JP3256184B2 - Method for producing ultra-free-cutting steel rods and parts, and ultra-free-cutting steel rods and parts using them - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for manufacturing a super free-cutting steel material, in which graphite is precipitated by a normalizing process after hot rolling of a steel material, and a soft ferrite or ferrite + The present invention relates to a technology for manufacturing a super-free-cutting steel rod and a super-free-cutting steel part having excellent machinability using pearlite.

[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.
UM23L and SUM24L are typical. In these free-cutting steels, machinability is more important than strength and toughness, but as the machinability required for them, the life of the cutting tool and the handling of chips are particularly important.

[0003] Since recent machining of steel materials is performed at a much higher speed than in the past in order to increase productivity, steel materials having low tool wear, that is, free-cutting steel having a long tool life, are required.

Recently, automatic machines are often used for unmanned machining, and when chips are long and entangled, it is necessary to stop the machine or perform extra work for removing the chips, which leads to a problem in production. Will decrease the performance.
Therefore, there is a need for a free-cutting steel having excellent chip disposability, in which chips are finely divided into appropriate sizes.

[0005] In order to improve the tool life and chip disposability, conventionally, as shown in SUM23L and SUM24L, lead, which is a free-cutting element, is added to sulfur and phosphorus composite free-cutting steel in an amount of 0.10-0. The machinability has been further improved by adding 0.35%.

[0006] Since the melting point of Pb is as low as 327 ° C, Pb melts during cutting and the steel becomes brittle, thereby improving the 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.

[0007] As an element for improving the machinability of steel,
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 the machinability, but when a large amount of S is added, 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 or to decrease toughness. For this reason, in the conventional SUM23L and SUM24L, 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. Also SUM23L, SUM
Since 24L 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.

[0009] 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.

[0010] However, such a long-time graphitization heat treatment not only causes an increase in cost, but also causes adverse effects such as decarburization of the steel material during the heat treatment, which adversely affects the performance of the final part. Therefore, a lead-free ultra-free-cutting steel excellent in machinability by obtaining a desired graphite and structure by a simple heat treatment is desired.

[0011]

The prior art 1 described above,
2 has any of the following problems unresolved. Problem 1: The free-cutting elements used are toxic and have problems in environmental measures.

Problem 2: Since a large amount of S and P are contained in combination, the hot workability is poor, and special machining is required on the steel slab before rolling to prevent the tip from cracking. Problem 3: In order to improve wear resistance, it is necessary to perform carburizing and quenching for a long time.

Problem 4: Machinability can be improved by utilizing carbon as a non-toxic free-cutting element and precipitating it as graphite, but it requires a long time graphitizing anneal and costs. Increases.

The present invention solves the above-mentioned problems and provides a steel rod or wire used as a material for parts of an automobile or an industrial machine, and a machined product of the steel rod or wire to obtain a product by a simple heat treatment. In order to produce the above components with excellent properties, by normalizing for a short time, the machinability is good, and the surface can be hardened by induction hardening,
The purpose is to develop a method that is inexpensive and has no environmental health problems.

[0015]

SUMMARY OF THE INVENTION In view of the above, the present inventors have developed a super free-cutting steel having machinability equivalent to or higher than that of a conventional sulfur-lead composite free-cutting steel without adding lead. Intensive research to develop. As a result, the following findings were obtained.

That is, in order to precipitate graphite by a simple heat treatment such as normalizing, C is set to 1.00 with respect to the composition of steel.
% To make hypereutectoid steel, and Si is increased to 1.00% or more to promote graphitization. Further, a steel in which an appropriate amount of Mn is added to secure the ductility of the steel and impurity elements such as P and S are suppressed to a low level is prepared. Next, a slab or a slab having the above-mentioned chemical composition is prepared by an ingot-making method or a continuous casting method, and is hot-rolled to produce a steel rod wire.
After heating to a temperature between 0 and 1000 ° C., preferably between 650 and 950 ° C. for 3 hours or less, air cooling is performed.

By the above-described treatment, a steel rod or wire having an appropriate size and amount of graphite and a soft ferrite or a ferrite + pearlite structure is manufactured. As a result, it has been found that it is possible to produce a super-free-cutting steel wire having machinability equal to or higher than that of a conventional sulfur-lead composite free-cutting steel without adding lead.

