JP3255611B2 - Free-cutting steel rod and wire excellent in drilling workability and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-free-cutting steel rod or wire having excellent drilling workability at the center of a cross section of a steel material. Technology for producing ultra-free-cutting steel rods and wires with excellent drilling properties that do not cause misalignment by adjusting the hardness so that the central part is lower than the surroundings so that the steel can be easily precipitated. It is.

[0002]

2. Description of the Related Art Conventionally, super free-cutting steels having excellent machinability include JIS G 4804 S with a combined addition of sulfur and lead.
UM24L or SUM43L with Pb added to SUM43
Are typical examples. In these free-cutting steels, machinability is more important than toughness such as impact value and elongation, but the machinability required for these is not only turning and parting off the outer periphery, but also tools such as drills. It is important that the steel has excellent drilling workability at the center of the cross section of the steel material.

On the other hand, in the final solidified portion at the center of the axial center of a slab or a steel ingot cast by a continuous casting method or an ingot casting method, center segregation of components is generally formed. And even if these are hot worked, the initial center segregation remains. For the production of rods and wires, slabs cast by the continuous casting method or ingot casting method and slabs obtained by slab-rolling the slabs are used, but both the slabs and the slabs have a high component concentration. A center segregation part is formed. The hardness of such a center segregation portion is usually higher than the surrounding portion.

[0004] In drilling, if the center of the work material is hard, wear of the tool tip is large, and the tool life is shortened.
Also, if the hardness at the center is harder than the surroundings, the tool tip will shift to the soft surrounding part in drilling,
The product cannot be used as a product due to so-called misalignment. Therefore, as for a free-cutting steel rod wire to be subjected to drilling, it is necessary that the hardness at the center is equal to or lower than that of the surrounding part.

[0005] In drilling, if the chips are not properly divided into small pieces, the discharge of chips from the holes is poor.
It may cause tool breakage. In recent years, automatic machines are often used for unmanned machining.If chips are long and entangled, it becomes necessary to stop the machine and perform extra work to remove the chips, thereby reducing productivity. Will be reduced. Therefore, along with the long tool life,
There is a need for a free-cutting steel with excellent chip disposability, in which the chips are finely divided into appropriate sizes.

[0006] In order to improve the tool life and chip disposability, conventionally, as shown in SUM23L and SUM43L, lead, which is a free-cutting element, is added to sulfur-phosphorus combined free-cutting steel in an amount of 0.10-0. The machinability has been improved by adding 0.35%.

[0007] Since the melting point of Pb is as low as 327 ° C, Pb melts during cutting and the steel becomes brittle, thereby improving chip controllability. In addition, the lubricating action of Pb is added, and the life of the tool is extended. However, the use of Pb in free-cutting steel
Due to environmental health problems such as generation of fume, lead-free ultra-free-cutting steel is required today.

Elements that improve the machinability of steel materials include:
Elements other than Pb such as S, Ca, Bi, Se, and Te are known, but these elements alone have a small machinability improvement effect, are expensive, and have environmental health problems.
Has at least one of the drawbacks described above, which limits its use as an alternative to lead.

For example, S is effective in improving machinability, but when a large amount of S is added, M elongates in the hot working direction.
There is a problem that a large amount of nS is formed to cause anisotropy in mechanical properties or to decrease toughness. For this reason, in the conventional SUM24L and SUM43L, the tip crack is easily caused at the time of hot rolling, which causes a rolling trouble. In order to avoid this trouble, it was necessary to perform a complicated operation such as sharpening the tip of the steel slab before rolling into a shape similar to the tip of a pencil. Further, since SUM24L is a low-carbon steel, when imparting wear resistance to a machined part, it was necessary to perform carburizing and quenching at around 900 ° C. for several hours for a long time.

[0010] On the other hand, graphite is an element that greatly improves machinability, as seen in cast iron. However, when carbon is added to steel, cementite precipitates, so that it is not easy to obtain graphite. In the case of steel having a carbon concentration of 0.10 to 1.5% in the conventional invention, for example, Japanese Patent Application Laid-Open No. 2-107742 (hereinafter referred to as Prior Art 1)
And JP-A-3-140411 (hereinafter referred to as prior art 2).
) At a temperature of 600 to 800 ° C for several hours to 20 hours.
A steel material that precipitates graphite by performing annealing for as long as 0 hours, or a method for producing such a steel material is disclosed.

However, such a long-time graphitization heat treatment not only causes an increase in cost, but also causes a problem that the steel material is decarburized during the heat treatment and adversely affects the performance of the final part. Therefore, there is a demand for a lead-free ultra-free-cutting steel excellent in the desired center drilling property by a simpler heat treatment than before, preferably without heat treatment.

[0012]

The prior art 1 described above,
2 has any of the following problems unresolved. Problem 1: Due to the center segregation, the hardness of the center is high, so that the tool life is short and the center is misaligned.

Problem 2: The free-cutting elements used are toxic and have problems in environmental measures. Problem 3: Since a large amount of S and P are contained in a complex manner,
It has poor hot workability and requires special machining to prevent cracks at the end of the slab before rolling.

Problem 4: It is necessary to perform carburizing and quenching for a long time in low-carbon free-cutting steel in order to improve wear resistance. Problem 5: Machinability can be improved by using carbon as a non-toxic free-cutting element and precipitating it as graphite, but it must be subjected to long-time graphitizing annealing.
Cost increases.

Therefore, in order to solve the above-mentioned problems and to produce parts for automobiles and industrial machines excellent in machinability, the present invention is intended to gradually cool steel after hot rolling or to heat it for a short time. The purpose of the present invention is to provide a free-cutting steel rod and wire that does not contain lead, is excellent in not only peripheral machining but also has excellent drilling workability at the center of the cross section, is inexpensive, and has no environmental health problems. I do.

[0016]

SUMMARY OF THE INVENTION In view of the above-mentioned background and viewpoint, the present inventors have achieved excellent drilling workability equal to or higher than that of conventional sulfur-lead composite free-cutting steel without adding lead. We worked diligently to develop super free cutting steel. As a result, the following findings were obtained.

That is, in order to precipitate graphite by air-cooling after rolling or by a simple heat treatment such as normalizing, a steel is added to the steel in a composition of more than 1.00% to obtain a hypereutectoid steel, Si is increased to 1.00% or more to promote graphitization. Also, an appropriate amount of Mn is added to secure the ductility of the steel,
Further, a steel in which impurity elements such as P and S are suppressed to a low level is prepared. In the solidification process of molten steel in the continuous casting method or the ingot casting method, component segregation is inevitably formed at the center of the axis of the slab. Conventionally, this center segregation was harmful to the quality of the steel material. However, in the present invention, the technology utilizing the center segregation as follows is abolished. Degree of segregation of carbon concentration [C] / [C] _{0 at the} center segregation part (where [C] is the carbon content of the target position, and [C] ₀ is the carbon content of raw steel analysis)
In the range of 1.01 to 2.00, or a slab obtained by slab rolling the slab. However, the steel ingot may be treated in the same manner as the slab.

Next, the slab or the slab is heated at a predetermined temperature and then hot-rolled, and then slowly cooled at a predetermined cooling rate or lower, or cooled at an arbitrary cooling rate, and then cooled to 60 ° C.
After heating to a temperature between 0 and 900 ° C. for 3 hours or less, air cooling or any of the treatments is performed.

By the above-mentioned treatment, a graphite rod having an appropriate size and amount necessary for achieving the object of the present invention and a soft ferrite or a ferrite or ferrite + cementite structure steel bar is obtained. In the center of the cross section of the rod and wire, C, Si, etc. are segregated, so that the graphitization index CE is large, so that the precipitation of graphite is promoted, the amount of graphite is larger than the surrounding portion, and the amount of cementite is reduced and ferrite is increased. And the hardness decreases.

[0020] Thus, by utilizing the center segregation, which has been regarded as harmful, without adding lead, a super free-cutting steel whose drilling workability is equal to or higher than that of the conventional sulfur-lead composite free-cutting steel. It has been found that it is possible to produce rods and wires. .

