JP3854878B2 - Low carbon sulfur-based free-cutting steel wire and method for producing the same - Google Patents

Description

【００４０】
【表２】

【００４１】
【表３】

【００４２】
［実施例２］
表１に記載の鋼種Ａを使用し、ビレット加熱温度１０００〜１０２５℃とし、仕上圧延温度７５０〜８００℃とし、圧延後、ステルモアラインでの平均冷却速度を表４に示すように種々変化させて、線径９．５mmの熱間圧延線材を得た。その後、伸線加工率を２９％として伸線し、線径８．０mmの伸線材を得た。
【００４３】
得られた線材について、実施例１と同様、硫化物の平均幅を測定したところ２．７〜３．１μm であり、log ｄ（ｄ＝８．０）＝０．９０より大きい値であった。また、機械的性質および被削性を調べた。これらの結果を表４に併せて示す。log Ｖmin ＝１．１３（１−log Ｄ）で求められる限界冷却速度は１．０６℃／ｓである。
【００４４】
表４より、熱間圧延後のステルモアライン上での平均冷却速度を限界冷却速度以上で冷却することにより、所定の降伏比が得られ、良好な被削性が得られることが確認された。
【００４５】
【表４】

【００４６】
【発明の効果】
本発明の低炭素硫黄快削鋼線材は、所定成分、所定降伏比の下で、硫化物系介在物の平均幅を線材の直径ｄ（伸線材）、Ｄ（熱間圧延線材、熱間鍛造線材）とするとき２．８＊log ｄ（Ｄ）以上とするので、Ｐｂなどの特殊元素を用いない場合であっても、良好な被削性を得ることができる。また、本発明の製造方法によれば、本発明にかかる鋼線材を工業的に容易に製造することができ、生産性に優れる。
【図面の簡単な説明】
【図１】硫化物の形態を規定する寸法の測定要領説明図である。
【図２】硫化物の観察領域を示す線材の横断面図である。
【図３】実施例における被削性試験の切削要領説明図である。
【図４】実施例における硫化物の平均幅および２．８＊log ｄ（Ｄ）と、発明例・比較例（被削性の優劣）との関係を示すグラフである。[0001]
[Technical field to which the invention belongs]
The present invention relates to a low-carbon sulfur-based free-cutting steel wire excellent in machinability and a method for producing the same even when a special element such as Pb is not contained. In addition, the steel wire described here includes not only a hot-rolled or hot-forged steel wire but also a steel wire that has been drawn (cold drawing) thereafter.
[0002]
[Prior art]
Low-carbon sulfur free-cutting steel with a large amount of S added is used for screws, nipples, etc., which are parts that do not place much emphasis on mechanical properties, and are mainly small parts manufactured by cutting. Further, as a free-cutting steel having excellent machinability, a composite free-cutting steel containing Pb in addition to S is also widely used. However, since Pb is a harmful substance that is harmful to health, it is desired to reduce the amount of Pb used in free-cutting steel. Te may also be used, but it is toxic and at the same time hinders hot workability, so a reduction is required.
[0003]
Many studies have been made to improve the machinability of low-carbon sulfur-based free-cutting steel. For example, as described in Japanese Patent No. 1605766, Japanese Patent No. 19007099, Japanese Patent No. 2129869, Japanese Patent Laid-Open No. 9-157771, and Japanese Patent Laid-Open No. 11-293391, most of them are the number and size of sulfide inclusions. This relates to the control of the form. Further, for example, as described in Japanese Patent No. 1605766, Japanese Patent No. 19007099 (Japanese Patent Publication No. 4-54736), Japanese Patent No. 2922105, Japanese Patent Laid-Open No. 9-71838, Japanese Patent Laid-Open No. 10-158781, There are many that define oxide inclusions.
On the other hand, the structure and characteristics (matrix characteristics) other than inclusions also have an important influence on machinability, but there are few techniques that focus on these, for example, Japanese Patent No. 2125814 (Japanese Patent Publication No. 1-11069) A striped pearlite structure continuous in the rolling direction is specified in Japanese Patent No. 2740982 to define the amount of solid solution C in pro-eutectoid ferrite.
