JP2000063988A - Free cutting steel bar wire rod excellent in punching workability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a bar wire rod excellent in punching workability by hot- rolling and cooling a cast or steel slab specifying component composition, a graphitization index and the degree of central segregation of carbon under specified conditions. SOLUTION: A cast or steel slab which has a composition consisting of by weight, >1.00-1.50% C, 1.00-2.80% Si, 0.01-2.00% Mn, <=0.050% P, <=0.10% S, <=0.0050% O and the balance Fe with inevitable impurities and in which a graphitization index CE determined by the equation is >=1.30 and also the degree of central segregation of carbon, [C]/[C]0 (where [C] is carbon content in an object position and [C]0 is carbon content at ingot steel analysis), is 1.01-2.00, is prepared. The slab is then heated to 850 to 1,150 deg.C, hot rolled and cooled down to 800 to 600 deg.C in >=5 min. By this method, graphite of >=1.0 μm average grain size is precipitated by >=100 pieces/mm2, the metallic structure is composed of >=20% ferrite and the balance cementite, further, the hardness in the cross-sectional central part becomes not higher than that in the cross-sectional intermediate part and the hardness in the cross-sectional intermediate part becomes <=HY300.

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 and wire rod which is excellent in hole formability at the center of the cross section of a steel material. Related to the technology for manufacturing ultra-free-cutting steel rods and wires with excellent drilling workability, in which misalignment does not occur by adjusting the hardness so that the center part is lower than the surroundings. Is.

[0002]

2. Description of the Related Art As a conventional super free-cutting steel excellent in machinability, JISG 4804 S containing a combination of sulfur and lead is added.
SUM43L in which Pb is added to UM24L and SUM43
Is a typical example. The machinability of these free-cutting steels is emphasized more than the ductility such as impact value and elongation, but the machinability required for them is the turning and parting of the outer periphery as well as tools such as drills. It is important that the steel has excellent drilling workability at the center of the steel section.

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

In drilling, if the center of the work material is hard, the tip of the tool is greatly worn and the life of the tool is shortened.
Also, if the hardness in the center is harder than the surroundings, the tool tip will shift to the soft surrounding part during drilling,
So-called core misalignment occurs and the product cannot be used as a product. Therefore, as a free-cutting steel rod wire to be perforated, the hardness of its central portion must be equal to or lower than that of the surrounding portion.

Further, in the drilling process, unless the chips are appropriately divided into small pieces, the chip dischargeability from the holes is poor,
This may cause tool breakage. In addition, recently, unmanned machining is often performed by an automatic lathe, and if the chips are connected for a long time and become entangled, it is necessary to stop the machine and perform extra work to remove the chips, which increases productivity. Will be lowered. Therefore, along with the long life of the tool,
There is a demand for free-cutting steel that has excellent chip disposability so that the chips can be finely divided into appropriate sizes.

In order to improve the tool life and the chip disposability, sulfur and phosphorus composite free-cutting steels are additionally provided with 0.10 to 0 of lead, which is a free-cutting element, as conventionally found in SUM23L and SUM43L. Machinability has been improved by adding 0.35%.

Since the melting point of Pb is as low as 327 ° C., Pb melts during cutting to embrittle the steel and improve chip disposability. Further, the lubricating effect of Pb is also added, and the tool life is extended. However, the use of Pb in free cutting steel is
Due to environmental hygiene issues such as generation of fume, lead-free ultra-free cutting steel is required today.

As elements for improving the machinability of steel materials,
In addition to Pb, elements such as S, Ca, Bi, Se and Te are known, but these elements alone have a small machinability improving effect, are expensive, and have a problem in environmental hygiene.
Since it has at least one of the above-mentioned drawbacks, its use as a lead substitute element is limited.

For example, S is effective in improving machinability, but when S is added in a large amount, M which is elongated in the hot working direction is elongated.
There is a problem that a large amount of nS is formed to cause anisotropy in mechanical properties and to reduce toughness. Therefore, in the conventional SUM24L and SUM43L, tip cracks are likely to occur during hot rolling, causing rolling troubles. In order to avoid this trouble, it has been necessary to perform a complicated work such as cutting the tip of the unrolled steel strip into a fine shape like a pencil tip. Further, since SUM24L is a low carbon steel, in order to impart wear resistance to machined parts, it was necessary to carry out carburizing and quenching at 900 ° C. for a long time of several hours.

On the other hand, graphite is an element that significantly improves machinability as seen in cast iron. However, when carbon is added to steel, cementite precipitates, so 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, JP-A-2-107742 (hereinafter referred to as Prior Art 1).
JP-A-3-140411 (hereinafter, referred to as prior art 2
), A temperature of 600 to 800 ° C. for several hours to 20
It discloses a steel material in which graphite is precipitated by performing annealing for a long time of 0 hours, or a method for manufacturing such a steel material.

However, such a long-time graphitization heat treatment not only causes an increase in cost, but also causes a detrimental effect of decarburizing the steel material during the heat treatment and adversely affecting the performance of the final part. Therefore, there has been a demand for a lead-free super free-cutting steel which is simpler than conventional heat treatment, and which is excellent in desired center hole drilling property without heat treatment if possible.

[0012]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
No. 2 has one of the following problems unsolved. Problem 1: Due to center segregation, the hardness of the center part is high, so the tool life is short and misalignment occurs.

Problem 2: The free-cutting elements used are toxic and pose a problem in terms of environmental measures. Problem 3: Since a large amount of S and P are contained in combination,
It has poor hot workability and requires special machining to prevent tip cracking before rolling.

Problem 4: In low carbon free-cutting steel, it is necessary to carry out carburizing and quenching for a long time in order to improve wear resistance. Problem 5: By using carbon as a non-toxic free-cutting element and precipitating it as graphite, machinability can be improved, but long-time graphitization annealing must be performed.
High cost.

Therefore, according to the present invention, in order to solve the above-mentioned problems and manufacture parts for automobiles and industrial machines having excellent machinability, the steel material is gradually cooled after hot rolling or baked for a short time. The purpose is to provide a free-cutting steel rod wire that does not contain lead, is excellent not only in peripheral machining, but also in punching workability at the center of the cross section, is inexpensive, and has no environmental hygiene problems due to smoothing. To do.

[0016]

From the background and viewpoints described above, the inventors of the present invention have excellent drilling workability equal to or higher than conventional sulfur-lead composite free-cutting steel without adding lead. We have conducted extensive research to develop ultra-free cutting steel. As a result, the following findings were obtained.

That is, in order to precipitate graphite in the air-cooled state after rolling or by a simple heat treatment such as normalizing, C is added in an amount of more than 1.00% with respect to the composition of the steel to obtain a hypereutectoid steel, Si is set to a high value of 1.00% or more to promote graphitization. In addition, an appropriate amount of Mn is added to secure the ductility of steel,
In addition, a steel in which the 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 making method, segregation of components is inevitably formed in the center of the axial center of the slab. Conventionally, this center segregation has been harmful to the quality of steel materials, but in the present invention, the technology of utilizing center segregation as follows was abolished. Degree of segregation of carbon concentration in the center segregation part [C] / [C] ₀ (where [C] is the carbon content of the target position, and [C] ₀ is the carbon content of raw steel analysis)
Is adjusted within the range of 1.01 to 2.00, or a slab obtained by slab-rolling the slab is manufactured. However, the steel ingot may be treated in the same manner as the cast piece.