The present invention has been made based on the above findings, and has the following features. Claim 1
According to the invention described, C: more than 1.00 to 1.50 by weight%.
%, Si: 1.00 to 2.80%, Mn: 0.01 to
2.00%, P: 0.050% or less, S: 0.10% or less, O: 0.0050% or less, and N: 0.020%
It contains the following, has a chemical component composition consisting of the balance of Fe and inevitable impurities, has a graphitization index CE of 1.30 or more determined by the following equation (1), and has a carbon segregation degree [C] / [C] ₀ (where [C] is the carbon content at the target position, and [C] ₀ (the carbon content of the raw steel analysis) is 1.0
A slab or slab within the range of 1 to 2.00
The steel is heated to a temperature in the range of 1150 ° C., hot-rolled, and the hot-rolled hot steel is heated to 800 ° C. to 600 ° C.
The cooling time was adjusted to 5 minutes or more, and with respect to the characteristic values of the steel material thus obtained, 100 particles / mm ² or more of graphite having an average particle size of 1.0 μm or more were precipitated, and the metal structure was reduced to 70% or more of ferrite. or consisting of the remainder cementite, or only consists of ferrite, the hardness of the cross-sectional center portion is equal to or less than the hardness of the cross-section intermediate portion, and the cross-sectional hardness of the intermediate portion becomes Vickers hardness H _V 300 or less Is characterized by the fact that The graphitization index CE is calculated by the following equation.

CE = C + Si / 3-Mn / 12 (1) where: Element symbols represent weight% of each element. Since graphite grows by aggregating carbon in the base iron, ferrite precipitates in a form that erodes layered cementite around the graphite. Since the graphitization index CE is higher at the center due to component segregation, the precipitation of graphite is further promoted, and the amount of graphite is large, the amount of ferrite is large, and the periphery is softer than its surroundings.

[0023] By the above-mentioned treatment, a Vickers hardness of the intermediate portion is set to a soft value of 300 or less, and an ultra-free-cutting steel rod having excellent drilling workability can be obtained. The free-cutting steel rod or wire according to claim 2 is particularly applied when the diameter of the rod or rod is small and the cooling rate is not sufficiently small.

C: more than 1.00 to 1.50% by weight
Si: 1.00 to 2.80%, Mn: 0.01 to 2.0
0%, P: 0.050% or less, S: 0.10% or less,
It contains O: 0.0050% or less and N: 0.020% or less, has a chemical composition composed of a balance of Fe and unavoidable impurities, and has a graphitization index C obtained by the following equation (1).
E is not less than 1.30 and the degree of central segregation of carbon [C] /
[C] ₀ (where [C] is the carbon content of the target position, [C] ₀
Is the carbon content in raw steel analysis) of 1.01 to 2.00
Is heated to a temperature in the range of 850 to 1150 ° C., hot-rolled, and the hot-rolled hot steel is heated from 800 ° C. to 600 ° C. for an arbitrary time. And then further heated to 600-900
After holding for 3 hours or less at a temperature within the range of ° C., air-cooled,
Regarding the characteristic values of the steel material thus obtained, the average particle size is 1.0
100 μm / mm ² or more of graphite of μm or more is precipitated, and the metal structure is composed of 80% or more of ferrite and the remaining cementite, or is composed only of ferrite, and the hardness at the center of the cross section is less than the hardness at the middle of the cross section. And the hardness of the intermediate portion of the cross section is Vickers hardness H _V 250 or less. The graphitization index CE is calculated by the following equation.

CE = C + Si / 3-Mn / 12— (1) where: Element symbols represent weight% of each element. In the above, after hot rolling, cooling is performed at an arbitrary time between 800 and 600 ° C. The method may be simple air cooling. The use of site, bainite or fine pearlite promotes the precipitation of graphite, and the subsequent processing time is short.

[0026] The free-cutting steel rod or wire according to the third aspect is the first aspect of the invention.
Or in the invention according to 2, as the slab or steel slab,
Furthermore, one or more selected from the group consisting of the following components are added and contained, and the following equation (2) is used in place of the equation for calculating the graphitization index CE. is there.

Here, the group consisting of the component compositions of the above-mentioned elements means, by weight%, Cu: 0.01 to 2.0%, Ni: 0.
01 to 1.0%, Co: 0.01 to 0.50%, Cr:
0.01 to 0.50%, Mo: 0.01 to 0.50%,
And B: 0.0005 to 0.010%. Then, the above equation (2) means that CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B ----------------- ------------ (2) Here, the symbol of the element in the above formula indicates the weight% of each element.

[0028] The free-cutting steel rod wire according to the fourth aspect is the invention according to the first, second or third aspect, wherein the slab or the steel slab is one type selected from the group consisting of the following components. In addition to the above, the graphitization index CE
Is characterized by using the following equation (3) instead of the calculation equation.

Here, the group consisting of the component compositions of the above elements means, by weight%, Al: 0.001 to 0.10%, Ti:
0.005 to 0.050%, Zr: 0.005 to 0.0
50%, V: 0.01 to 0.20%, and Nb: 0.
01 to 0.20%. Then, the above equation (3) means that CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 -------------------------- (3) However, the symbol of the element in the above formula indicates the weight% of each element.

[0030] The free-cutting steel rod or wire according to the fifth aspect is the first aspect of the invention.
In any one of the inventions of (1) to (4), the cast slab or the steel slab further contains at least one element selected from the group consisting of the following element compositions, and has a graphitization index CE. It is characterized by using the following equation (4) instead of the calculation equation.

Here, the group consisting of the component compositions of the above-mentioned elements means, by weight%, Ca: 0.0010 to 0.0100%,
Mg: 0.0010 to 0.10%;
0010 to 0.10%. And, the equation (4) is: CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 + 0.07- ---------------------------- (4) However, the element symbol in the above formula indicates the weight% of each element. .

C: more than 1.00 to 1.50% by weight
Si: 1.00 to 2.80%, Mn: 0.01 to 2.0
0%, P: 0.050% or less, S: 0.10% or less,
It contains O: 0.0050% or less and N: 0.020% or less, has a chemical composition composed of a balance of Fe and unavoidable impurities, and has a graphitization index C obtained by the following equation (1).
E is not less than 1.30 and the degree of central segregation of carbon [C] /
[C] ₀ (where [C] is the carbon content of the target position, [C] ₀
Is the carbon content in raw steel analysis) of 1.01 to 2.00
Is heated to a temperature in the range of 850 to 1150 ° C., hot-rolled, and the hot-rolled hot steel is gradually cooled from 800 ° C. to 60 ° C.
The time for cooling to 0 ° C. was adjusted to 5 minutes or more, and graphite having an average particle size of 1.0 μm or more was precipitated in the steel material at 100 pieces / mm ² or more, and the metal structure was reduced from 70% or more of ferrite and the remaining cementite. made or only made of ferrite, the hardness of the cross-sectional center portion and below the hardness of the cross-section intermediate portion, and the hardness of the cross-section intermediate portion having a feature to adjust the following Vickers hardness H _V 300 It is. The graphitization index CE is calculated by the following equation.

CE = C + Si / 3-Mn / 12 ----------------------------- (1) where: Element symbols represent weight% of each element. According to a seventh aspect of the present invention, in the method for manufacturing a free-cutting steel rod or wire according to the sixth aspect, at least one selected from the group consisting of the following elements is added as the slab or the steel slab. And the following formula (2) is used in place of the calculation formula of the graphitization index CE.

Here, the group consisting of the component compositions of the above-mentioned elements means, in terms of% by weight, Cu: 0.01 to 2.0%, Ni: 0.
01 to 1.0%, Co: 0.01 to 0.50%, Cr:
0.01 to 0.50%, Mo: 0.01 to 0.50%,
And B: 0.0005 to 0.010%. Then, the above equation (2) means that CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B ----------------- ------------ (2) Here, the symbol of the element in the above formula indicates the weight% of each element.

According to a eighth aspect of the present invention, in the method for producing a free-cutting steel rod or wire according to the sixth or seventh aspect, the slab or the slab is at least one selected from the group consisting of the following element compositions. And the graphitization index CE
Is characterized by using the following equation (3) instead of the calculation equation.

Here, the group consisting of the component compositions of the above-mentioned elements means, by weight%, Al: 0.001 to 0.10%, Ti:
0.005 to 0.050%, Zr: 0.005 to 0.0
50%, V: 0.01 to 0.20%, and Nb: 0.
01 to 0.20%. Then, the above equation (3) means that CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 -------------------------- (3) However, the symbol of the element in the above formula indicates the weight% of each element.