[0004]
[Problems to be solved by the invention]
Although the technique disclosed in the above publication is important, sufficient machinability has not been obtained yet. For example, in the technique disclosed in Japanese Patent No. 19007099, for inclusions in steel, MnS having a major axis of 5 μm or more, a minor axis of 2 μm or more, and a major axis / minor axis ratio of 5 or less is 50% or more of all MnS inclusions. The content of Al ₂ O _{3 in} the oxide inclusions is defined as 15% or less on average, but it is essential that the total amount of Pb, Bi and Te be 0.2% or more. Without the addition of these elements, sufficient machinability cannot be obtained.
The present invention has been made in view of such a problem, and even when a special element such as toxic Pb, Bi, or Te is not added, the low carbon sulfur-based free cutting steel wire having excellent machinability. And it aims at providing the suitable manufacturing method.
[0005]
[Means for Solving the Problems]
The amount and distribution of sulfide inclusions affect machinability, but these are almost determined by the steel components and melting / casting conditions. The present invention does not control the amount and distribution of sulfide inclusions, but in the form of sulfide inclusions, the width of the inclusions that particularly affect machinability is expanded at the hot rolling stage. In this way, the present invention has succeeded in obtaining excellent machinability by maintaining it larger than a predetermined size according to the wire diameter. That is, the low carbon sulfur based free cutting steel wire of the present invention is mass%,
C: 0.02 to 0.15%,
Mn: 0.50 to 2.0%,
P: 0.05-0.20%
S: 0.15-0.50%,
Si: 0.01% or less,
Al: 0.01% or less,
N: 0.002 to 0.02%,
O: 0.01 to 0.03%
Or further containing at least one of Bi: 0.3% or less, Pb: 0.4% or less, Te: 0.1% or less, B: 0.01% or less, the balance Fe and inevitable impurities Is a steel wire drawn after hot rolling or hot forging, and when the diameter of the steel wire is d (mm), in the region from the outer peripheral surface to a depth of 0.1 mm to d / 8 The sulfide inclusions have an average width (μm) of 2.8 * logd or more, and the yield ratio of the wire is 0.96 or more. The “*” means a multiplication symbol, and “log” means a common logarithm. The same applies below.
[0006]
Another sulfur low-carbon free-cutting steel wire rod according to the present invention is a steel wire rod having the above-described components and hot-rolled or hot-forged, and the diameter of the steel wire rod is D (mm). The average width (μm) of sulfide inclusions in the region from the outer surface to a depth of 1 μm to D / 8 is 2.8 * logD or more, and the yield ratio of the wire is 0.68 or more It is.
[0007]
Moreover, the manufacturing method of this invention heats the steel slab which has the said component to 1000 degreeC or more, performs hot rolling by making a finishing rolling temperature into 700 degreeC or more and less than 800 degreeC in surface temperature, and hot-rolled steel wire rod When the average cooling rate V (° C./s) from immediately after being placed on the stealmore line to at least 500 ° C. is D (mm), 1) A method for producing a low-carbon sulfur-based free-cutting steel wire that is air-cooled at a critical cooling rate Vmin or higher that satisfies the equation (1) In addition, another production method of the present invention is a low carbon sulfur system in which a steel wire having a yield ratio of 0.96 or more is obtained by further drawing after obtaining a hot-rolled steel wire by the production method. This is a method of manufacturing a free-cutting steel wire.
log Vmin = 1.13 (1-log D)
[0008]
DETAILED DESCRIPTION OF THE INVENTION
First, the composition (unit mass%) of the low-carbon sulfur-based free-cutting steel wire of the present invention will be described.
C: 0.02-0.15%
C is added to secure the strength of the steel, but if it is less than 0.02%, the strength is insufficient, and at the same time, the toughness and ductility become excessive and the machinability also decreases. On the other hand, if it exceeds 0.15%, the strength becomes excessively high, and the machinability deteriorates. For this reason, the lower limit of C is 0.02%, preferably 0.04%, and the upper limit is 0.15%, preferably 0.12%.
[0009]
Mn: 0.50 to 2.0%
Mn combines with S in the steel to form sulfides and improves machinability. Moreover, the red hot embrittlement by FeS production | generation is suppressed. In order to exert these effects, the lower limit is made 0.50%, preferably 0.75%. However, even if added in excess of 2.0%, the strength increases excessively, and the machinability decreases on the contrary, so the upper limit is made 2.0%, preferably 1.8%.