The present invention has been made based on the above findings, and has the following features. According to the first aspect of the present invention, 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%
A slab or a slab having the following composition, having a chemical composition consisting of the balance of iron (Fe) and unavoidable impurities, and having a graphitization index CE of 1.30 or more determined by the following equation (1):
Heated to a temperature in the range of 50-1150 ° C, hot rolled and cooled to room temperature, the hot rolled steel thus obtained was heated at a temperature in the range of 600-1000 ° C for no more than 3 hours. after air-cooling, the average particle diameter 1.0μm or more graphite is deposited 100 / mm ² or more, and either made of metal structure from at least 70% of ferrite and the balance pearlite, or made of ferrite alone in said steel And a Brinell hardness of 200 or less.

According to a second aspect of the present invention, there is provided the method for manufacturing a super-cuttable steel rod or wire according to the first aspect, wherein the slab or the steel slab is one type selected from the group consisting of the following elements. In addition to the above, the graphitization index CE
Is characterized by using a compound having a chemical component composition whose calculated value is 1.30 or more when the following formula (2) is used as the calculation formula. 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
-0.50%, Cr: 0.01-0.50%, Mo:
0.01-0.50%, and B: 0.0005-0.
010%. The equation (2) is expressed as follows: 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 that it represents the weight% of each element.

According to a third aspect of the present invention, in the method for manufacturing a super-cuttable steel rod or wire according to the first or second aspect, the slab or the slab is further selected from the group consisting of the following elements. And one having a chemical component composition whose calculated value is 1.30 or more when the following formula (3) is used as a calculation formula of the graphitization index CE. In particular, it has features. 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 0.05
0%, Zr: 0.005 to 0.050%, V: 0.01
And Nb: 0.01 to 0.20%. And, the equation (3) 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 weight% of each element.

According to a fourth aspect of the present invention, there is provided a method for manufacturing a super free-cutting steel rod or wire according to the first, second or third aspect, wherein the slab or the slab is further selected from the group consisting of the following elements. One or more of the above, and having a chemical component composition whose calculated value is 1.30 or more when the following formula (4) is used as the calculation formula of the graphitization index CE. Is characterized by using. Here, the group consisting of the component compositions of the above-mentioned elements refers to C% by weight.
a: 0.0010 to 0.0100%, Mg: 0.001
0 to 0.10% and REM: 0.0010 to 0.1
0%. 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. .

According to a fifth aspect of the present invention, there is provided a method for manufacturing a super free-cutting steel part, comprising the steps of: The present invention is characterized in that a steel rod or wire is machined directly or after cold working and then finished into a part shape. Here, the case of performing direct machining corresponds to, for example, a method of performing direct machining after straightening, and the case of performing machining after cold working is represented by, for example, hexagonal drawing. A method of machining after cold working is applicable.

A super-cutting steel rod according to a sixth aspect of the present invention is characterized by being manufactured by any one of the methods according to the first to fourth aspects of the present invention. Claim 7
The ultra-free-cutting steel rod or wire described above is characterized by being produced by any one of the methods according to the fifth aspect of the present invention.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is to reheat a hot-rolled bar and wire made of a high Si hypereutectoid carbon steel and its low alloy steel to a predetermined temperature for 3 hours or less, and then air-cool. , To deposit graphite and produce a steel rod or wire having a desired metal structure and hardness. That is, a suitable steel component composition is found, and a super-free-cutting steel part having excellent lead-free machinability is manufactured by such a simple process.

Next, the reasons for limiting the constituent elements of the present invention will be described. (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. Further, Mn improves quenching properties, refines pearlite, and improves ductility of steel. Since it is used for this purpose, Mn needs to be added in an amount of 0.01% or more. However, since Mn inhibits graphite precipitation, the upper limit is set to 2.00%. If the Mn 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 set to 0.1.
It is limited to the range of 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 that 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 strength by dissolving in ferrite. In addition, it increases the fluidity of the molten metal and improves castability. Since Cu is used for these purposes,
Addition of 0.01% or more is required. However, when the Cu content exceeds 2.0%, the solid solubility in steel is exceeded, so that undissolved Cu remains, lowers the hot ductility and promotes the generation of surface defects. Therefore, the Cu content is 0.01 to 2.0.
% Is desirable.