According to a ninth aspect of the present invention, in the method for producing a free-cutting steel rod or wire according to the sixth, seventh or eighth aspect, the slab or the steel slab is further selected from the group consisting of the following elements. In addition, one or more of these are added and contained, and the following formula (4) is used in place of the calculation formula of the graphitization index CE.

Here, the group consisting of the component compositions of the above-mentioned elements means, by weight%, Ca: 0.0010 to 0.0100%,
Mg: 0.0010 to 0.10%;
0010 to 0.10%. And, the equation (4) is: CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 + 0.07- ---------------------------- (4) However, the element symbol in the above formula indicates the weight% of each element. .

By weight%, C: more than 1.00 to 1.50
%, Si: 1.00 to 2.80%, Mn: 0.01 to
2.00%, P: 0.050% or less, S: 0.10% or less, O: 0.0050% or less, and N: 0.020%
It contains the following, has a chemical component composition consisting of the balance of Fe and inevitable impurities, has a graphitization index CE of 1.30 or more determined by the following equation (1), and has a carbon segregation degree [C] / [C] ₀ (where [C] is the carbon content of the target position and [C] ₀ is the carbon content of the raw steel analysis) is 1.0
A slab or slab within the range of 1 to 2.00
Heating to a temperature in the range of 1150 ° C., hot rolling, and adjusting the time for cooling from 800 ° C. to 600 ° C. to 1 minute or less by cooling the hot-rolled hot steel with a cooling medium, After further heating and holding at a temperature within the range of 600 to 900 ° C. for 3 hours or less, air-cooled, and 100 graphite / m 2 having an average particle size of 1.0 μm or more in the steel material.
m ² is deposited over, become the metal structure of 80% or more of ferrite and the balance cementite, or made of ferrite alone, the hardness of the cross-sectional center portion and below the hardness of the cross-section intermediate portion, and the cross-section intermediate portion Vickers hardness H _V 2
It is characterized in that it is adjusted to 50 or less. The graphitization index CE is calculated by the following equation.

CE = C + Si / 3−Mn / 12— (1) where: Element symbols represent weight% of each element. In the above method, in particular, in order to promote the precipitation of graphite by the reheating treatment, the rod or wire after hot rolling is 800 to 60%.
Rapid cooling in less than 1 minute between 0 ° C.

[0041] The method for producing a free-cutting steel rod or wire according to the eleventh aspect is characterized in that, in the invention according to the tenth aspect, the slab or the slab further comprises at least one selected from the group consisting of the following element compositions. And the graphitization index C
It is characterized by using the following equation (2) instead of the equation for calculating E.

Here, the group consisting of the component compositions of the above-mentioned elements means, by weight%, Cu: 0.01 to 2.0%, Ni: 0.
01 to 1.0%, Co: 0.01 to 0.50%, Cr:
0.01 to 0.50%, Mo: 0.01 to 0.50%,
And B: 0.0005 to 0.010%. Then, the above equation (2) means that CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B ----------------- ------------ (2) Here, the symbol of the element in the above formula indicates the weight% of each element.

According to a twelfth aspect of the present invention, there is provided a method for manufacturing a free-cutting steel rod or wire according to the tenth or eleventh aspect, wherein the slab or the slab is further selected from the group consisting of the following components. It is characterized by containing at least one or more species and using the following equation (3) instead of the equation for calculating the graphitization index CE.

Here, the group consisting of the component compositions of the above-mentioned elements means, by weight%, Al: 0.001 to 0.10%, Ti:
0.005 to 0.050%, Zr: 0.005 to 0.0
50%, V: 0.01 to 0.20%, and Nb: 0.
01 to 0.20%. Then, the above equation (3) means that CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 -------------------------- (3) However, the symbol of the element in the above formula indicates the weight% of each element.

The method for producing a free-cutting steel rod or wire according to the thirteenth aspect is the invention according to the tenth, eleventh or twelfth aspect, wherein the slab or the slab is further selected from the group consisting of the following elements. In addition, one or more of these are added and contained, and the following formula (4) is used in place of the calculation formula of the graphitization index CE.

Here, the group consisting of the component compositions of the above-mentioned elements means, by weight%, Ca: 0.0010 to 0.0100%,
Mg: 0.0010 to 0.10%;
0010 to 0.10%. And, the equation (4) is: CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 + 0.07- ---------------------------- (4) However, the element symbol in the above formula indicates the weight% of each element. .

[0047]

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a hot-rolled rod or wire from a high Si hypereutectoid carbon steel having a predetermined degree of center segregation and a low alloy steel slab or slab thereof. For at least 3 hours or within a predetermined temperature range for 3 hours.
Thereafter, a rod or wire having a structure in which the center is soft and has a lot of graphite is manufactured by air cooling after reheating. By such a simple process, a super-free-cutting rod or wire having excellent lead-free drilling properties is manufactured.

Next, the reasons for limiting the constituent elements of the present invention will be described below. (1) Carbon (C) C is an important element for precipitating graphite and securing strength. After reheating of steel material manufactured by hot rolling,
In order to precipitate graphite by air cooling without slow cooling such as furnace cooling, a C content of more than 1.00% is required. However, if the C content exceeds 1.50%, the hot ductility is greatly reduced, and the occurrence of surface flaws during bar rolling increases. In addition, graphite particles precipitated after air cooling become coarse, and the toughness is reduced. Therefore, the C content is more than 1.00 to 1.5.
Limit within the range of 0%.

(2) Silicon (Si) Si is an element that plays an important role in the present invention.
That is, Si is an element that promotes graphitization of cementite. However, if it is less than 1.00%, the effect is small. On the other hand, if Si exceeds 2.80%, nonmetallic inclusions increase and not only decrease in toughness, but also increase decarburization in hot rolling or reheating for graphitization. Therefore, Si
The content is limited to the range of 1.00 to 2.80%.

(3) Manganese (Mn) Mn detoxifies S in steel as MnS and improves the hot ductility of steel. For this purpose, Mn is 0.0
Requires addition of 1% or more. However, since Mn inhibits graphite precipitation, the upper limit is set to 2.00%. Note that M
If the n content is reduced, the amount of Si required for graphitization can be reduced. If the Mn content is high, high strength and toughness can be imparted to the component. Therefore, the Mn content is limited to the range of 0.01 to 2.00%.

(4) Phosphorus (P) P is an element that promotes graphitization. However, P segregates at the grain boundaries, lowers hot ductility, and promotes generation of surface defects. In order to prevent such adverse effects, the P content is set to 0.0
Limited to 50 or less. More preferably, the content is 0.030% or less.

(5) Sulfur (S) S is an element which greatly inhibits graphitization. If its content exceeds 0.100%, it is necessary to add a large amount of a graphitization promoting element such as Si. In addition, a decrease in hot ductility is caused.
Therefore, the S content is limited to 0.100% or less. More desirably, the content should be 0.050% or less.

(6) Oxygen (O) Since O is an element that lowers the cleanliness of steel and inhibits graphitization, it should be kept as low as possible. However, the O content is acceptable up to 0.0050%. Therefore, the upper limit of the O content is set to 0.0050%. More desirably, the content is 0.0030% or less.

(7) Nitrogen (N) When N alone exists in steel, it inhibits graphitization. If the N content exceeds 0.020%, precipitation of graphite becomes difficult, and a large number of blowholes are formed due to generation of nitrogen gas during solidification of steel, which causes surface defects after rolling. Therefore, the N content is set to 0.020% or less. More preferably, the content is 0.010% or less.

(8) Copper (Cu) Cu promotes the precipitation of graphite and increases the toughness of steel by dissolving in ferrite. Since Cu is used for this purpose, it is necessary to add 0.01% or more. However, Cu
If the content exceeds 2.0%, the solid solubility in steel is exceeded, so that undissolved Cu remains, reducing hot ductility and promoting the generation of surface flaws. Therefore, the Cu content is 0.01 to
It is desirable to be within the range of 2.0%.

(9) Nickel (Ni) Ni also promotes the precipitation of graphite, like Cu,
Solid solution in ferrite to increase steel toughness. Since Ni is added for these purposes, 0.01% or more of Ni needs to be added. However, when the content exceeds 2.0%, the effect is saturated. Ni is an expensive element. Therefore, it is desirable that the Ni content be in the range of 0.01 to 2.0%.