[0010]
P: 0.05-0.20%
P has an effect of improving machinability, and addition of 0.05% or more is effective. On the other hand, even if added over 0.20%, the effect is saturated, so 0.20% is made the upper limit.
[0011]
S: 0.15-0.50%
S is an element that improves the machinability by forming a sulfide, and if it is less than 0.15%, such an effect is insufficient. On the other hand, if added over 0.50%, the hot workability may be degraded. Thus, the lower limit is 0.15%, preferably 0.25%, while the upper limit is 0.50%, preferably 0.45%.
[0012]
Si: 0.01% or less, Al: 0.01% or less Si and Al may be used as deoxidizers, but if both are added in excess of 0.01%, a hard oxide is formed and the material is cut. Sexually begins to decline. For this reason, each is stopped to 0.01% or less.
[0013]
N: 0.002 to 0.02%
N has an effect of improving machinability, particularly surface roughness. If it is less than 0.002%, such an effect is insufficient. On the other hand, adding over 0.02% not only saturates the effect, but also reduces hot workability. For this reason, the lower limit is made 0.002% and the upper limit is made 0.02%.
[0014]
O: 0.01 to 0.03%
O is an element that affects the size and form of MnS and improves the machinability. For this purpose, 0.01% or more is necessary, but if added over 0.03%, hard oxides increase and machinability deteriorates. For this reason, the upper limit is made 0.03%.
[0015]
In addition to the above components, the steel wire rod of the present invention is composed of the remaining Fe and unavoidable impurities. In addition, one or more of Bi, Pb, Te, or further B can be added within the following range.
[0016]
Bi: 0.3% or less Bi is an element that improves machinability by the same effect as Pb. However, even if added over 0.3%, the effect is saturated, so 0.3% or less.
[0017]
Pb: 0.4% or less Pb is an element that should be avoided because it is toxic, but may be added because it has a significant effect on improving machinability. However, even if added over 0.4%, the effect is saturated, so it is limited to 0.4% or less.
[0018]
Te: 0.1% or less Te may be added because the machinability is remarkably improved due to the effect of suppressing expansion during hot working of MnS. However, even if it exceeds 0.1%, the effect is saturated, so the upper limit is made 0.1%.
[0019]
B: 0.01% or less B is an element that improves hot workability and may be added. However, if it exceeds 0.01%, the hot workability is conversely reduced, so it is limited to 0.01% or less.
[0020]
Next, the form of the sulfide inclusions in the steel wire will be described in detail.
The amount and distribution of sulfide inclusions such as MnS (hereinafter simply referred to as sulfide) is almost determined by the composition and melting / casting conditions, but the form is determined by the hot rolling and hot forging processes after casting. Change. The more spherical the sulfide is, the harder it is to stretch during rolling and forging, and a larger width form after processing. The width of the sulfide has a great influence on the machinability even in a hot-rolled, hot-forged steel wire or a wire drawn thereafter, and in general, the larger the width, the better the machinability. . However, the required average width differs depending on the wire diameter. For example, when sulfides having the same volume, number, and form (width) are present in the wire, the smaller the wire diameter, the better the machinability, and the larger the wire diameter, the lower the machinability. If attention is paid to the form, the machinability can be improved by using a sulfide having a sufficient width even if the wire diameter is large.
[0021]
There are various methods for defining the form of sulfide. In the present invention, the average width is defined. As shown in FIG. 1, the average width means that the maximum length of each sulfide is the major axis L, and the largest width in the direction perpendicular to the major axis L is the maximum width W. The average of the maximum width W of all sulfides. The reason why the average width is used as a form index is that the measurement reliability and reproducibility are the highest. On the other hand, the major axis of the sulfide and the area of the inclusion have a large error due to measurement, and it is difficult to obtain reliable data.
[0022]
The present inventor investigated the relationship between the average width of sulfide and the wire diameter (diameter) of the steel wire on the machinability. As is apparent from the examples described later, the required average width is It was found that when the diameter was d (drawn wire) or D (hot rolled wire, hot forged wire), it was 2.8 * log d or more or 2.8 * log D or more. If the maximum width of the sulfide inclusions is less than this, the machinability, particularly the surface roughness, is lowered.