(9) Nickel (Ni) Like Ni, Ni promotes the precipitation of graphite.
Increases strength by dissolving in ferrite. 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 strengthens ferrite. 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 strengthen 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, strengthens ferrite. 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 0.50
%, A large amount of the 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, reduces the effect of N on graphitization, and acts as graphite precipitation nuclei to promote the precipitation of graphite. 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 is 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 Ti, Zr also precipitates nitrides and carbides, refines crystal grains, and promotes 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 where the symbol of the element in the above formula represents the content% by weight of the element. When at least one of Ca, Mg, and REM is contained by 0.0010% or more, 0.07 is added to the right side of the above equation.

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) Heating temperature during hot rolling If the heating temperature of the slab or slab before hot rolling is less than 850 ° C., the deformability of the steel is insufficient, and surface defects are apt to occur 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, and the hot ductility is also insufficient, and cracks occur in the rod or rod. Therefore, the heating temperature during hot rolling is 850.
の 間 1150 ° C.

(23) Graphitization heating temperature In order to promote the precipitation of graphite by reheating the steel,
Perform graphite normalization. The heating temperature in the graphitizing normalization is an important factor for graphite precipitation. Heating temperature is 6
Heating at less than 00 ° C. results in a low carbon diffusion rate. However, when heated to a temperature higher than 1000 ° 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, the heating temperature for graphitization normalization is limited to the range of 600 to 1000 ° C. More preferably, the temperature is in the range of 650 to 950 ° C., and within this temperature range, graphite of a desired size and number can be obtained by heating for a shorter time.

(24) Graphitization Heating Time The shorter the graphitization heating time, the better the productivity is.
Heating of 3 hours or less can provide the desired size and number of graphite, metal structure, and hardness. Therefore, the heating time is limited to 3 hours or less.

After the 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.

(25) Particle Size of Graphite 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 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 the form of a lump is less than 1.0 μm, the effect of breaking small chips during cutting is small, and the contribution to improving the machinability is small. Therefore, the average particle size of graphite is set to 1.0 μm or more. 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.

(26) 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 .

(27) To extend the life of the metallographic tool, it is necessary to lower the hardness of the steel material. In order to obtain such a low-hardness steel material, it is necessary that the metallographic structure of the graphitized normalizing steel be only ferrite + pearlite or ferrite, and the amount of ferrite must be 70% or more. is there. In order to control the metal structure in this way,
The cooling rate after the graphitization heating is reduced, so that the graphite grows larger and the amount of pearlite is reduced. By cooling at a sufficiently low cooling rate, the metal structure becomes a soft ferrite single phase containing no pearlite. In order to obtain the above metal structure under the conditions according to the other constitutional requirements of the present invention, as described in the above section (24), it is sufficient to air-cool after heating for graphitization.

(28) Hardness When the Brinell hardness (HB) after graphitizing and normalizing exceeds 200, the wear of the cutting tool increases and the tool life is shortened. Therefore, the Brinell hardness needs to be 200 or less. When the metal structure becomes a ferrite single phase, the Brinell hardness decreases to about 130.

[0057]

Next, the present invention will be described in more detail by way of examples. Tables 1 and 2 show the chemical composition and the graphitization index CE of the test steel used in the test. The steel No. in Table 1 was used .
1 to 11 and 21 to 23 are all steels having a chemical composition within the scope of the present invention. In Table 2, steel No. Nos. 24 to 46 are all steels whose chemical composition is out of the range of the present invention,
Nos. 4 to 45 are comparative component steels. 46 is a conventional component steel SUM24L.

[0058]

[Table 1]

[0059]

[Table 2]

The test steels (Steel Nos. 1 to 46) of each of the above components were melted in a 130-ton electric furnace and then cast into slabs by continuous casting or ingot casting. After slab rolling the cast slab into a 160 mm square steel slab, after heating in a slab heating furnace, the diameter was 24 mm.
Or hot-rolled into a 32 mm steel bar, which is 5.5 m long
After cutting into steel bars, the steel bars were inspected for surface flaws. Then, the steel bars were united in units of 1 ton, charged into a heat treatment furnace, and subjected to a graphitization normalizing treatment.

Tables 3 and 4 show the steel No. of the test steel used in the steel bar production test. , The heating temperature of the billet, the presence or absence of surface flaws in the rolled bar, and the graphitizing heating temperature and graphitizing heating time of the bar. That is, the tests in Tables 3 and 4 are Examples 1-1 to 1-11 which are tests within the scope of the present invention, and Comparative Examples 1-21 to Comparative Examples 1 which are tests outside the scope of the present invention. −46
Consists of

[0062]

[Table 3]

[0063]

[Table 4]

The following test was carried out on the graphitized and normalized steel bars obtained in the above test. The graphite precipitation state and metal structure were observed with an optical microscope. As the graphite precipitation state, the average particle size of graphite and the number of graphite particles were measured. As the metal structure, ferrite% (area%) in ferrite + pearlite structure was measured. Brinell hardness was tested with a hardness tester.