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

By the above treatment, a graphite rod having an appropriate size and amount necessary for achieving the object of the present invention and a steel rod wire having a soft ferrite or a ferrite + cementite structure are obtained. Since C, Si, etc. are segregated at the center of the rod wire cross section, the graphitization index CE is large, which promotes the precipitation of graphite, the amount of graphite is larger than that of the surrounding portion, and the amount of cementite decreases and ferrite increases. The hardness decreases.

As a result, by utilizing the central segregation, which has been conventionally regarded as harmful, without adding lead, the drilling workability is equal to or more than that of the conventional sulfur-lead composite free-cutting steel. We have found that it is possible to manufacture rods and wires. .

The present invention is based on the above findings and has the following features. The free-cutting steel rod wire according to claim 1, in weight%, is C: more than 1.00 to 1.5.
0%, 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 composition of the balance iron (Fe) and unavoidable impurities, has a graphitization index CE of 1.30 or more determined by the following formula (1), and has a central segregation degree of carbon [ C] / [C] ₀ (where [C] is the carbon content of the target position, and [C] ₀ is the carbon content of raw steel analysis) is 1.0
Cast pieces or steel pieces in the range of 1 to 2.00 are
It is heated to a temperature in the range of 1150 ° C., hot-rolled, and the hot-rolled high-temperature steel material is heated to 800 ° C. to 600 ° C.
The cooling time is adjusted to 5 minutes or more, and with respect to the characteristic values of the steel material thus obtained, 100 pieces / mm ² or more of graphite having an average particle size of 1.0 μm are precipitated, and the metallographic structure is 20% or more of ferrite. The remainder is cementite or only ferrite, the hardness of the center of the cross section is less than or equal to the hardness of the middle of the cross section, and the hardness of the middle of the cross section is Vickers hardness H _V 300 or less. It is characterized by being present. The graphitization index CE is calculated by the following formula.

CE = C + Si / 3-Mn / 12 ----------------------------- (1) However, in the above equation The element symbol represents the weight% of each element. Since graphite grows by aggregating carbon in base iron, ferrite precipitates around graphite in such a manner as to erode layered cementite. Since the graphitization index CE is high in the central part due to the segregation of the components, the precipitation of graphite is further promoted, and the amount of graphite is large and the amount of ferrite is large and the surroundings are softer.

By the above treatment, it is possible to obtain a super free-cutting steel rod wire having a Vickers hardness of 300 or less in the middle portion and having a softness of 300 or less and having excellent drilling workability. The free-cutting steel rod wire according to claim 2 is applied particularly when the diameter of the rod wire is thin and the cooling rate is not sufficiently small.

% By weight, C: more than 1.00 to 1.50%,
Si: 1.00 to 2.80%, Mn: 0.01 to 2.0
0%, P: 0.050% or less, S: 0.10% or less,
O: 0.0050% or less and N: 0.020% or less, having a chemical composition of the balance iron (Fe) and unavoidable impurities, and having a graphitization index determined by the following formula (1). CE is 1.30 or more, and the center segregation degree of carbon [C] / [C] ₀ (where [C] is the carbon content at the target position, and [C] ₀ is the carbon content in raw steel analysis). ) Is 1.0
Cast pieces or steel pieces in the range of 1 to 2.00 are
It is heated to a temperature in the range of 1150 ° C., hot-rolled, and the hot-rolled high-temperature steel material is heated to 800 ° C. to 600 ° C.
Up to 6 after cooling down to any time.
After maintaining at a temperature in the range of 00 to 900 ° C. for 3 hours or less, air-cooling, and regarding the characteristic value of the steel material thus obtained, 100 pieces / mm ² or more of graphite having an average particle size of 1.0 μm or more are precipitated, The structure consists of 30% or more of ferrite and the remainder cementite, or consists of only ferrite, and the hardness of the center of the cross section is less than the hardness of the middle of the cross section, and the hardness of the middle of the cross section is Vickers hardness. The feature is that it is H _V 250 or less. The graphitization index CE is calculated by the following formula.

CE = C + Si / 3-Mn / 12 ----------------------------- (1) However, in the above equation The element symbol represents the weight% of each element. In the above, after hot rolling, cooling is performed at a temperature of 800 to 600 ° C. for an arbitrary time, but the method may be simple air cooling, but it may be cooled by water or an air blast, and martens containing a large amount of lattice defects such as dislocations. If site, bainite, or fine pearlite is used, the precipitation of graphite is promoted and the subsequent processing time is short.

The free-cutting steel rod wire according to claim 3 is the same as in claim 1.
Or in the invention described in 2, as the cast piece or steel piece,
Further, it is characterized by additionally containing at least one selected from the group consisting of component compositions of the following elements, and using the following formula (2) instead of the formula for calculating the graphitization index CE. is there.

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

The free-cutting steel rod wire according to claim 4 is, in the invention according to claim 1, 2 or 3, one type selected from the group consisting of the component compositions of the following elements as the cast piece or the steel piece. In addition to the above, the graphitization index CE is contained.
It is characterized by using the following formula (3) instead of the calculation formula.

Here, the group consisting of the component composition of the above elements is, by weight%, Al: 0.001 to 0.10%, Ti:
0.005-0.050%, Zr: 0.005-0.0
50%, V: 0.01 to 0.20%, and Nb: 0.
It is 01 to 0.20%. And the above formula (3) means 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 element symbol in the above formula indicates that it represents the weight% of each element.

The free-cutting steel rod wire according to claim 5 is the same as in claim 1.
In any one of the inventions 1 to 4, as the cast slab or steel slab, at least one selected from the group consisting of the composition of elements of the following elements is further added and contained, and the graphitization index CE is It is characterized by using the following formula (4) instead of the calculation formula.

Here, the group consisting of the component composition of the above elements is, by weight%, Ca: 0.0010 to 0.0100%,
Mg: 0.0010 to 0.10%, and REM: 0.
It is 0010 to 0.10%. And the expression (4) means 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. .

In the method for producing a free-cutting steel rod wire according to claim 6, the weight% is C: more than 1.00 to 1.50%, and Si:
1.00 to 2.80%, Mn: 0.01 to 2.00%,
P: 0.050% or less, S: 0.10% or less, O: 0.
It contains 0050% or less and N: 0.020% or less, has a chemical composition of the balance Fe and inevitable impurities, and has a graphitization index CE of 1.30 or more determined by the following formula (1). The center segregation degree of carbon [C] /
[C] ₀ (where [C] is the carbon content of the target position, [C]
₀ is the carbon content of raw steel analysis) is 1.01 to 2.0
Cast pieces or steel pieces within the range of 0 to 850 to 1150 ° C
Is heated to a temperature within the range of, and hot-rolled.
The time for cooling to 00 ° C. was adjusted to 5 minutes or more, and 100 pieces / mm ² of graphite having an average particle size of 1.0 μm or more in the above steel material.
Precipitated as described above, the metallographic structure consists of 20% or more of ferrite and the remaining cementite, or consists of only ferrite, and the hardness of the center of the cross section is set to be equal to or less than the hardness of the middle part of the cross section
Also, the hardness of the intermediate portion of the cross section is Vickers hardness H _V 300
It is characterized by making the following adjustments.