[0023]
In addition to compounds mainly composed of S typified by MnS, sulfides include sulfides in which oxygen is dissolved or complexed with oxides. This is because these are also effective in improving machinability. The maximum width of each sulfide is obtained by image analysis of an optical microscope observation result at a magnification of 100 times, but the observation position is important, and the following regions are observed. As shown in FIG. 2, the most important part for the machinability is a region from the position of a depth of 0.1 mm to the depth of d / 8 or D / 8 from the outer peripheral surface, and this region is observed. At the time of observation, the area of the measurement region is 6 mm ² or more on a plane parallel to the rolling / forging direction. Moreover, it is only necessary to observe with polishing, and etching is not necessary. Exclude inclusions with a major axis of less than 1 μm, and measure and analyze the maximum width. This is because inclusions having a major axis of less than 1 μm have a large measurement error and a small effect on machinability.
[0024]
In addition, in Japanese Patent No. 19007099 (Japanese Patent Publication No. 4-54736), it is prescribed that the minor axis is 2 μm or more as one of the defining elements of sulfide, but the same provision is used regardless of the diameter of the wire. Then, when the wire diameter is large, the machinability improvement effect cannot be expected unless the maximum width of the sulfide is increased. In addition, the publication is unquestioned about the yield ratio described later, which is an important requirement in the present invention, and is different from the present invention.
[0025]
In order to control the maximum width of the sulfide, it is necessary to optimize the slab heating temperature at the time of hot rolling and hot forging, and it is necessary to set at least 1000 ° C. or higher, preferably 1040 ° C. or higher. It is good to do. If it is less than 1000 degreeC, it will become difficult to suppress extension of sulfide in subsequent rolling and forging, and to enlarge the average width of sulfide. The heating temperature of the billet is measured when the billet leaves the furnace. In addition to controlling the billet heating temperature, measures such as slowing the cooling rate during solidification, increasing the amount of oxygen in the molten steel, and reducing strong deoxidizing elements such as Al and Si are simultaneously performed. It can also be expected to increase the average width of the sulfide. However, if the amount of oxygen in the molten steel is too large, hard oxides that adversely affect the machinability are generated, so care must be taken.
[0026]
In order to improve machinability, it has been found that it is important to set not only the average width of sulfides but also the yield ratio (yield strength / tensile strength) YR to a specific value or more.
That is, in the drawn steel wire rod, YR needs to be 0.96 or more. If it is less than 0.96, not only the surface roughness but also the dimensional change in the machinability becomes large, and the tool life defined by the dimensional change is reduced. Desirably, it is 0.97 or more. By increasing the yield ratio, the shear deformation energy in the shear region where chips are generated decreases, and at the same time, shearing of the chips occurs stably. As a result, cutting resistance is reduced, tool wear is reduced, and dimensional change is also improved. In addition to the above, the surface roughness is improved by stabilizing the cutting. In addition, YR in the low carbon sulfur free-cutting steel equivalent to the present invention (SAE, AISI standard) is usually about 0.77 to 0.84 (JSSC, VOL.8, NO.75, 1972, p55, Table 3-1).
[0027]
On the other hand, YR is set to 0.68 or more in the wire rod after hot rolling before hot drawing and hot forging. Desirably, it should be 0.70 or more. In order to satisfy the machinability when cutting after rolling / forging, it is necessary to make it 0.66 or more. Even when wire drawing is performed after hot working, unless this characteristic is satisfied, it is difficult to set the YR of the wire after drawing to 0.96 or more under normal cold drawing conditions. In steels with low tensile strength, the yield ratio tends to be low. Therefore, in order to achieve the above YR value, it is preferable to set it to 440 MPa or more from the viewpoint of tensile strength. In addition, YR of the steel material as hot-rolled in low carbon sulfur free-cutting steel (SAE, AISI standard) equivalent to the present invention is usually about 0.50 to 0.60 (Table 3-1 in the above-mentioned document). ).