(10) Cobalt (Co) Co, like Cu and Ni, promotes the precipitation of graphite and increases the toughness of steel. Since Co is added for these purposes, 0.01% or more of Co must be added. However, Co is an element more expensive than Ni. Therefore, the Co content is desirably in the range of 0.01 to 0.50%.

(11) Chromium (Cr) When a small amount of Cr is also added, it forms a solid solution with ferrite to increase the toughness of the steel. Since it is used for this purpose, it is necessary to add 0.01% or more. However, Cr has a greater effect of inhibiting graphitization than Mn. Therefore, when Cr exceeds 0.50%, a large amount of the graphitization promoting element is required, and the cost is increased. Therefore, it is desirable that the Cr content be in the range of 0.01 to 0.50%.

(12) Molybdenum (Mo) Mo, when added in a small amount, increases the toughness of the steel. Since it is used for this purpose, it is necessary to add 0.01% or more. However, Mo is also an element that inhibits graphitization, and if it exceeds 0.50%, a large amount of graphitization promoting element is required. Therefore, it is desirable to set the Mo content within the range of 0.01 to 0.50%.

(13) Boron (B) B fixes N in steel as BN to reduce the graphitization inhibiting effect of N, and BN acts as a graphite nucleus to promote graphite precipitation. Because it is used for this purpose, 0.00
It requires an addition of at least 05%. However, B is 0.01
Adding more than 0% not only saturates the effect,
Precipitates a large amount of BN and carbon borides and reduces hot ductility. Therefore, the B content is desirably in the range of 0.0005 to 0.010%.

(14) Aluminum (Al) Al is an important element as a deoxidizing agent and an element that precipitates AlN to make crystal grains fine. It is an element that promotes graphitization like Si. For these purposes, it is necessary to add Al at least 0.001% or more. However, when Al is added in excess of 0.10%, the amount of oxide-based inclusions increases, thereby deteriorating the cleanliness of the steel and causing cracking during hot working. Further, in continuous casting, Al ₂ O ₃ accumulates on the nozzle and causes clogging of the nozzle, so that the Al content is 0.001 to 0.10%.
It is desirable to be within the range.

(15) Titanium (Ti) Ti precipitates TiN and TiC and refines crystal grains. Further, TiN and TiC act as nuclei for graphite precipitation and promote graphite deposition. 0.005% Ti added
If the amount is less than 0.10%, the effect is small. On the other hand, if Ti is added in excess of 0.10%, a large amount of hard TiN or TiC is generated to promote tool wear. Therefore, the Ti content is
It is desirable to set it in the range of 0.005 to 0.050%.

(15) Zirconium (Zr) Like Zr, Zr also precipitates nitrides and carbides, refines crystal grains, and promotes the precipitation of graphite. Zr
If the added amount is less than 0.005%, the effect is small. On the other hand, if Zr exceeds 0.050%, tool wear is promoted. Therefore, the Zr content is 0.005 to 0.5.
It is desirable to be within the range of 050%.

(16) Vanadium (V) V also precipitates nitrides and carbides and refines crystal grains. In addition, since the precipitate is fine, it increases the yield stress of the steel,
Improve fatigue limit stress. If the V content is less than 0.01%, the effect is small. On the other hand, V is an element that inhibits the precipitation of graphite, and if added in excess of 0.20%, a large amount of the graphitization promoting element is required. Therefore, the V content is 0.0
It is desirable to set it in the range of 1 to 0.20%.

(17) Niobium (Nb) Nb also precipitates nitrides and carbides, refines crystal grains, and increases the yield stress. Nb carbonitride is 115
It does not dissolve in steel even at a high temperature of 0 ° C., prevents coarsening of austenite grains, refines crystal grains after forging, and improves toughness. If the amount of Nb is less than 0.01%, the effect is small. On the other hand, if it exceeds 0.20%, the precipitation of graphite is inhibited and a large amount of the graphitization promoting element is required. Therefore, it is desirable that the Nb content be in the range of 0.01 to 0.20%.

(18) Calcium (Ca) Ca is used as an inoculant in cast iron to promote graphitization. This is thought to be because the vapor pressure of Ca at the temperature level of the molten steel is high, and the vapor of Ca forms small cavities in the solidified steel during casting, which becomes the core of graphite precipitation and precipitates spheroidal graphite. . In steel, Ca behaves similarly to cast iron, and facilitates graphite precipitation after hot working. Also,
When Ca is present as an oxide-based inclusion, Ca forms a belag in the cutting of a cemented carbide tool, and has a great effect of extending the tool life. Therefore, Ca is an element desirably added to free-cutting steel. For this purpose, Ca needs to be added in an amount of 0.0010% or more. However, even if it exceeds 0.010%, the effect is saturated. Therefore, the Ca content is 0.001.
It is desirable to set it in the range of 0 to 0.010%.

(19) Magnesium (Mg) Mg, like Ca, is used as an inoculant in cast iron, promotes graphitization, and also facilitates the precipitation of graphite after working in steel. If the added amount is less than 0.0010%, the effect is small. On the other hand, even if Mg is added in excess of 0.10%, the effect is saturated. Therefore, the Mg content is 0.0
It is desirable to be within the range of 010 to 0.10%.

(20) REM (Rare Earth Element) REM such as Ce and La also promotes graphite precipitation after forging.
If the added amount is less than 0.0010%, the effect is small. On the other hand, even if REM exceeds 0.10%, the effect is saturated. Therefore, the REM content is 0.0010 to 0.10
% Is desirable.

In addition to the above, steel contains elements inevitably mixed, such as Sn and As. If the environmental problem is small, it is also possible to supplementarily add a small amount of a free-cutting element such as Bi, Se, or Te.

(21) Graphitization Index Graphite in steel is effective in improving its free-cutting properties. In order to promote the precipitation of graphite in a steel material, it is important to increase the graphitization index CE. This CE is represented by the following formula for the main elements. That is, CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 ----------- ------------------ (3) However, the symbol of the element in the above formula indicates the content% by weight of the element. When at least one of Ca, Mg and REM is contained in an amount of 0.0010% or more, 0.07 is added to the right side of the above equation (3).

On the other hand, the precipitation of graphite depends on the heating temperature and the cooling rate, and is not uniquely determined by CE. However, when CE is not more than 1.30, the target high-temperature steel material must be subjected to a long-time graphitization treatment by furnace cooling or the like, and it becomes difficult to precipitate graphite by a short-time heat treatment. Therefore, the graphitization index CE is limited to 1.30 or more.

(22) Degree of central segregation of carbon [C] /
[C ₀ ] In general, when there is a center segregation of components in a slab or a slab,
In other words, when the content of the component element such as C in the central portion is higher than in other portions, the hardness of that portion becomes higher. for that reason,
When a hole is drilled at the center of the cross section of the rod or wire, the tip of the drill shifts around it and the center of the hole shifts. However,
In the present invention, the segregation of C and Si at the center increases the graphitization index CE according to the above equation (3), thereby increasing the amount of graphite precipitated at the center and the amount of ferrite. Also increase. Thus, the hardness at the central portion is lower than that at the periphery thereof, which is preferable for drilling. Thus, the present invention is also greatly characterized in that center segregation is actively utilized.

The degree of center segregation of C in a slab or a slab is 1.0
If the segregation is less than 1, the effect of precipitating a larger amount of graphite to an extent advantageous for drilling than in the surrounding area and making the metal structure have a higher ratio of soft ferrite cannot be exerted. On the other hand, if the degree of center segregation of C exceeds 2.00, the ductility of the center is insufficient and the center is torn at the time of hot rolling, which hinders the rolling operation. However, the degree of center segregation of C is limited to the range of 1.01 to 2.00.

The measurement of the degree of center segregation was performed as follows.
U. That is, the center of the cross section perpendicular to the length direction of the slab or slab
The part is sampled for analysis using a drill of about 5 mmφ,
This is analyzed to determine the C content rate [C] at the center. on the other hand,
Base steel with C analysis value or ladle analysis value of middle section
Analysis C content [C]₀And And [C] / [C]
₀Is calculated, and this is defined as the center segregation degree of C.