Thereafter, the steel bar was cut by an automatic lathe and machined into a piston pin which is a hydraulic component of an automobile. Using this, a machinability test was performed. The machinability was determined based on the chip disposability and the tool life. That is, the chip disposability is ranked 1 as “good” when the chip is divided in 2 or less volumes, and rank 2 as “normal” when the chip is divided in 3 to 6 volumes, and If the chips were longer than 7 rolls, they were ranked as “poor”. The tool life test was performed at a cutting speed of 150 m / mi with a high-speed tool.
n, cutting was performed at a feed rate of 0.20 mm / rev with cutting oil applied, and the time until the cutting edge was melted and became uncuttable was measured and defined as the tool life.

The test results are shown in Tables 3 and 4. From Tables 1 to 4 above, the following matters can be understood. (1) Example 1-1 which satisfies all the component composition of the steel of the present invention, hot rolling conditions and graphitizing and normalizing conditions.
~ 1-11 , there is no occurrence of surface flaws in the steel bar, the average particle size of graphite and the number of graphite particles meet the target value, the target is also satisfied for the metal structure, and the hardness is less than HB200 and soft bar Has become. For this reason, the chip handling properties were all small cut chips with an evaluation rank of 1, and the tool life was as long as 30 minutes or more.

(2) On the other hand, Comparative Examples 1-21 to 21-5 were tests in which at least one condition outside the scope of the present invention was included.
In 1-46, the goal of the present invention was not achieved. Details are as follows.

In Comparative Example 1-21, the chemical composition is within the range of the present invention, but since the heating temperature during hot rolling was higher than the range of the present invention, the hot-ductility was insufficient and the steel bar was large. Scratches occurred. Similarly, in Comparative Example 1-22, 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 1-23, both the chemical composition and the heating temperature during hot rolling are within the range of the present invention, but the heating temperature for graphitization is higher than the range of the present invention. For this reason, the graphite that once precipitated during the temperature rise was redissolved under high temperature conditions, so that the graphite particles became smaller than 1.0 μm, and the chip controllability was only obtained in the normal state of evaluation rank 2. could not.

In Comparative Example 1-24, the C content was low outside the range of the present invention, so that only graphite having a small size of 1.0 μm or less was obtained, the amount of ferrite in the metal structure was small, and Higher than HB200, chip processing,
The tool life was also inferior. In particular, since the chips were long and entangled with the work material, it was necessary to stop the machine and remove the chips. Conversely, in Comparative Example 1-25, in which the C content deviated from the present invention and was high, the hot ductility was insufficient, and the steel bar cracked. In the following comparative examples, a cutting test of a bar steel having a surface flaw was omitted.

In Comparative Example 1-26, the Si content was low outside the range of the present invention, so that the graphite particles were small and the machinability was poor. In Comparative Example 1-27, Si deviated from the present invention and was high, so that hot ductility was insufficient, and flaws occurred in the steel bar.

In Comparative Example 1-28, the Mn content was 2.0
%, And the hot ductility was also insufficient, so that the steel bar was flawed. -In Comparative Example 1-29, the P content was higher than the range of the present invention, the hot ductility was insufficient, and flaws occurred in the steel bar.

In Comparative Example 1-30, the S content was higher than the range of the present invention, the hot ductility was insufficient, and flaws occurred in the steel bar. -In Comparative Example 1-31, the Cu content was higher than the range of the present invention, the hot ductility was insufficient, and flaws occurred in the steel bar.

In Comparative Example 1-32, since the Cr content was higher than the range of the present invention, and the graphitization index CE was lower than 1.30, only small graphite could be obtained. The machinability was poor.

Comparative Example 1 In No. 33, 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. In Comparative Example 1-34, the contents of Co and O were higher than those of the present invention, and the bar also had flaws.

In Comparative Example 1-35, the B content was higher than the range of the present invention, a large amount of carbon boride was precipitated, and defects were generated due to insufficient ductility. -In Comparative Example 1-36, the N content was higher than the range of the present invention, and thus, due to blow holes generated in the slab,
Many linear flaws occurred on the surface of the steel bar.