[0028]
In order to control the yield ratio of the wire rod by hot rolling, it is important to control the finish rolling conditions and the cooling rate after rolling. In addition, in the case of hot forging, it is important to control the finish forging conditions and the cooling rate after forging. However, these conditions are the same as those in the case of hot rolling, so the case of hot rolling will be described in detail below. .
[0029]
In the present invention, the finish rolling temperature is 700 ° C. or more and less than 800 ° C. as the surface temperature. If it is less than 700 degreeC, it will roll in a two-phase area | region, and the structure | tissue after rolling will become non-uniform | heterogenous, As a result, mechanical properties, such as a yield ratio, will deteriorate. On the other hand, in rolling at 800 ° C. or higher, a fine structure cannot be obtained, and a predetermined yield ratio cannot be obtained. In order to constantly obtain a yield ratio of 0.68 or more with a wire rod obtained by hot rolling and hot forging and 0.96 or more with a wire rod drawn by drawing as in the present invention, the structure is further increased as compared with the conventional level. Miniaturization is indispensable, and for that purpose, it is necessary to perform the finish rolling temperature at 700 ° C. or more and less than 800 ° C. on the surface of the steel material as described above. Under such conditions, the tensile strength of the hot rolled material and the hot forged material can be set to 440 MPa or more. Conventionally, the finish rolling temperature and the slab heating temperature are often linked, but in order to obtain the average width and yield ratio of the sulfide according to the present invention, it is necessary to control both independently. That is, the steel slab heating temperature is preferably high in order to increase the average width of the sulfide inclusions, but the finish rolling temperature needs to be low.
[0030]
In addition, as described in JP-A-1-224103, JP-A-5-247585, and JP-A-5-333192, the finish rolling temperature has conventionally been 800 to 1000 ° C, or 800 to 900 ° C. Generally, the finish rolling temperature is too high as compared with the present invention. In JP-A-1-224103, the finish rolling temperature is lowered than the heating temperature of the steel slab, and the rolling is performed in the range of 800 to 1000 ° C., thereby making the microscopic structure after rolling uniform with fine grains, It describes the need to increase the yield ratio. However, the actual yield ratio and tensile strength are not described, and it is clear that the yield ratio according to the present invention cannot be obtained.
[0031]
Regarding the cooling rate after hot rolling, in the present invention, when the hot-rolled steel wire rod is cooled by the stealmore line, the average cooling rate V (from 500 to at least 500 ° C. immediately after the steel wire material is substantially placed on the stealmore line. When the diameter of the steel wire rod is D (mm), the air cooling is performed at a critical cooling rate Vmin or more that satisfies the following formula (1). “Mounting in real time” means mounting at the first location where the air-cooling facility is located. Strictly speaking, the cooling rate of the wire rod when cooled by the stealth conveyor is different depending on the sparse part and the dense part of the wire coil coil, but means an average cooling rate of these cooling rates.
log Vmin = 1.13 (1-log D) (1)
[0032]
In cooling in the stealth line, the average cooling rate up to 500 ° C is a problem. Below 500 ° C, phase transformation and crystal grain growth are difficult to occur, and the effect on machinability is reduced. It is because the influence of can be ignored. The present inventor considered that the cooling rate has an influence on the structure of the hot-rolled wire rod, and in turn has an influence on the machinability under the finishing rolling temperature, and thus various cooling operations. As a result of observing the structure under conditions, the yield ratio was hot-rolled by air-cooling the average cooling rate V (° C / s) up to 500 ° C at or above the critical cooling rate Vmin that satisfies the above equation (1). It has been found that 0.68 or more can be secured with a wire, or 0.96 or more can be secured with a further drawn wire. This is because the structure of the rolled material can be uniformly refined by increasing the cooling rate as described above, and the yield ratio is increased by obtaining such a uniform microstructure.