(23) Heating temperature at the time of hot rolling If the heating temperature of the steel material before the hot rolling is lower than 850 ° C., the deformability of the steel is insufficient, and surface flaws are liable to be generated on the rod or wire. on the other hand,
If the heating temperature exceeds 1150 ° C., the temperature becomes close to the solidus temperature of steel, so that the hot ductility is also insufficient and cracks occur in the rod or wire. Therefore, the heating temperature during hot rolling is 850 to 1150.
C. The slab or the slab is hot-rolled after being heated within the above-mentioned temperature range. A lower hot-rolling ending temperature is preferable because the number of lattice defects increases and the precipitation of graphite is promoted. Therefore, a means of lowering the finishing temperature by water cooling in the middle of the rolling mill group is effective.

(24) Cooling time after hot rolling After hot rolling, cooling is performed as slowly as possible.
The precipitation of graphite is promoted, the amount of ferrite increases, and the steel softens. If the cooling time from 800 ° C. to 600 ° C. is less than 5 minutes, the desired size and number of graphite cannot be obtained and the hardness will be high. Therefore, 8
The cooling time from 00 ° C to 600 ° C is 5 minutes or more.
That is, the average cooling rate is 0.67 ° C./sec or less.

In the case where the diameter of the rod or wire is as large as about 80 mmφ, the above cooling time is 5 hours even if the air is simply cooled after hot rolling.
Since it takes more than a minute, the above condition can be satisfied. However, in the case of a thin steel bar or wire having a diameter of about 5 to 25 mmφ, the cooling time cannot be set to 5 minutes or more by air cooling. Therefore, when the diameter of a straight bar (a single straight bar) is small, the cooling wire is covered with a cover to gradually cool the bar or the hot rolled material is wound into a coil. And cool. Further, the rod-shaped wire wound into a coil is covered with a cover and gradually cooled. In the case of a cooling facility such as a stermore line in which a hot-rolled wire flows on a conveyor in a non-concentric ring state, the conveyor is covered with a cover and the hot-rolled wire is gradually cooled. By the means as described above, it is possible to cool even a rod having a small diameter from 800 ° C. to 600 ° C. in 5 minutes or more.

In the case where the rod or wire is reheated to promote graphitization, the rod or wire after hot rolling has a fine metal structure.
It is desirable to contain a large amount of lattice defects such as dislocations.
In this case, when the hot rolled wire is cooled from 800 ° C. to 600 ° C. in a short time of 1 minute or less to adjust the metal structure to martensite, bainite, or fine pearlite, the effect of promoting graphitization is improved. There is.

(25) Heating temperature for graphitization In the case where a rod having a small diameter is not gradually cooled by the cover as described above, the cooling time at 800 to 600 ° C. is short, and graphite cannot be sufficiently precipitated. It is not possible to obtain a rod wire having excellent drilling workability. Therefore, graphite is promoted by reheating. In order to obtain the desired size and number of graphite, if the heating temperature is lower than 600 ° C., the carbon diffusion rate is low, so that it takes more than 3 hours for graphitization. On the other hand, when heated to a temperature higher than 900 ° C., the graphite once precipitated during the temperature rise is redissolved during the high-temperature heating, and the size of the graphite obtained after air cooling is reduced. Therefore, it is necessary to cool the furnace slowly after heating, for example, so that the graphitization processing time becomes longer. Therefore, the reheating temperature is in the range of 600 to 900 ° C. The precipitation of graphite is most promoted at temperatures between 650 and 800 ° C,
By processing within this temperature range, the object can be achieved in a shorter time.

(26) Graphitization Heating Time The heating time within the above temperature range is prolonged in which the heating time exceeds 3 hours, so that the cost increases. Therefore, the processing time is set to 3 hours or less. Then, the desired size and number of graphite, metal structure and hardness can be sufficiently obtained by heating for 3 hours or less. Therefore, the heating time is limited to 3 hours or less.

After heating, the steel material is taken out of the furnace and air-cooled, which is sufficient without slow cooling. In the case of a coil material, it is cooled in the state of a wound coil of 1 to 3 tons.
In the case of a straight rod, it is mostly cooled in a state of binding several to about 100 rods. Therefore, in the case of the steel material according to the present invention, the cooling rate by air cooling in such a state is sufficiently small to precipitate and grow graphite, which is satisfactory.

(27) Graphite Particle Size The precipitation form of graphite in the present invention is generally expressed as a lump, but it may be spherical, granular or elliptical, and the average length is There is no particular problem if the thickness / thickness ratio is 5 or less. As described above, when the average particle size of the graphite precipitated in a lump is less than 1.0 μm, the effect of pulverizing the chips into small pieces during drilling is small, and the contribution to the improvement of the cutting processability is small. Therefore, the average particle size of graphite is 1.0 μm
Make adjustments as described above. On the other hand, the upper limit of the average particle size is not particularly limited, but precipitation of a large amount of graphite exceeding 30 μm causes a decrease in toughness. Therefore, it is desirable that the average particle size of the graphite be 30 μm or less.

(28) Number of Graphite The number of graphite per unit sectional area is important for breaking chips into small pieces. If the number is less than 100 / mm ² , the effect of improving the chip controllability is small.
Set to 00 pieces / mm ² or more. The number of graphite depends on the size of graphite, and decreases as the particle size increases and increases as the particle size decreases. In the present invention, when graphite having a particle size of 10 to 25 μm is deposited, the number thereof is approximately between 100 to 1000 / mm ² , but when graphite having a particle size of 1.0 to 5 μm is deposited, it is approximately 3000 to 50,000. Pcs / mm ² .

(29) Metal Structure In order to prolong the tool life, it is necessary to reduce the hardness of the steel material, and it is necessary that the metal structure of the steel rod or wire is only ferrite + pearlite or ferrite. At this time, the amount of ferrite was 70%
It is necessary to do the above. As the cooling rate after hot rolling decreases, the amount of cementite decreases because the graphite grows larger and the amount of ferrite increases. By cooling at a sufficiently low cooling rate, the metal structure becomes a soft ferrite single phase containing no layered cementite. Therefore, the metal structure is adjusted to be 70% or more of ferrite and the remaining cementite or only ferrite.

Further, in the case of performing the graphitization heat treatment, the metal structure further contains 80% or more ferrite in order to prolong the service life of the tool, that is, to further reduce the hardness of the rod or wire. It is necessary to include cementite as the remainder or ferrite only.

[0086] By heating to a temperature in the range from ₁ point A to the vicinity of A ₁ point -100 ° C., and proceeds spheroidized cementite remaining without graphitization, cementite layered spherical or granular cementite become.

When heating to a temperature higher than the point A ₁ and cooling, ferrite and cementite precipitate from austenite, but by cooling at a sufficiently low cooling rate, a graphite + ferrite structure can be obtained.

(30) Hardness After hot rolling, the Vickers hardness (H
_{When V} ) exceeds 300, tool wear increases,
Tool life is shortened. Therefore, the Vickers hardness at the intermediate portion needs to be 300 or less. In the case of graphitized heat treatment materials, in order to further extend tool life,
Vickers hardness is adjusted to 250 or less. When the metal structure becomes a ferrite single phase, the Vickers hardness is 13
It drops to about 0. By adjusting the amounts of graphite and ferrite such that the center portion is larger than the middle portion, the hardness of the center portion is set to be equal to or less than the hardness of the middle portion.

[0089]

Next, the present invention will be described in more detail with reference to examples. Tables 1 and 2 show the chemical composition and the graphitization index CE of the test steel used in the test.

[0090]

[Table 1]

[0091]

[Table 2]

Steel Nos. 1 to 17 in Table 1 are all steels having a component composition (components of the present invention) within the scope of the present invention.
-27 are steels outside the scope of the present invention. Among them, Steel No.2
In No. 4, although the component composition was within the range of the present invention, the central segregation degree of C was 2.00 because the molten steel temperature during solidification of the slab was too high.
In steel No. 25, negative segregation was formed at the center part due to too large roll reduction with respect to the slab during continuous casting, and the degree of central segregation of C was less than 1.0. is there. Steel Nos. 26 and 27 both have individual component compositions within the scope of the present invention, but have a graphitization index CE of less than 1.30, which is lower than the scope of the present invention. Steel No. 2 in Table 2
Nos. 8 to 50 are steels having a component composition outside the scope of the present invention. Among these, steel No. 48 is SUM24L of the conventional component steel,
Steel No. 49 is a conventional component steel SUM43L, and steel No. 49.
50 is a hypoeutectoid graphite steel of the conventional component steel.