CE = C + Si / 3-Mn / 12 ----------------------------- (1) However, in the above equation The element symbol represents the weight% of each element. In the method for producing a free-cutting steel rod wire according to claim 7, in the invention according to claim 6, at least one selected from the group consisting of the component compositions of the following elements is further added as the cast piece or the steel piece. It is characterized in that the following formula (2) is used instead of the formula for calculating the graphitization index CE.

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

The method for producing a free-cutting steel rod wire according to claim 8 is the method according to claim 6 or 7, wherein the cast piece or the steel piece is one or more selected from the group consisting of the component compositions of the following elements. Is added and contained, and the graphitization index CE
It is characterized by using the following formula (3) instead of the calculation formula.

Here, the group consisting of the component composition of the above elements is, by weight%, Al: 0.001 to 0.10%, Ti:
0.005-0.050%, Zr: 0.005-0.0
50%, V: 0.01 to 0.20%, and Nb: 0.
It is 01 to 0.20%. And the above formula (3) means 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 element symbol in the above formula indicates that it represents the weight% of each element.

In the method for producing a free-cutting steel rod wire according to claim 9, in the invention according to claim 6, 7 or 8, the cast piece or the steel piece is further selected from the group consisting of component compositions of the following elements. It is characterized by additionally containing one or more kinds and using the following formula (4) instead of the formula for calculating the graphitization index CE.

Here, the group consisting of the component composition of the above elements is, by weight%, Ca: 0.0010 to 0.0100%,
Mg: 0.0010 to 0.10%, and REM: 0.
It is 0010 to 0.10%. And the expression (4) means 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. .

The manufacturing method of the free-cutting steel rod wire according to claim 10 is, 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.
It contains 0050% or less and N: 0.020% or less, has a chemical composition of the balance Fe and inevitable impurities, and has a graphitization index CE of 1.30 or more determined by the following formula (1). The center segregation degree of carbon [C] /
[C] ₀ (where [C] is the carbon content of the target position, [C]
₀ is the carbon content of raw steel analysis) is 1.01 to 2.0
Cast pieces or steel pieces within the range of 0 to 850 to 1150 ° C
The temperature for cooling from 800 ° C. to 600 ° C. is adjusted to 1 minute or less by heating to a temperature within the range, hot rolling, and cooling the hot-rolled high temperature steel material with a cooling medium, and further heating to the temperature after the 3hr following time holding in the range of 600 to 900 ° C., air cooled, the steel material of 100 the average particle diameter 1.0μm or more graphite in / mm ² to precipitate more, the metal structure 30% Consisting of the above ferrite and the remainder cementite, or consisting of only ferrite,
It is characterized in that the hardness of the central portion of the cross section is set to be equal to or lower than the hardness of the intermediate portion of the cross section, and the hardness of the intermediate portion of the cross section is adjusted to Vickers hardness H _V 250 or lower.

CE = C + Si / 3-Mn / 12 ----------------------------- (1) However, in the above equation The element symbol represents the weight% of each element. In the above method, in particular, in order to promote the precipitation of graphite by the reheating treatment, the bar wire rod after hot rolling is set to 800-60.
It rapidly cools between 0 ° C in 1 minute or less.

The method for producing a free-cutting steel rod wire according to claim 11 is the method according to claim 10, wherein the cast piece or the steel piece is one or more selected from the group consisting of the component compositions of the following elements. Is added, and the graphitization index C is included.
It is characterized by using the following formula (2) instead of the calculation formula of E.

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

According to a twelfth aspect of the present invention, in the method for producing a free-cutting steel rod and wire according to the tenth or eleventh aspect of the invention, the cast piece or the steel piece is further selected from the group consisting of the component compositions of the following elements. It is characterized in that it contains more than one species and contains the following formula (3) instead of the formula for calculating the graphitization index CE.

Here, the group consisting of the component composition of the above elements is, by weight%, Al: 0.001 to 0.10%, Ti:
0.005-0.050%, Zr: 0.005-0.0
50%, V: 0.01 to 0.20%, and Nb: 0.
It is 01 to 0.20%. And the above formula (3) means 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 element symbol in the above formula indicates that it represents the weight% of each element.

The method for producing a free-cutting steel rod wire according to claim 13 is the method according to claim 10, 11 or 12, wherein the cast or steel piece is further selected from the group consisting of the following elemental compositions. It is characterized by additionally containing one or more kinds and using the following formula (4) instead of the formula for calculating the graphitization index CE.

Here, the group consisting of the component composition of the above elements is, by weight%, Ca: 0.0010 to 0.0100%,
Mg: 0.0010 to 0.10%, and REM: 0.
It is 0010 to 0.10%. And the expression (4) means 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]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, a high Si hyper-eutectoid carbon steel having a predetermined center segregation degree and a low alloy steel cast or steel slab are used as a raw material for hot rolling a wire rod. Or slowly cool it over 3 hours, or keep it within the specified temperature range for 3 hours.
Then, depending on whether it is reheated and then air-cooled, a rod-shaped wire having a soft central part and a structure rich in graphite is manufactured. By such a simple process, a super free-cutting rod wire excellent in lead-free drilling workability is manufactured.

Next, the reasons for limiting the constituent features of the present invention will be described below. (1) Carbon (C) C is an important element for precipitating graphite and ensuring strength. After reheating the steel material produced by hot rolling,
In order to precipitate graphite by air cooling without performing slow cooling such as furnace cooling, a C content rate 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 defects during bar rolling increases. Further, the graphite particles precipitated after air cooling become coarse, and the toughness is reduced. Therefore, the C content exceeds 1.00 to 1.5.
It is limited to 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, when Si exceeds 2.80%, not only the non-metallic inclusions increase and the toughness decreases, but also decarburization is increased during hot rolling or reheating for graphitization. Therefore, Si
The content is limited within the range of 1.00 to 2.80%.

(3) Manganese (Mn) Mn makes S in the steel harmless as MnS and improves the hot ductility of the steel. Since it is used for this purpose, Mn is 0.0
Addition of 1% or more is required. However, Mn hinders the precipitation of graphite, so the upper limit is 2.00%. In addition, M
By lowering the n content, the amount of Si required for graphitization can be reduced. When 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 to reduce hot ductility and promotes the generation of surface defects. In order to prevent such an adverse effect, the P content is 0.0
Limited to 50 or less. It is more preferably 0.030% or less.