[0033]
In addition, there are few examples examined about the influence of the cooling process after hot rolling and hot forging on the machinability of the low carbon sulfur free cutting steel wire. Japanese Patent No. 2125814 (Japanese Patent Publication No. 7-11069) describes that it is important to control striped pearlite, and it is desirable that the cooling rate after rolling and forging is slower, but the cooling rate is fixed. Although not specifically shown, as an example of cooling, an example in which the wire is covered and gradually cooled on a steermore line is shown, and it can be seen that the cooling is quite slow. For example, a sample having a diameter of 18 mm, which is one of the examples, is expected to have a slow cooling rate of 0.41 ° C./s or less, which is slower than the limit value of 0.51 ° C./s according to the equation (1). . In addition, in Japanese Patent No. 2740982, machinability is improved by gradually cooling to 500 to 700 ° C. to precipitate pro-eutectoid ferrite and then rapidly cooling to leave solid solution C in the pro-eutectoid ferrite. Has been shown to let However, the heat treatment to austenite may be after reheating from room temperature, or just after hot working, and the final rolling temperature (finish rolling temperature) is not a requirement as in the present invention, No disclosure or suggestion.
[0034]
In addition to the finish rolling temperature and cooling rate in the above hot rolling, the cold drawing ratio after hot rolling affects the yield ratio of the wire drawing material. The drawing rate is usually about 5 to 30%. Outside this range, it becomes difficult to ensure roundness and linearity, and there are problems such as excessive mechanical impossibility, and it is difficult to make significant changes. For this reason, it is necessary to ensure the yield ratio of 0.68 at the stage of obtaining a hot-rolled wire that is a material for the wire drawing material. Thereby, a yield ratio of 0.96 or more can be ensured by the normal wire drawing rate.
[0035]
【Example】
[Example 1]
Average cooling rate up to 500 ° C. after melting steels with various compositions (mass%) shown in Table 1 in an actual furnace and then being placed on the various rolling conditions and Stealmore lines shown in Table 2 (Hereinafter, simply referred to as an average cooling rate), a hot-rolled wire was produced. Thereafter, a part of the wire was removed and further wire drawing was performed. The drawing rate and final wire diameter in this case are also shown in the same table.
[0036]
For the obtained wire, the average width of the sulfide was measured in the manner described above. Further, mechanical properties were measured by a tensile test (JISZ2241). Moreover, the machinability test was done in the following manner. As shown in FIG. 3, a wire rod W is fixed to an automatic lathe so as to be rotatable about its axis, and a high speed tool (SKH4) B is fed perpendicularly to the wire rod W to form it, and then a wire rod piece after cutting The finished surface roughness of the forming part F was measured. Moreover, after 3000 pieces were cut , the dimension of the forming part F of the wire piece cut first and the wire piece cut last was measured, and the dimensional change was calculated | required. In addition, the machinability test was carried out after removing the scale by cutting or centerless grinding with respect to the hot-rolled wire. The forming conditions were a cutting speed of 92 m / min, a tool feed speed of 0.03 mm / rev, and a cutting depth of 1.0 mm.
[0037]
These measurement results are shown in Table 3. In the table, a finished surface roughness of less than 42 μm and a dimensional change of less than 20 μm were used as indicators of excellent machinability, and samples satisfying this were taken as invention examples. Further, for a sample that satisfies only the yield ratio condition of the present invention, log d (d: diameter mm of the drawn wire) or log D (D: diameter mm of the hot rolled wire) and the average sulfide width μm A graph in which the relationship is divided between the invention example and the comparative example is shown in FIG. The broken line in FIG. 4 is a straight line having an average width (μm) = 2.8 * log d (D).
[0038]
From Table 3 and FIG. 4, in the sample with the yield ratio of the rolled material of 0.68 or more, the yield ratio of the wire after the drawing has also achieved 0.96 or more, and the average width of the sulfide is 2.8 log. Those having d (D) or more have good surface roughness and dimensional change, and are excellent in machinability. Steel type D of sample No. 12 has a large amount of Si and Al, which are strong deoxidizing elements, and a small amount of oxygen. Therefore, an appropriate sulfide cannot be obtained, and the average width is smaller than the standard. For this reason, machinability is also reduced.
[0039]
[Table 1]

[0040]
[Table 2]

[0041]
[Table 3]

[0042]
[Example 2]
Steel type A listed in Table 1 was used, billet heating temperature was 1000 to 1025 ° C., finishing rolling temperature was 750 to 800 ° C., and after rolling, the average cooling rate in the Stemmore line was varied as shown in Table 4. Thus, a hot rolled wire rod having a wire diameter of 9.5 mm was obtained. Thereafter, the wire was drawn at a wire drawing rate of 29% to obtain a wire drawing material having a wire diameter of 8.0 mm.