The test steels (Steel Nos. 1 to 17, 2
Nos. 4 to 50 ) are obtained by melting in a 130-ton electric furnace and then casting into slabs by continuous casting or ingot making. Steel No.1
Was cast directly into a 160 mm square slab and then directly subjected to bar rolling. The other steel No. was subjected to bar-rolling after slab-rolling a slab or a steel ingot into a 160 mm square steel slab. In all the bar and wire rolling, a slab or a slab was heated to a predetermined temperature in a heating furnace, and then hot-rolled into a bar or a wire having various diameters.

[0094] In the [Test 1] Test 1, steel No. 1~1
7 , 24-50 , Examples 1-1 , 1-3-1-9 , 1-11-1-14 which are tests within the scope of the present invention, and comparisons which are tests outside the scope of the present invention Examples 1-15-1
-17, 1-24 to 1-47 and Conventional Examples 1-48 to 1-
50 were tested. In any case, after hot rolling to a rod wire,
Cooling was performed under predetermined test conditions, thus producing a rod or wire. That is, all rolled materials having a diameter of 20 mm or more were rolled into straight bars and air-cooled, or gradually cooled with a cooling floor. With respect to the rod and wire thus manufactured, a property test and a drilling test of a steel material were performed. The test contents were as follows: (1) The rolled material was visually checked for surface flaws or cracks, and the rod and wire cooled after rolling was tested. (2) Measurement and observation test of graphite precipitation state and metal structure by optical microscope: Graphite precipitation The average particle size of graphite and the number of graphite particles were measured as the state, and the ferrite + ferrite% (area%) in the cementite structure was measured as the metal structure. (3) Vickers hardness at the center and middle of the cross section A measurement test was performed, and (4) a drilling test of a rod or wire was performed as follows. Drilling was performed on the rod and wire using an automatic lathe. Diameter 13-20mm
Use a 5 mmφ drill for the rod and wire, and use a 10 mmφ drill for the steel bar with a diameter of 25 mm or more.
Drilled a hole twice as deep. The drilling conditions were as follows: the outer peripheral speed of the drill was 150 m / min, the feed was 0.30 mm / rev,
A high speed drill was used. The life of the drill was determined by measuring the life when the tool was melted and the hole could not be drilled, and the number of drilled holes was defined as the life. Also, the misalignment of the hole after processing was checked.

Tables 3 and 4 show the steel No. of the test steels, the conditions and the test results for the production of rods and rods.

[0096]

[Table 3]

[0097]

[Table 4]

From the above test, the following items can be understood. (1) Examples 1-1, 1-3 to 1-9, 1-1 of the present invention
Nos. 1 to 1-14 have a target composition since the component composition and the rolling heating temperature are within the range of the present invention, so that there is no occurrence of surface flaws and the cooling time at 800 to 600 ° C. after rolling is 5 minutes or more. has a size and number of graphite and metal organization requirements are met, also Vickers hardness it is low in the central portion than the middle portion, H _V 300 or less is satisfied and an intermediate portion, soft A rod is obtained.

[0099] Figure 1, as an example showing the microstructure of the free-cutting steel bar wire rod of the present invention, the graphite of Example 1-1 and ferrite +
The figure which shows the microstructure which consists of granular cementite is shown. In addition, by utilizing moderate center segregation, the center of the drill did not shift, and the tool life was as good as 50 or more drilled holes.

(2) On the other hand, in the comparative example and the conventional example in which at least one condition out of the range of the present invention was included, the target of the present invention was not achieved. Details are as follows.

In Comparative Example 1-15 , the chemical composition was within the range of the present invention, but the heating temperature at the time of hot rolling was lower than the range of the present invention. There has occurred. Similarly, in Comparative Example 1-16 , the chemical composition was within the range of the present invention, but the heating temperature during hot rolling was conversely lower than the range of the present invention. Large flaws occurred.

In Comparative Example 1-17 , both the chemical composition and the heating temperature during hot rolling were within the scope of the present invention, but the cooling time after rolling was as short as 3.1 minutes. sufficient and not average particle size of the graphite is smaller than 1.0 .mu.m, less ferrite content of metal structure, also the hardness of the insufficient heart softening is higher than the intermediate portion, H _V 30 also hardness of the intermediate portion
Exceeded zero. For this reason, misalignment occurred during drilling, and the drill life was as short as 24 holes.

In Comparative Example 1-24, since the center segregation degree of C exceeded 2.00, the center part was insufficient in ductility, and thus was torn from the center during hot rolling. In Comparative Example 1-25, on the contrary, the center segregation degree of C was smaller than 1.01, so that the hardness of the center part was higher than the hardness of the middle part, and thus the center of the drill was misaligned.

Comparative Examples 1-26 and 1-27
Although the chemical components were within the range of the present invention, no graphite deposition was observed because the graphitization index CE was lower than the range of the present invention.

In Comparative Example 1-28, the C content was low outside the scope of the present invention, so that only graphite having a small size of less than 1.0 μm was obtained, and the amount of ferrite in the metal structure was small.
The hardness of the intermediate portion was higher than _HV 300, and the center of the drill was misaligned, and the tool life was short. Comparative Example 1
No. 29, on the contrary, had a high C content outside of the present invention, resulting in insufficient hot ductility and flaws in the steel bar.

In Comparative Example 1-30, the Si content was lower than the range of the present invention, so that the graphite grains were small and the drilling workability was poor. In Comparative Example 1-31, the Si content was higher than the range of the present invention, so that the hot ductility was insufficient, and the steel bar had flaws.

In Comparative Example 1-32, the Mn content was 2.0
It was higher than 0%, so that the hot-ductility was insufficient, and the steel bar had flaws. Comparative Example 1-33 has a P content higher than the range of the present invention, Comparative Example 1-34 has an S content higher than the range of the present invention, and Comparative Example 1-35 has a Cu content of the present invention. Since they were higher than the range, the hot ductility was insufficient in all cases, and the steel bars were flawed.

In Comparative Example 1-36, since the Cr content was higher than the range of the present invention, and the graphitization index CE was smaller than 1.30, only small graphite could be obtained. The hardness of the middle portion was high, and the drilling workability was poor.

In Comparative Example 1-37, the content of Ni and Mo was higher than the range of the present invention, so that the hot ductility was insufficient and the steel bar was flawed. Comparative Example 1 No. 38 had Co and O contents higher than the range of the present invention, and also had insufficient hot ductility, and the steel bar had flaws.

In Comparative Example 1-39, the B content was higher than the range of the present invention, a large amount of carbon boride was precipitated, the ductility was insufficient, and the steel bar was flawed. Comparative Example 1 No. 40 has a higher N content than the range of the present invention, and therefore, due to blow holes generated in the slab,
Many linear flaws occurred on the surface of the steel bar.

Comparative Example 1 In No. 41, the content of Zr and Ti was higher than the range of the present invention, and the steel bar had flaws. -In Comparative Example 1-42, the V content was higher than the range of the present invention, and only a small graphite having a graphitization index CE of less than 1.30 and 1.0 µm or less could be obtained. Therefore, the drilling workability was poor.

In Comparative Example 1-43, the AlM content was higher than the range of the present invention, and the steel bar had flaws. In Comparative Example 1-44, the Nb content was higher than the range of the present invention, and only a small graphite having a graphitization index CE of less than 1.30 and 1.0 μm or less could be obtained. Therefore, the drilling workability was poor.

Comparative Example 1-45 has a Ca content, Comparative Example 1-46 has a Mg content, and Comparative Example 1-47 has a REM content higher than the range of the present invention. A large amount of material-based inclusions were involved, and this remained as rolling scratches on the steel bar.

Comparative Example 1-48 shows that the conventional component steel SUM
24L, and Comparative Example 1-49 shows S of the conventional component steel.
UM43L, and the drilling tool life was good. However, the hardness of the central portion was higher than the hardness of the middle portion, causing misalignment. When hot rolling into steel bars, the tip of the billet is made thinner like a pencil to prevent the tip from tearing and cracking during rolling, resulting in misrolling. Had to be bitten.