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

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

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

(8) Copper (Cu) Cu accelerates the precipitation of graphite and forms a solid solution with ferrite to enhance the toughness of steel. Since Cu is used for this purpose, addition of 0.01% or more is required. However, Cu
If the content exceeds 2.0%, the solid solubility limit in the steel is exceeded, so undissolved Cu remains, hot ductility is reduced, and surface defects are promoted. Therefore, the Cu content is 0.01 to
It is desirable to set it within the range of 2.0%.

(9) Nickel (Ni) Ni, like Cu, promotes the precipitation of graphite, and
Improves the toughness of steel by forming a solid solution in ferrite. Since Ni is added for these purposes, Ni needs to be added in an amount of 0.01% or more. However, the effect is saturated when the content exceeds 2.0%. Further, Ni is an expensive element. Therefore, it is desirable that the Ni content is within the range of 0.01 to 2.0%.

(10) Cobalt (Co) Co, like Cu and Ni, promotes precipitation of graphite and enhances toughness of steel. Since Co is added for these purposes, Co needs to be added in an amount of 0.01% or more. However, Co is a more expensive element than Ni. Therefore, it is desirable that the Co content is within the range of 0.01 to 0.50%.

(11) Chromium (Cr) When Cr is also added in a small amount, it forms a solid solution with ferrite to enhance the toughness of steel. Since it is used for this purpose, addition of 0.01% or more is required. However, Cr has a larger effect of inhibiting graphitization than Mn. Therefore, if Cr exceeds 0.50%, a large amount of graphitization-promoting element is required, resulting in high cost. Therefore, it is desirable that the Cr content is within the range of 0.01 to 0.50%.

(12) Molybdenum (Mo) Mo enhances the toughness of steel when added in a small amount. Since it is used for this purpose, addition of 0.01% or more is required. 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 that the Mo content is in the range of 0.01 to 0.50%.

(13) Boron (B) B fixes N in steel as BN, reduces the graphitization inhibiting effect of N, and BN acts as graphite precipitation nuclei to promote the precipitation of graphite. Since it is used for this purpose, 0.00
Addition of 05% or more is required. However, B is 0.01
Even if added over 0%, not only is the effect saturated,
Precipitates a large amount of BN or carbon boride and reduces hot ductility. Therefore, it is desirable that the B content is within the range of 0.0005 to 0.010%.

(14) Aluminum (Al) Al is an important element as a deoxidizing agent and is an element for precipitating AlN to make crystal grains fine. Further, it is an element that promotes graphitization similarly to Si. For these purposes, it is necessary to add at least 0.001% of Al. However, when Al is added in excess of 0.10%, the amount of oxide-based inclusions increases, which deteriorates the cleanliness of steel and causes cracking during hot working. Further, in continuous casting, Al ₂ O ₃ is deposited on the nozzle and causes nozzle clogging, so 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 to refine the crystal grains. In addition, TiN and TiC act as nuclei for graphite precipitation and accelerate the precipitation of graphite. Ti addition amount is 0.005%
If it is less than 0.1%, 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 wear of the tool. Therefore, the Ti content is
It is desirable to set it within the range of 0.005 to 0.050%.

(15) Zirconium (Zr) Zr also precipitates nitrides and carbides in the same manner as Ti to refine the crystal grains and promote the precipitation of graphite. Zr
If the addition amount is less than 0.005%, the effect is small. On the other hand, when Zr is added in an amount of more than 0.050%, wear of the tool is promoted. Therefore, the Zr content is 0.005 to 0.
It is desirable to set it within the range of 050%.

(16) Vanadium (V) V also precipitates nitrides and carbides to refine the crystal grains. Also, since the precipitates are fine, the yield stress of steel is increased,
Improve fatigue limit stress. If the amount of V added 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 graphitization promoting element is required. Therefore, the V content is 0.0
It is desirable to set it within the range of 1 to 0.20%.

(17) Niobium (Nb) Nb also precipitates nitrides and carbides to refine the crystal grains and increase 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, and makes the crystal grains after forging finer to improve toughness. If the amount of Nb added is less than 0.01%, its effect is small. On the other hand, if it is added in excess of 0.20%, precipitation of graphite is hindered and a large amount of graphitization promoting element is required. Therefore, it is desirable that the Nb content is within the range of 0.01 to 0.20%.

(18) Calcium (Ca) Ca is used as an inoculant in cast iron and promotes graphitization. It is considered that this is because the vapor pressure of Ca at the temperature level of the molten steel is high and the vapor of Ca forms minute cavities in the solidified steel during casting, which serves as nuclei for graphite precipitation to deposit spherical graphite. . Also in steel, Ca behaves similarly to cast iron and facilitates graphite precipitation after hot working. Also,
If Ca is present as an oxide inclusion, it has a large effect of forming a berag in cemented carbide tool cutting and prolonging the tool life, so addition of Ca to free-cutting steel is a desirable element. For these purposes, it is necessary to add Ca in an amount of 0.0010% or more. However, even if added over 0.010%, the effect is saturated. Therefore, the Ca content is 0.001
It is desirable to set it within the range of 0 to 0.010%.

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

(20) REM (Rare Earth Element) REM such as Ce and La also promotes precipitation of graphite after forging.
If the addition amount is less than 0.0010%, the effect is small. On the other hand, the effect is saturated even if REM is added in excess of 0.10%. Therefore, the REM content is 0.0010 to 0.10.
It is desirable to set it within the range of%.

In addition to the above, steel contains elements such as Sn and As that are inevitably mixed. Further, when the environmental problem is small, it is possible to supplementarily add a small amount of a free-machining property improving element such as Bi, Se or Te.

(21) Graphitization index Graphite in the steel material is effective in improving its free-cutting property. In order to promote the precipitation of graphite in the steel material, it is important to increase the graphitization index CE. This CE is represented by the following formula for major 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 element symbol in the above formula represents 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 formula (3).

On the other hand, the precipitation of graphite also 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 graphitized for a long time by furnace cooling or the like, and it becomes difficult to precipitate graphite by a short heat treatment. Therefore, the graphitization index CE is limited to 1.30 or more.

(22) Degree of center segregation of carbon [C] /
[C ₀ ] Generally, the presence of center segregation of the components in the cast or steel,
That is, when the content ratio of the elemental component such as C in the central portion is higher than that in other portions, the hardness of that portion becomes high. for that reason,
When drilling a hole in the center of the cross section of a rod and wire, the tip of the drill shifts to its periphery and the core of the hole shifts. However,
In the present invention, the segregation of C and Si in the central portion is used to increase the graphitization index CE according to the above equation (3), so that the graphite precipitation amount in the central portion is increased and the amount of ferrite is increased accordingly. Also increases. In this way, the hardness of the central portion is lower than that of its periphery, which is preferable for drilling. Thus, the present invention is also characterized in that the center segregation is positively utilized.

The center segregation degree of C in the cast or steel slab is 1.0.
If the segregation is less than 1, it is not possible to exert the effect of precipitating a large amount of graphite to an extent advantageous for punching and making the metal structure have a large proportion of soft ferrite, as compared with the surrounding portion. On the other hand, when the center segregation degree of C exceeds 2.00, the ductility of the central portion is insufficient and the center is torn during hot rolling, which hinders the rolling operation. However, the center segregation degree of C is limited to the range of 1.01 to 2.00.