[0043]
About the obtained wire, the average width of the sulfide was measured in the same manner as in Example 1, and was 2.7 to 3.1 μm, which was larger than log d (d = 8.0) = 0.90. . In addition, mechanical properties and machinability were investigated. These results are also shown in Table 4. The critical cooling rate obtained by log Vmin = 1.13 (1-log D) is 1.06 ° C./s.
[0044]
From Table 4, it was confirmed that a predetermined yield ratio was obtained and good machinability was obtained by cooling the average cooling rate on the steermore line after hot rolling at or above the critical cooling rate. .
[0045]
[Table 4]

[0046]
【The invention's effect】
The low-carbon sulfur free-cutting steel wire of the present invention has a predetermined component and a predetermined yield ratio, and the average width of sulfide inclusions is the wire diameter d (drawn wire), D (hot rolled wire, hot forged) Since it is 2.8 * log d (D) or more when the wire is used, good machinability can be obtained even when a special element such as Pb is not used. Moreover, according to the manufacturing method of this invention, the steel wire concerning this invention can be manufactured easily industrially, and it is excellent in productivity.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is an explanatory diagram of a measuring procedure of dimensions that define the form of sulfide.
FIG. 2 is a cross-sectional view of a wire showing an observation region of sulfide.
FIG. 3 is an explanatory diagram of a cutting procedure of a machinability test in an example.
FIG. 4 is a graph showing the relationship between the average width of sulfide and 2.8 * log d (D) in Examples and Invention Examples / Comparative Examples (Machinability).

Claims (5)

mass%
C: 0.02 to 0.15%,
Mn: 0.50 to 2.0%,
P: 0.05-0.20%
S: 0.15-0.50%,
Si: 0.01% or less,
Al: 0.01% or less,
N: 0.002 to 0.02%,
O: 0.01 to 0.03%
A steel wire rod that is drawn after hot rolling or hot forging, comprising the balance Fe and inevitable impurities,
When the diameter of the steel wire is d (mm) , “*” is a multiplication symbol, and “log” is a common logarithm , the sulfide inclusions in the region from the outer surface to a depth of 0.1 mm to d / 8 A low-carbon sulfur-based free-cutting steel wire having an average width (μm) of 2.8 * logd or more and a yield ratio of the wire of 0.96 or more. mass%
C: 0.02 to 0.15%,
Mn: 0.50 to 2.0%,
P: 0.05-0.20%
S: 0.15-0.50%,
Si: 0.01% or less,
Al: 0.01% or less,
N: 0.002 to 0.02%,
O: 0.01 to 0.03%
A hot-rolled or hot-forged steel wire consisting of the balance Fe and inevitable impurities,
The average width of sulfide inclusions in the region from the depth of 1 μm to D / 8 when the diameter of the steel wire is D (mm) , “*” is a multiplication symbol, and “log” is a common logarithm. A low-carbon sulfur-based free-cutting steel wire having a (μm) of 2.8 * log D or more and a yield ratio of the wire of 0.68 or more. The steel component further contains at least one of Bi: 0.3% or less, Pb: 0.4% or less, Te: 0.1% or less, and B: 0.01% or less. The low-carbon sulfur-based free-cutting steel wire described in 1. A steel wire rod obtained by heating a steel slab having the component described in claim 2 or 3 to 1000 ° C or higher, performing hot rolling at a surface rolling temperature of 700 ° C or higher and lower than 800 ° C, and hot rolling. During cooling with the Stealmore line, the average cooling rate V (° C / s) from immediately after mounting on the Stealmore line to at least 500 ° C is used, the diameter of the steel wire is D (mm) , and "log" is commonly used A method for producing a low-carbon sulfur-based free-cutting steel wire that is air-cooled at a critical cooling rate Vmin or higher that satisfies the following formula (1) when logarithmic .
log Vmin = 1.13 (1-log D)   A low carbon sulfur free-cutting steel wire for producing a steel wire having a yield ratio of 0.96 or more by further drawing after obtaining a hot-rolled steel wire by the production method according to claim 4. Production method.

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