SUM24L and SUM of the above conventional component steels
For 43L, Examples 1-1, 1-3 to 1-1-1 of the present invention were used.
9 , 1-11 to 1-14 did not require any special steel slab tip processing, and could be rolled without any trouble using a steel slab that had been cut by a shear.

Comparative Example 1-50 is an example of conventional graphite steel. After winding into a coil, the cover was gradually cooled.
No precipitation of graphite was observed, and the drilling workability was poor.

[Test 2] In Test 2, steel Nos. 2 to 15 were tested in Examples 2-2 to 2-2, which are tests within the scope of the present invention.
15 and steel Nos. 16 , 17 , 26, 27 and 50
Comparative Examples 2-16, which are tests outside the scope of the present invention ,
2-17, 2-26, 2-27 and Conventional Example 2-50 were tested. In each case, after being hot-rolled into a rod or wire, the rod was cooled under predetermined test conditions, then reheated for graphitization treatment, held for a predetermined time, and air-cooled. Thus, a rod wire was manufactured. For the rod and wire manufactured in this way,
A property test and a drilling test of the steel material were performed. The test items were (1) visual judgment of the presence or absence of surface flaws and cracks of the rolled material, (2) measurement observation test of the graphite precipitation state and metal structure by an optical microscope, and (3) Vickers hardness at the center and intermediate part of the cross section. Measurement test and (4) Drilling test of rods and wires, the contents of which are the same as in Test 1.

Table 5 shows the steel No. of the test steels, the test conditions for the production of rods and wires, and the test results.

[0119]

[Table 5]

From the above test, the following matters can be understood. (1) In Examples 2-2 to 2-15 of the present invention, the rod or wire after hot rolling is set to a temperature within the range of 600 to 900 ° C.
By reheating in the range of 5 minutes to 180 minutes, a desired size and number of graphite and metal structure can be obtained, and the hardness of the central portion is lower than the hardness of the intermediate portion, and the hardness of the intermediate portion is H _V
It was 250 or less. Therefore, drilling workability was also good.

(2) On the other hand, the comparative example and the conventional example, which are tests in which at least one condition outside the scope of the present invention is included, did not achieve the objective of the present invention. Details are as follows.

In Comparative Example 2-16 , since the heating temperature was higher than 900 ° C., only small graphite of less than 1.0 μm could be obtained, and the drilling workability was poor.
On the contrary, in Comparative Example 2-17 , since the heating temperature was low at less than 600 ° C., only graphite as small as less than 1.0 μm could be obtained as in the above, and the drilling workability was poor.

The comparative examples 2-26 and 2-27
Since the graphitization index CE was less than 1.30, no precipitation of graphite was observed, and the drilling workability was poor. Comparative Example 2-50 is an example in which conventional graphite steel was subjected to a graphitization heat treatment at 700 ° C. for 900 minutes for a long time. Only by performing such a long-time heat treatment, graphite was precipitated and the steel material was softened, so that excellent drilling workability was obtained.

[Test 3] Examples 1-9 Performed in Test 1
And the wires manufactured in Conventional Examples 1-48 (both having a diameter of 13
mmφ) to process a connector bolt which is an engine part of an automobile. The process of processing the connector bolt in the conventional example is as follows: wire rod → cold drawing → bolt head cold forging →
Thread rolling → peripheral cutting → drilling → deburring → carburizing and quenching. Conventionally, when SUM24L was used, it was necessary to perform rasping manually to remove burrs generated after drilling. In addition, to improve wear resistance, after carburizing and quenching at 930 ° C. × 5 hours,
It was necessary to perform tempering for 0 minutes, which was a major cost increase factor.

On the other hand, when the wire of Example 1-9 was used, the deburring step could be omitted, and the labor could be saved. In addition, the tool life of cutting and drilling has been greatly extended to about twice that of conventional ones. Further, the wear resistance of the parts could be improved by switching the long-time carburizing quenching and tempering to simple induction hardening. As a result, significant cost reduction was achieved.

[0126]

As described above, according to the present invention,
Without the addition of lead, it is possible to manufacture ultra-free-cutting steel parts with excellent drilling workability equal to or better than conventional sulfur-lead composite free-cutting steel, and carburizing and quenching after machining the parts. Even if not performed, the abrasion resistance can be improved by simple induction hardening. Compared to conventional graphite steel,
No graphitization heat treatment is required, or graphitization becomes possible with a short heat treatment. It is possible to provide a manufacturing technique for a free-cutting steel rod or wire excellent in such drilling workability, and an industrially useful effect is obtained.

[Brief description of the drawings]

FIG. 1 is an example showing a microstructure of a free-cutting steel bar of the present invention, and is a diagram showing a microstructure composed of graphite and ferrite + granular cementite of Example 1-1 .

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-11-2933387 (JP, A) JP-A-11-293388 (JP, A) JP-A-11-293389 (JP, A) JP-A-11-293389 JP-A-11-350067 (JP, A) JP-A-11-350068 (JP, A) JP-A-49-67817 (JP, A) JP-A-3-146618 (JP, A) JP-A-6-279849 (JP, A) JP-A-8-127845 (JP, A) Edited by The Japan Institute of Metals, Theory and Practice of Spheroidal Graphite Cast Iron, published by Maruzen Co., Japan, June 1966 30 days, p. 438 (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 6/00, 8/06, 8/08

Claims (13)