The center segregation degree is measured as follows.
U That is, the center of the cross section perpendicular to the length direction of the slab or steel
Sample for analysis with a drill of about 5 mmφ,
This is analyzed to obtain a C content rate [C] in the central portion. on the other hand,
Raw steel with C analysis value or ladle analysis value in the middle of cross section
Analysis C content [C]₀And And [C] / [C]
₀And the center segregation degree of C is calculated.

(23) Heating Temperature During Hot Rolling If the heating temperature of the steel material before hot rolling is less than 850 ° C., the deformability of the steel is insufficient, and surface defects are likely to occur on the rod wire. on the other hand,
When the heating temperature exceeds 1150 ° C., the solidus temperature of the steel is approached, and the hot ductility is also insufficient and cracks occur in the rod wire. Therefore, the heating temperature during hot rolling is 850 to 1150.
Between ℃. The slab or steel slab is heated within the above temperature range and then hot-rolled. A lower hot-rolling end temperature is preferable because it has many lattice defects and promotes precipitation of graphite. Therefore, a means of lowering the finishing temperature by cooling with water in the middle of the rolling mill group is effective.

(24) Cooling time after hot rolling By cooling as slowly as possible after hot rolling,
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, graphite of the desired size and number cannot be obtained, and the hardness becomes high. Therefore, 8
The cooling time from 00 ° C to 600 ° C is 5 minutes or more.
That is, the average cooling rate is set to 0.67 ° C./sec or less.

When the diameter of the rod wire is as thick as about 80 mmφ, the above cooling time is 5 even if it is simply air-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 a wire having a diameter of about 5 to 25 mmφ, the cooling time cannot be set to 5 minutes or longer by air cooling. Therefore, if the diameter of a straight rod (one straight rod) is small, cover the cooling floor with a cover to gradually cool the rod wire, or wind the hot rolled material into a coil. To cool. Further, a rod wire rod wound in a coil shape is covered with a cover and gradually cooled. In the case of a cooling facility such as a Stelmore line, in which the hot-rolled wire rod flows on the conveyor in a non-concentric ring state, the conveyor is covered with a cover to gradually cool the hot-rolled wire rod. By means of the above means, it is possible to cool a wire rod having a small diameter between 800 ° C. and 600 ° C. over 5 minutes or more.

When reheating the rod wire to promote graphitization, the rod 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 rod is cooled from 800 ° C. to 600 ° C. in a short time of 1 minute or less and the metal structure is adjusted to martensite, bainite, or fine pearlite, graphitization is promoted. There is.

(25) Graphitization If the rod wire having a small heating temperature diameter is not gradually cooled with the cover as described above, the cooling time at 800 to 600 ° C. is short and graphite cannot be sufficiently deposited. It is not possible to obtain a rod wire having excellent drilling workability. Therefore, reheating accelerates the precipitation of graphite. In order to obtain graphite of a desired size and number, if the heating temperature is lower than 600 ° C., the diffusion rate of carbon is low, so that it takes a long time of 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 increase is redissolved during the high temperature heating, and the size of the graphite obtained after air cooling becomes small. Therefore, it becomes necessary to cool the furnace slowly after heating, so that the graphitization treatment time becomes long. Therefore, the reheating temperature is set within the range of 600 to 900 ° C. Since graphite precipitation is most accelerated at temperatures between 650 and 800 ° C,
By treating within this temperature range, the purpose can be achieved in a shorter time.

(26) Graphitizing heating time Since the heating time in the above temperature range is long and the treatment is longer than 3 hours, the cost is high, and therefore the treatment time is 3 hours or less. Then, heating for 3 hours or less is sufficient to obtain desired size and number of graphite, metallographic structure and hardness. Therefore, the heating time is limited to 3 hours or less.

After heating, if the steel material is taken out of the furnace and air-cooled, it is sufficient without slow cooling. In the case of a coil material, it is cooled in a wound coil state of 1 to 3 tons.
In the case of straight rods, most of them are cooled in a state in which several to 100 wires are bound together. 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 low to precipitate and grow graphite, which is satisfactory.

(27) Particle size of graphite The precipitation shape of graphite in the present invention is generally expressed as a lump, but it may be spherical, granular or ellipsoidal, and has an average length. If the thickness / thickness ratio is 5 or less, there is no particular problem. As described above, when the average particle diameter of the graphite precipitated in the form of lumps is less than 1.0 μm, the effect of crushing the chips into small pieces at the time of 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
Adjust so that it is above. On the other hand, the upper limit of the average particle size is not particularly limited, but if a large number of graphite particles having a particle size exceeding 30 μm are deposited, the toughness will be reduced. Therefore, it is desirable that the average particle size of graphite is 30 μm or less.

(28) Number of Graphites The number of graphites per unit cross-sectional area is important for dividing the chips into small pieces. If the number is less than 100 pieces / mm ² , the effect of improving the chip disposability is small, so the number of graphite is 1
00 pieces / mm ² or more. The number of graphite depends on the size of the graphite, and the smaller the particle size, the smaller the particle size. In the present invention, when graphite having a particle size of 10 to 25 μm is deposited, the number is approximately 100 to 1000 pieces / mm ² , but when graphite having a particle size of 1.0 to 5 μm is deposited, it is approximately 3000 to 50,000. Reaching the number of pieces / mm ² .

(29) Metallographic Structure In order to extend the tool life, it is necessary to lower the hardness of the steel material, and it is necessary that the metallographic structure of the steel rod wire material is only ferrite + pearlite or ferrite. On this occasion,
The amount of ferrite must be 20% or more.
As the cooling rate after hot rolling becomes smaller, graphite grows larger and the amount of ferrite increases, so the amount of cementite decreases. Then, 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 20% or more of ferrite and the remaining cementite, or
Adjust so that only ferrite is used.

Further, when the graphitization heat treatment is applied, in order to further prolong the life of the tool, that is, to further lower the hardness of the rod and wire, the metallographic structure contains 30% or more of ferrite. It is necessary to include cementite for the balance or only ferrite.

By heating to a temperature in the range from A ₁ point to around A ₁ point-100 ° C., spheroidizing of cementite remaining without graphitization proceeds, and layered cementite becomes spherical or granular cementite. become.

Further, when heated to a temperature higher than the A ₁ point and cooled, precipitation of ferrite and cementite from austenite occurs, but by cooling at a sufficiently low cooling rate, a structure of graphite + ferrite can be obtained.

(30) Hardness After hot rolling, the Vickers hardness (H
_{If V} ) exceeds 300, tool wear will increase,
Tool life is shortened. Therefore, the Vickers hardness in the middle part needs to be 300 or less. Further, in the graphitized heat treatment material, in order to further extend the tool life,
Vickers hardness is adjusted to 250 or less. When the metal structure becomes ferrite single phase, the Vickers hardness is 13
It drops to about 0. By adjusting the amounts of graphite and ferrite so that the central portion is larger than the intermediate portion, the hardness of the central portion is made equal to or less than the hardness of the intermediate 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 graphitization index CE of the test steel used in the test.

