold working. For example, the steel wire for mechanical structural parts of this embodiment is used for the manufacture of various mechanical structural parts such as automobile parts and construction machinery parts by providing cold working such as cold forging, cold head, cold rolling, etc. . Specific examples of such mechanical structural parts include bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch boxes, cages, housings, hubs, covers, outer boxes, gaskets, Tappets, saddles, valves, inner boxes, clutches, bushings, outer races, sprockets, cores, stators, anvils, spiders, rockers, bodies, flanges, drums, joints, connectors , pulleys, hardware, yokes, metal covers, valve ejector rods, spark plugs, pinions, steering shafts, common rails and other mechanical parts, electrical parts, etc.

[ Fig. 1] Fig. 1 is an explanatory diagram of spheroidizing annealing conditions in a method of manufacturing a steel wire for machine structural parts according to the present embodiment. [FIG. 2] It is an explanatory drawing of the heat treatment process of a comparative example. [ Fig. 3 ] is an explanatory diagram of a heat treatment process in the prior art. [FIG. 4] It is an explanatory drawing of the heat treatment process of another prior art. [FIG. 5] It is an explanatory drawing of the heat treatment process of another prior art.

Claims (5)

A steel wire for mechanical structural parts, comprising: 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 not more than 0.050% by mass, S: More than 0% by mass and not more than 0.050% by mass, Al: 0.001% by mass to 0.10% by mass, Cr: more than 0% by mass and not more than 1.5% by mass, and N: More than 0% by mass and not more than 0.02% by mass, The remainder is composed of iron and unavoidable impurities, The total content (mass %) of Cr and Mn in the metal structure of snow clear carbon iron is represented by {Cr+Mn}, and the total content (mass %) of Cr and Mn in steel is represented by [Cr+Mn] , and when the amount of C (mass %) in the steel is represented by [C], the concentration ratio {Cr+Mn}/[Cr+Mn] is (0.5[C]+0.040) or more, And when the amount of C (mass %) in steel is represented by [C], the average circle equivalent diameter of all Xueming carbon iron is (1.668-2.13[C]) μm~(1.863-2.13[C]) μm. The steel wire for mechanical structural parts as mentioned in claim 1, which satisfies one or more of the following conditions (a) to (c): (a) further comprising: selected from Cu: More than 0% by mass and not more than 0.25% by mass, Ni: More than 0% by mass and not more than 0.25% by mass, Mo: More than 0% by mass and not more than 0.50% by mass, and B: More than 0% by mass and not more than 0.01% by mass More than one species in the group formed, (b) further comprising: selected from Ti: More than 0% by mass and not more than 0.2% by mass, Nb: more than 0% by mass and not more than 0.2% by mass, and V: More than 0% by mass and not more than 0.5% by mass More than one species in the group formed, (c) further comprising: selected from Mg: More than 0% by mass and not more than 0.02% by mass, Ca: More than 0% by mass and not more than 0.05% by mass, Li: More than 0% by mass and not more than 0.02% by mass, and REM: More than 0% by mass and not more than 0.05% by mass More than one species in the group formed. The steel wire for mechanical structural parts according to claim 1 or 2, wherein, The average value of ferrite grain size is 30 μm or less. A method of manufacturing steel wire for mechanical structural parts as described in Claim 1 or 2, comprising: performing the following (1)~( 3) The spheroidizing annealing process of the process, (1) After heating to the temperature T1 of (A1+8°C)~(A1+31°C), heating and maintaining at the temperature T1 for more than 1 hour and less than 6 hours, (2 ) is cooled to a temperature T2 exceeding 650°C and below (A1-17°C), and then heated to a temperature T3 of (A1+8°C)~(A1+31°C) at an average heating rate of 75°C/hour to 160°C/hour The cooling-heating process, and the cooling-heating process is implemented for a total of 2 to 6 times, (3) cooling is carried out from the last temperature T3 of the cooling-heating process. Here, A1 is according to the following formula (1) Work out:

Here, [element] represents the content (mass %) of each element, and the content of elements not included is zero. The manufacturing method of the steel wire for mechanical structural parts as described in claim 4, wherein, The aforementioned bar steel is a steel wire obtained by drawing a wire rod at an area reduction rate of more than 5%.

Images