(57) [Claims] 1. Weight%: C: more than 1.00 to 1.50%, Si: 1.00 to 2.80%, Mn: 0.01 to 2.00%, P: 0.050% or less , S: 0.10% or less, O: 0.0050% or less, and N: 0.020% or less, and has a chemical component composition comprising the balance of Fe and inevitable impurities, and has the following formula (1) The graphitization index CE obtained by 1.3 is 1.3.
0 or more, and the center segregation degree of carbon [C] / [C]
₀ (however, [C] is the carbon content of the target position, and [C] ₀ (the carbon content of the raw steel analysis) is in the range of 1.01 to 2.00. １1150 ° C., heated to a temperature in the range of 1150 ° C., hot-rolled, and the time for cooling the hot-rolled hot steel from 800 ° C. to 600 ° C. was adjusted to 5 minutes or more. Regarding the characteristic values, 100 particles / mm ² of graphite having an average particle size of 1.0 μm or more were used.
Precipitated above, the metal structure consists of 70% or more of ferrite and the remainder cementite, or consists only of ferrite, the hardness of the center of the cross section is not more than the hardness of the middle of the cross section, and the middle of the cross section is Hardness is Vickers hardness H _V 3
A free-cutting steel rod and wire excellent in drilling workability, characterized in that it is not more than 00. The graphitization index CE is calculated by the following equation. CE = C + Si / 3-Mn / 12 --- (1) However, the symbol of the element in the above formula represents the weight% of each element. 2. In% by weight, C: more than 1.00 to 1.50%, Si: 1.00 to 2.80%, Mn: 0.01 to 2.00%, P: 0.050% or less , S: 0.10% or less, O: 0.0050% or less, and N: 0.020% or less, and has a chemical component composition comprising the balance of Fe and inevitable impurities, and has the following formula (1) The graphitization index CE obtained by 1.3 is 1.3.
0 or more, and the center segregation degree of carbon [C] / [C]
₀ (however, [C] is the carbon content of the target position, [C] ₀ is the carbon content of the raw steel analysis) in the range of 1.01-2.00, After heating to a temperature in the range of １1150 ° C. and hot rolling, the hot-rolled high-temperature steel material is cooled from 800 ° C. to 600 ° C. over an arbitrary period of time, and further heated to 600６００ after holding the temperature in 3hr following times in the range of 900 ° C., air cooled, thus relates characteristic values of the resulting steel, precipitated average particle diameter 1.0μm or more graphite 100 / mm ² or more, the metal structure 8
0% or more of ferrite and the remainder of cementite or only ferrite, the hardness of the center of the cross section is not more than the hardness of the middle of the cross section, and the hardness of the middle of the cross section is Vickers hardness H A free-cutting steel wire rod excellent in drilling workability, having a _{V of} 250 or less. The graphitization index CE is calculated by the following equation. CE = C + Si / 3-Mn / 12 --- (1) However, the symbol of the element in the above formula represents the weight% of each element. 3. The slab or steel slab further contains at least one element selected from the group consisting of the following elements, and the following formula is used instead of the graphitization index CE. The free-cutting steel rod or wire according to claim 1 or 2, characterized by using formula (2). weight%
Cu: 0.01 to 2.0%, Ni: 0.01 to 1.0%, Co: 0.01 to 0.50%, Cr: 0.01 to 0.50%, Mo: 0. 01 to 0.50%, and B: 0.0005 to 0.010%. The graphitization index CE is given by the following equation. CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9 -Cr / 9-Mo / 9 + B --------------------------- -(2) However, the symbol of the element in the above formula represents the weight% of each element. 4. The slab or steel slab further contains at least one element selected from the group consisting of the following element compositions, and, instead of the formula for calculating the graphitization index CE, 3. The method according to claim 1, wherein equation (3) is used.
Or a free-cutting steel rod or wire excellent in drilling workability described in 3. % By weight, Al: 0.001 to 0.10%, Ti: 0.005 to 0.050%, Zr: 0.005 to 0.050%, V: 0.01 to 0.20%, and Nb: 0.01 to 0.20%. The graphitization index CE is given by the following equation. CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 ------------- ---------------- (3) Here, the symbol of the element in the above formula represents the weight% of each element. 5. The slab or the steel slab further contains at least one element selected from the group consisting of the following element compositions, and the following formula is used instead of the graphitization index CE. 5. The method according to claim 1, wherein equation (4) is used.
A free-cutting steel rod or wire according to any of the described inventions, which is excellent in drilling workability. Ca: 0.0010-0.0100%, Mg: 0.0010-0.10%, and REM: 0.0010-0.10% by weight. The graphitization index CE is given by the following equation. CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 + 0.07 ---------- ------------------- (4) However, the symbol of the element in the above formula represents the weight% of each element. 6. In% by weight, C: more than 1.00 to 1.50%, Si: 1.00 to 2.80%, Mn: 0.01 to 2.00%, P: 0.050% or less , S: 0.10% or less, O: 0.0050% or less, and N: 0.020% or less, and has a chemical component composition consisting of a balance of Fe and unavoidable impurities. The graphitization index CE obtained by the above is 1.30 or more, and the degree of central segregation of carbon [C] / [C] ₀ (where [C] is the carbon content at the target position, and [C] ₀ is the elemental steel analysis The slab or slab having a carbon content of 1.01 to 2.00 is heated to a temperature in the range of 850 to 1150 ° C., hot-rolled, and thus hot-rolled. The time for cooling the high-temperature steel material from 800 ° C. to 600 ° C. by gradually cooling the cover was adjusted to 5 minutes or more, and the average particle size in the steel material was 1 μm. The above graphite 0μm to precipitate 100 / mm ² or more, 70 metallographic
% Or more of ferrite and the balance cementite,
Or made of ferrite alone, the hardness of the cross-sectional center portion and below the hardness of the cross-section intermediate portion, characterized in that it and adjusting the hardness of the cross-section intermediate portion below Vickers hardness H _V 300, drilling Method for manufacturing free-cutting steel rods and wires with excellent properties. The graphitization index CE is calculated by the following equation. CE = C + Si / 3-Mn / 12 --- (1) However, the symbol of the element in the above formula represents the weight% of each element. 7. The slab or steel slab further contains at least one element selected from the group consisting of the following elements, and, instead of the formula for calculating the graphitization index CE, The method for producing a free-cutting steel rod and wire having excellent drilling workability according to claim 6, wherein the formula (2) is used. By weight%, Cu: 0.01 to 2.0%, Ni: 0.01 to 1.0%, Co: 0.01 to 0.50%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and B: 0.0005 to 0.010%. The graphitization index CE is given by the following equation. CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9 -Cr / 9-Mo / 9 + B --------------------------- -(2) However, the symbol of the element in the above formula represents the weight% of each element. 8. The slab or steel slab further contains at least one element selected from the group consisting of the following elements, and the following formula is used instead of the formula for calculating the graphitization index CE. The method for producing a free-cutting steel rod and wire having excellent drilling workability according to claim 6 or 7, wherein the formula (3) is used. % By weight, Al: 0.001 to 0.10%, Ti: 0.005 to 0.050%, Zr: 0.005 to 0.050%, V: 0.01 to 0.20%, and Nb: 0.01 to 0.20%. The graphitization index CE is given by the following equation. CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 ------------- ---------------- (3) Here, the symbol of the element in the above formula represents the weight% of each element. 9. The slab or steel slab further contains one or more elements selected from the group consisting of the following element compositions, and, instead of the formula for calculating the graphitization index CE, 8. The method according to claim 6, wherein equation (4) is used.
Or a method for producing a free-cutting steel rod or wire excellent in drilling workability according to item 8. Ca: 0.0010-0.0100%, Mg: 0.0010-0.10%, and REM: 0.0010-0.10% by weight. The graphitization index CE is given by the following equation. CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 + 0.07 ---------- ------------------- (4) However, the symbol of the element in the above formula represents the weight% of each element. 10. In% by weight, C: more than 1.00 to 1.50%, Si: 1.00 to 2.80%, Mn: 0.01 to 2.00%, P: 0.050% or less , S: 0.10% or less, O: 0.0050% or less, and N: 0.020% or less, and has a chemical component composition consisting of a balance of Fe and unavoidable impurities. The graphitization index CE obtained by the above is 1.30 or more, and the degree of central segregation of carbon [C] / [C] ₀ (where [C] is the carbon content at the target position, and [C] ₀ is the elemental steel analysis The slab or slab having a carbon content of 1.01 to 2.00 is heated to a temperature in the range of 850 to 1150 ° C., hot-rolled, and thus hot-rolled. The time for cooling from 800 ° C. to 600 ° C. by cooling the hot steel material with a cooling medium is adjusted to 1 minute or less. Heated to 600
After holding for 3 hours or less at a temperature within the range of ~ 900 ° C,
After air cooling, graphite having an average particle size of 1.0 μm or more was precipitated in the steel material at a rate of 100 particles / mm ² or more, and the metal structure was composed of 80% or more of ferrite and the remainder cementite, or consisted only of ferrite, the hardness of the parts is below the hardness of the cross-section intermediate portion, and the hardness of the cross-section intermediate portion and adjusting the following Vickers hardness H _V 250, free cutting steel bars with excellent drilling processability Wire rod manufacturing method. The graphitization index CE is calculated by the following equation. CE = C + Si / 3-Mn / 12 --- (1) However, the symbol of the element in the above formula represents the weight% of each element. 11. The slab or slab further contains one or more elements selected from the group consisting of the following elements and the following slabs or steel slabs: 11. The method according to claim 10, wherein equation (2) is used.
A method for producing a free-cutting steel rod and wire having excellent drilling workability as described.
By weight%, Cu: 0.01 to 2.0%, Ni: 0.01 to 1.0%, Co: 0.01 to 0.50%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and B: 0.0005 to 0.010%. The graphitization index CE is given by the following equation. CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9 -Cr / 9-Mo / 9 + B --------------------------- -(2) However, the symbol of the element in the above formula represents the weight% of each element. 12. The slab or steel slab further contains at least one element selected from the group consisting of the following element compositions, and, instead of the formula for calculating the graphitization index CE, 11. The method according to claim 10, wherein equation (3) is used.
Or the method for producing a free-cutting steel rod or wire excellent in drilling workability described in 11; % By weight, Al: 0.001 to 0.10%, Ti: 0.005 to 0.050%, Zr: 0.005 to 0.050%, V: 0.01 to 0.20%, and Nb: 0.01 to 0.20%. The graphitization index CE is given by the following equation. CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 ------------- ---------------- (3) Here, the symbol of the element in the above formula represents the weight% of each element. 13. The slab or steel slab further contains at least one element selected from the group consisting of the following elements and the following slab or steel slab: 2. The method according to claim 1, wherein equation (4) is used.
13. A method for producing a free-cutting steel rod or wire according to 0, 11 or 12, which is excellent in drilling workability. Ca: 0.0010-0.0100%, Mg: 0.0010-0.10%, and REM: 0.0010-0.10% by weight. The graphitization index CE is given by the following equation. CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 + 0.07 ---------- ------------------- (4) However, the symbol of the element in the above formula represents the weight% of each element.