[0090]

[Table 1]

[0091]

[Table 2]

Steel Nos. 1 to 23 in Table 1 are all steels having a composition within the scope of the present invention (components of the present invention).
27 are steels outside the scope of the present invention. Of these, Steel No. 2
No. 4 had a composition within the range of the present invention, but the center segregation degree of C was 2.00 because the molten steel temperature during solidification of the slab was too high.
In steel No. 25, the negative segregation was formed in the center part because the amount of roll reduction to the slab during continuous casting was too large, and the center segregation degree 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
All of 8 to 50 are steels having a composition outside the scope of the present invention. Among them, Steel No. 48 is SUM24 which is a conventional composition steel.
L and steel No. 49 are SUM43L which is a conventional component steel, and steel No. 50 is a hypoeutectoid graphite steel which is a conventional component steel.

The sample steels (steel Nos. 1 to 50) having the above-mentioned respective components were melted in a 130 ton electric furnace and then cast into a slab by continuous casting or ingot casting. Steel No. 1, 5 and 10
Was cast into a 160 mm square slab and then directly subjected to bar wire rolling. Other steel No. was subjected to slab rolling of a cast slab or a steel ingot into a 160 mm square steel slab and then subjected to bar wire rolling. In all the bar wire rolling, a cast slab or a steel slab was heated to a predetermined temperature in a heating furnace, and then hot rolled into a bar steel or a wire rod having various diameters.

[Test 1] In Test 1, steel Nos. 1 to 50 were tested within the scope of the present invention, Examples 1-1 to 1-.
20 and Comparative Example 1 which is a test outside the scope of the present invention
21 to 1-47 and Conventional Examples 1-48 to 1-50 were tested. In each case, after hot rolling into a rod wire, cooling was performed under predetermined test conditions, and thus a rod wire was manufactured. That is,
All rolled materials having a diameter of 20 mm or more were rolled into a straight rod and air-cooled, or the cover was gradually cooled on a cooling floor. The bar wire rod thus manufactured was subjected to a characteristic test and a hole punching test of a steel material. The content of the test is to visually inspect the rolled material for surface flaws and cracks, and for the rod and wire cooled after rolling, measure the graphite precipitation state and the metallographic structure with an optical microscope. The diameter and the number of graphite particles are measured, and the ferrite% (area%) in the ferrite + cementite structure as a metal structure is measured, and the Vickers hardness measurement test of the center and middle parts of the cross section is performed. The perforation test was conducted in the following manner. The wire rod was perforated with an automatic lathe. A 5 mmφ drill was used for a rod wire having a diameter of 13 to 20 mm, and a 10 mmφ drill was used for a steel bar having a diameter of 25 mm or more, and a hole having a depth 5 times the drill diameter was drilled. Drilling condition is 1 perimeter speed of the drill
A high speed drill was used with a feed rate of 50 m / min and a feed rate of 0.30 mm / rev. The life of the drill was judged by measuring the life of the tool that melted and became unpierceable, and the number of drilled holes was taken as the life. Moreover, the misalignment of the hole after processing was checked.

Tables 3 and 4 show the steel No. of the sample steel, the test conditions for rod and wire rod manufacturing, and the test results.

[0096]

[Table 3]

[0097]

[Table 4]

The following matters can be understood from the above test. (1) In Examples 1-1 to 1-20 of the present invention, since the component composition and the rolling heating temperature are within the scope of the present invention, no surface flaw is generated, and cooling at 800 to 600 ° C. after rolling is performed. Since the time is 5 minutes or more, it has the target size and number of graphite, the metal structure satisfies the requirements, and the Vickers hardness is lower in the central part than in the central part and in the intermediate part. H _V
A soft bar wire rod is obtained by satisfying 300 or less.

As an example showing the microstructure of the free-cutting steel rod wire according to the present invention in FIG. 1, graphite and ferrite of Example 2-1 +
The figure which shows the microstructure which consists of granular cementite is shown. In addition, due to the use of moderate center segregation, no misalignment of the drill occurred, and the tool life was good with 50 or more drilled holes.

(2) On the other hand, the target of the present invention was not achieved in the comparative example and the conventional example, which are tests in which even one condition outside the scope of the present invention is included. Details are as follows.

In Comparative Example 1-21, the chemical composition was within the range of the present invention, but the heating temperature during hot rolling was lower than the range of the present invention, so that the hot ductility was insufficient and the bar steel had a large content. A flaw has occurred. Further, in Comparative Example 1-22, similarly, the chemical composition is within the range of the present invention, but since the heating temperature during hot rolling is conversely lower than the range of the present invention, hot ductility is insufficient and bar steel is obtained. A big flaw occurred.

In Comparative Example 1-23, both the chemical composition and the heating temperature during hot rolling are within the scope of the present invention, but the cooling time after rolling was as short as 3.1 minutes. Is not sufficient and the average particle size of graphite is smaller than 1.0 μm,
The amount of ferrite in the metal structure is small, and the softening is insufficient, so the hardness of the central part is higher than that of the middle part, and the hardness of the middle part is also H _V 3
It exceeded 00. For this reason, misalignment occurred during drilling, and the life of the drill was as short as 24 holes.

In Comparative Example 1-24, since the center segregation degree of C exceeded 2.00, the ductility of the central portion was insufficient, and the carbon was torn from the center during hot rolling. On the contrary, in Comparative Example 1-25, the center segregation degree of C was smaller than 1.01, so that the hardness of the central portion was higher than the hardness of the intermediate portion, and therefore the drill was misaligned.

Comparative Examples 1-26 and 1-27
Although the chemical composition was within the range of the present invention, graphite precipitation was not seen 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 content of less than 1.0 μm was obtained, and the amount of ferrite in the metal structure was small,
The hardness of the intermediate part was also higher than H _V 300, the core of the drill was misaligned, and the tool life was short. Comparative Example 1-
On the other hand, in No. 29, the C content was high outside the present invention, so that the hot ductility was insufficient and the steel bar was flawed.

In Comparative Example 1-30, the Si content was lower than the range of the present invention, and therefore the graphite particles 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 the hot ductility was insufficient, and the steel bar was flawed.

In Comparative Example 1-32, the Mn content is 2.0.
It was higher than 0%, and due to this, hot ductility was insufficient and flaws occurred in the steel bar. 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 it was higher than the range, the hot ductility was insufficient in all cases, and the steel bar was flawed.

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

In Comparative Examples 1-37, the Ni and Mo contents were higher than the range of the present invention. Therefore, the hot ductility was insufficient and the steel bar was flawed. -In Comparative Examples 1-38, the Co and O contents were higher than the range of the present invention, the hot ductility was also insufficient, and the steel bar was flawed.

In Comparative Examples 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. In Comparative Examples 1-40, the N content is higher than the range of the present invention, and therefore the blowholes generated in the cast pieces cause
Many linear flaws occurred on the surface of the steel bar.

In Comparative Examples 1-41, the Zr and Ti contents were higher than the range of the present invention, and the steel bar was flawed. In Comparative Example 1-42, the V content was higher than the range of the present invention, and only small graphite having a graphitization index CE of less than 1.30 and 1.0 μm or less could be obtained. Therefore, it was inferior in drilling workability.

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

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

Comparative Example 1-48 is a conventional composition steel SUM
24L, and Comparative Example 1-49 is S of the conventional composition steel.
It was UM43L, and the drilling tool life was good. However, the hardness of the central portion was higher than the hardness of the intermediate portion, and misalignment occurred. Also, when hot rolling to steel bar, the tip of the steel slab is made thin like a pencil tip in order to prevent the tip from splitting and cracking during rolling and becoming a misroll, and rolling mill Had to bite into.

SUM24L and SUM of the above conventional composition steels
In contrast to 43L, in Examples 1-1 to 1-20 of the present invention, special tip processing of the steel piece is not required, and even if the steel piece cut by the shear is used, there is no problem and rolling is possible. It was

Comparative Examples 1-50, which are examples of conventional graphite steel, were wound into a coil and then gradually cooled by a cover.
No precipitation of graphite was observed, and the drilling workability was poor.

[Test 2] In Test 2, Steel Nos. 1 to 21 were tested within the scope of the present invention, Examples 2-1 to 2-.
21 and steel Nos. 22, 23, 26, 27 and 50
Comparative Example 2-22, which is a test outside the scope of the present invention.
2-27 and Conventional Example 2-50 were tested. In each case, after hot rolling to a rod wire, cooling was carried out under predetermined test conditions, then reheating was carried out for performing graphitization treatment, and after holding for a predetermined time, it was air cooled. Thus, a rod wire was manufactured.
The bar wire rod thus manufactured was subjected to a characteristic test and a hole punching test of a steel material. The test items include visual inspection for surface flaws and cracks in the rolled material, measurement and observation test of the graphite deposition state and metallographic structure with an optical microscope, Vickers hardness measurement test at the center and middle of the cross section, and holes in rods and wires. It is the dawn test, and the contents are the same as in test 1.

Table 5 shows the steel No. of the test steel, the test conditions for the rod and wire rod manufacturing, and the test results.

[0119]

[Table 5]

From the above test, the following matters can be found. (1) In Examples 2-1 to 2-21 of the present invention, the bar wire rod after hot rolling was set to 1 within the range of 600 to 900 ° C.
By reheating in the range of 5 minutes to 180 minutes, the desired size and number of graphite and metallographic structure are 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, the drilling workability was also good.

(2) On the other hand, the target of the present invention was not achieved in the comparative example and the conventional example, which are tests in which even one condition outside the scope of the present invention is entered. Details are as follows.

In Comparative Example 2-22, the heating temperature is 900 ° C.
Since it was too high, it was possible to obtain only small graphite of less than 1.0 μm, and the drilling workability was poor. On the contrary, in Comparative Example 2-23, since the heating temperature was low at less than 600 ° C., only small graphite of less than 1.0 μm could be obtained as in the above, and the drilling workability was poor.

Comparative Example 2-26 and Comparative Example 2-27
Since the graphitization index CE was less than 1.30, precipitation of graphite was not observed and the drilling workability was poor. -Comparative example 2-50 is an example in which conventional graphite steel was subjected to graphitization heat treatment at 700 ° C for a long time of 900 minutes. Only by carrying out such a long-time heat treatment, graphite was precipitated and the steel material was softened, and it was possible to obtain excellent drilling workability.

[Test 3] Example 1-1 performed in Test 1
4 and wire rods manufactured in Conventional Example 1-48 (both have a diameter of 1
3 mmφ) was used to process a connector bolt that is an automobile engine part. The processing steps of the connector bolt in the conventional example are wire rod → cold drawing → cold forging of bolt head → thread rolling → periphery cutting → drilling of holes → deburring → carburizing and quenching. In the case of using the SUM24L in the related art, it was necessary to manually grind the flash in order to remove burrs generated after drilling. In addition, in order to improve wear resistance, 180 ° C x 1 after carburizing and quenching at 930 ° C x 5 hr
It was necessary to perform tempering for 50 minutes, which was a major factor in increasing costs.

On the other hand, when the wire rod of Example 1-14 is used, the deburring step can be omitted, and the labor can be saved. In addition, the tool life of cutting and drilling has been greatly extended to about twice that of conventional products. Further, it was possible to improve the wear resistance of the parts by switching the long-time carburizing and tempering to simple induction hardening. As a result, it has been possible to significantly reduce costs.

[0126]

As described above, according to the present invention,
It is possible to manufacture ultra-free-cutting steel parts with excellent drilling workability equivalent to or better than that of conventional sulfur-lead composite free-cutting steel without adding lead, and carburizing and quenching after machining the parts. Even if it is not performed, it is possible to improve wear resistance by simple induction hardening. Compared with conventional graphite steel,
No graphitization heat treatment is required, or a short time heat treatment enables graphitization. It is possible to provide a manufacturing technique of such a free-cutting steel rod wire having excellent drilling workability, which brings industrially useful effects.

[Brief description of drawings]

FIG. 1 is an example showing a microstructure of a free-cutting steel bar according to the present invention, which is a diagram showing a microstructure composed of graphite and ferrite + granular cementite of Example 2-1.

Claims (13)

[Claims] 1. 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, having a chemical composition of balance Fe and unavoidable impurities, and having the following formula (1): Graphitization index C
E is 1.30 or more and the central segregation degree of carbon [C] /
[C] ₀ (where [C] is the carbon content of the target position, [C]
₀ (which is the carbon content rate of raw steel analysis) is 1.01 to 2.0
Cast pieces or steel pieces within the range of 0 to 850 to 1150 ° C
With respect to the characteristic values of the steel material thus obtained, the time for heating the hot-rolled steel material thus hot-rolled to 800 ° C. to 600 ° C. is adjusted to 5 minutes or more. , 100 pieces / mm ² or more of graphite with an average particle size of 1.0 μm or more are deposited, and the metallographic structure is composed of 20% or more of ferrite and the remaining cementite, or consists of ferrite alone, and the hardness of the central part of the cross section A free-cutting steel rod and wire having excellent drilling workability, which is less than or equal to the hardness of the intermediate portion and has a Vickers hardness H _{V of} 300 or less in the intermediate portion of the cross section. The graphitization index CE is calculated by the following formula. CE = C + Si / 3-Mn / 12 ----------------------------- (1) However, the element symbol in the above formula is Represents the weight% of each element. 2. 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, having a chemical composition of balance Fe and unavoidable impurities, and having the following formula (1): Graphitization index C
E is 1.30 or more and the central segregation degree of carbon [C] /
[C] ₀ (where [C] is the carbon content of the target position, [C]
₀ is the carbon content of raw steel analysis) is 1.01 to 2.0
Cast pieces or steel pieces within the range of 0 to 850 to 1150 ° C
Is heated to a temperature within the range, hot-rolled, and the hot-rolled high-temperature steel material is cooled from 800 ° C. to 600 ° C. over an arbitrary time, and then further heated to 600 to 90 ° C.
After maintaining at a temperature in the range of 0 ° C. for 3 hours or less, air-cooling, and regarding the characteristic value of the steel material thus obtained, 100 pieces / mm ² or more of graphite having an average particle size of 1.0 μm or more are precipitated,
The metal structure is composed of 30% or more of ferrite and the remaining cementite, or is composed of only ferrite, and 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. A free-cutting steel rod wire material with excellent hardness for drilling, which has a hardness of H _V 250 or less. The graphitization index CE is calculated by the following formula. CE = C + Si / 3-Mn / 12 ----------------------------- (1) However, the element symbol in the above formula is Represents the weight% of each element. 3. The cast slab or steel slab further contains at least one selected from the group consisting of component compositions of the following elements, and instead of the formula for calculating the graphitization index CE: A free-cutting steel rod wire rod having excellent drilling workability according to claim 1 or 2, characterized by using the 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 calculated by the following formula. CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B --------------------------- -(2) However, the element symbol in the above formula represents the weight% of each element. 4. The cast or steel slab further contains at least one selected from the group consisting of component compositions of the following elements, and instead of the formula for calculating the graphitization index CE: (3) Formula is used, Claim 1, 2 characterized by the above-mentioned.
Alternatively, a free-cutting steel rod wire excellent in drilling workability as described in 3. In 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 calculated by the following formula. 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 element symbol in the above formula represents the weight% of each element. 5. The cast or steel slab further contains at least one selected from the group consisting of component compositions of the following elements, and instead of the formula for calculating the graphitization index CE, Formula (4) is used, Claims 1-4 characterized by the above-mentioned.
A free-cutting steel rod wire rod having excellent drilling workability according to any one of the described inventions. By weight%, Ca: 0.0010 to 0.0100%, Mg: 0.0010 to 0.10%, and REM: 0.0010 to 0.10%. The graphitization index CE is calculated by the following formula. 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 represents the weight% of each element. 6. 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, having a chemical composition of balance Fe and unavoidable impurities, and having the following formula (1): Graphitization index C
E is 1.30 or more and the central segregation degree of carbon [C] /
[C] ₀ (where [C] is the carbon content of the target position, [C]
₀ is the carbon content of raw steel analysis) is 1.01 to 2.0
Cast pieces or steel pieces within the range of 0 to 850 to 1150 ° C
Is heated to a temperature within the range of, and hot-rolled.
The time for cooling to 00 ° C. was adjusted to 5 minutes or more, and 100 pieces / mm ² of graphite having an average particle size of 1.0 μm or more were contained in the steel material.
Precipitated as described above, the metallographic structure consists of 20% or more of ferrite and the remaining cementite, or consists of only ferrite, and the hardness of the center of the cross section is set to be equal to or less than the hardness of the middle part of the cross section
Also, the hardness of the intermediate portion of the cross section is Vickers hardness H _V 300
A method for producing a free-cutting steel rod wire excellent in drilling workability, which comprises the following adjustments. The graphitization index CE is calculated by the following formula. CE = C + Si / 3-Mn / 12 ----------------------------- (1) However, the element symbol in the above formula is Represents the weight% of each element. 7. The cast slab or steel slab further contains at least one selected from the group consisting of component compositions of the following elements, and instead of the calculation formula of the graphitization index CE, The method for manufacturing a free-cutting steel rod wire having excellent drilling workability according to claim 6, characterized by using the formula (2). 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 calculated by the following formula. CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B --------------------------- -(2) However, the element symbol in the above formula represents the weight% of each element. 8. The cast or steel slab further contains at least one selected from the group consisting of component compositions of the following elements, and instead of the formula for calculating the graphitization index CE: (3) Formula is used, The manufacturing method of the free-machining steel rod wire excellent in the drilling workability of Claim 6 or 7 characterized by the above-mentioned. % 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 calculated by the following formula. 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 element symbol in the above formula represents the weight% of each element. 9. The cast slab or steel slab further contains at least one selected from the group consisting of component compositions of the following elements, and instead of the formula for calculating the graphitization index CE: The formula (4) is used.
Alternatively, the method for producing a free-cutting steel rod wire having excellent drilling workability according to 8 above. By weight%, Ca: 0.0010 to 0.0100%, Mg: 0.0010 to 0.10%, and REM: 0.0010 to 0.10%. The graphitization index CE is calculated by the following formula. 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 represents the weight% of each element. 10. 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, having a chemical composition of balance Fe and unavoidable impurities, and having the following formula (1): Graphitization index C
E is 1.30 or more and the central segregation degree of carbon [C] /
[C] ₀ (where [C] is the carbon content of the target position, [C]
₀ is the carbon content of raw steel analysis) is 1.01 to 2.0
Cast pieces or steel pieces within the range of 0 to 850 to 1150 ° C
The temperature for cooling from 800 ° C. to 600 ° C. is adjusted to 1 minute or less by heating to a temperature within the range, hot rolling, and cooling the hot-rolled high temperature steel material with a cooling medium, and further heating to the temperature after the 3hr following time holding in the range of 600 to 900 ° C., air cooled, the steel material of 100 the average particle diameter 1.0μm or more graphite in / mm ² to precipitate more, the metal structure 30% Consisting of the above ferrite and the remainder cementite, or consisting of only ferrite,
Free cutting excellent in drilling workability, characterized in that the hardness of the central portion of the cross section is set to be equal to or less than the hardness of the intermediate portion of the cross section, and the hardness of the intermediate section is adjusted to Vickers hardness H _V 250 or less. Steel rod wire manufacturing method. The graphitization index CE is calculated by the following formula. CE = C + Si / 3-Mn / 12 ----------------------------- (1) However, the element symbol in the above formula is Represents the weight% of each element. 11. The cast slab or steel slab further contains at least one selected from the group consisting of component compositions of the following elements, and instead of the formula for calculating the graphitization index CE: The formula (2) is used, and
A method for producing a free-cutting steel rod wire excellent in the hole forming work 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 calculated by the following formula. CE = C + Si / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B --------------------------- -(2) However, the element symbol in the above formula represents the weight% of each element. 12. The cast slab or steel slab further contains at least one selected from the group consisting of component compositions of the following elements, and instead of the formula for calculating the graphitization index CE: The expression (3) is used, and the expression (10) is used.
Alternatively, the method for producing a free-cutting steel rod wire having excellent drilling workability according to 11 above. % 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 calculated by the following formula. 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 element symbol in the above formula represents the weight% of each element. 13. The cast slab or steel slab further contains at least one selected from the group consisting of the composition of elements of the following elements, and instead of the formula for calculating the graphitization index CE: The formula (4) is used.
A method for producing a free-cutting steel rod wire having excellent drilling workability according to 0, 11 or 12. By weight%, Ca: 0.0010 to 0.0100%, Mg: 0.0010 to 0.10%, and REM: 0.0010 to 0.10%. The graphitization index CE is calculated by the following formula. 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 represents the weight% of each element.