In Comparative Example 1-37, the Zr content was higher than the range of the present invention, and the steel bar had flaws. -Comparative Example 1 # 38 has a V content higher than the range of the present invention and a graphitization index CE of less than 1.30,
Only small graphite of 1.0 μm or less could be obtained. Therefore, the machinability was poor.

In Comparative Example 1-39, the Al content was higher than the range of the present invention, and the steel bar had flaws.・ Comparative Example 1-40 has an Nb content higher than the range of the present invention, and the graphitization index CE is less than 1.30, and is insufficient.
Only small graphite of 1.0 μm or less could be obtained. Therefore, the machinability was poor.

Comparative Example 1 # 41 had a Ca content, Comparative Example 1-42 had a Mg content, and Comparative Example 1-43 had a REM content higher than the range of the present invention. Inclusions were entangled in the steel in large amounts, which remained as rolling flaws on the bar.

Comparative Examples 1-44 and 1-45
Although the composition of each chemical component is within the range of the present invention, the graphite deposited was small because the graphitization index CE was lower than the range of the present invention. Therefore, the machinability was poor.

Comparative Example 1-45 shows that the conventional component steel SUM
24L, and the machinability was good. But,
Since this steel has a low carbon content, after carburizing and quenching at 920 ° C. for 5 hours, 170 ° C. × 1.
It was necessary to perform tempering for 5 hours. When hot rolling into steel bars, the tip of the steel slab is made thinner like a pencil to prevent the tip from tearing and cracking during rolling, resulting in misrolling. Had to be included.

For the SUM24L described above, the first embodiment
In Examples 1 to 1-11 , the abrasion resistance could be improved by performing simple induction hardening and tempering. Further, in any of the examples, hot rolling did not require any special tip processing, and the rolling could be performed without any problem using a steel piece that had been cut with a shear.

Next, the effects of the heating temperature and the heating time on the graphitizing normalization were examined in detail. As a test material, steel No. 2 tons of slab with chemical composition of 1
° C, hot rolled and coiled up to a diameter of 18
A mmφ wire was used. For normalization, set the heating temperature to 550.
Eleven levels of temperature were set at an interval of 50 ° C. between 〜１０1050 ° C., and heating was performed at three levels of 0.5 hr, 1 hr and 3 hr, and then air-cooled. About the obtained test material,
Brinell hardness, graphite precipitation and metal structure were measured and tested.

FIG. 1 shows the relationship between the normalizing temperature and the Brinell hardness for each heating time. As can be seen from the graph, heating for 0.5 hr resulted in a Brinell hardness HB of 200 or less between 700 and 1000 ° C. Between 650 and 950 ° C. for 1 hr heating, and 60 ° C. for 3 hr heating.
Between 0 and 900 ° C., each has a Brinell hardness HB200
The following were obtained:

In the wire rod having a hardness of HB 200 or less, the average particle size of graphite was between 2 and 10 μm, and the metal structure was a ferrite + pearlite structure having 70% or more of ferrite. . FIG. 2 shows the microstructure of a wire rod test material which had a chemical composition of steel No. 1 and which had been subjected to graphitizing normalization at a normalizing temperature of 850 ° C. for a heating time of 1 hour and then air-cooled.

The wire having the microstructure shown in FIG. 2 was used as a test material, pulled out to a diameter of 16 mm, and machined into a screw part called a door lock knob pin. Good test results were obtained for both chip disposability and tool life. Conventionally, in the case of using the conventional component steel SUM24L in the manufacture of the above screw parts, after the wire was pulled out, the door lock knob pin was machined, carburized and quenched, and tempered.
When the wire according to the present invention is used, the wear resistance of the screw component can be improved by simple induction hardening and tempering.

[0087]

As described above, according to the present invention,
Without adding lead, it is possible to manufacture ultra-free-cutting steel parts with excellent machinability equal to or higher than conventional sulfur-lead composite free-cutting steel, and after carburizing the parts, carburizing and quenching is performed. If not, it is possible to improve wear resistance by simple induction hardening. It is possible to provide a manufacturing technique for such a super-free-cutting steel rod or rod and a component, and an industrially useful effect is obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the graphitizing normalizing temperature of a wire and Brinell hardness, stratified by heating time.

FIG. 2 is a view showing an example of a microstructure of a super free-cutting steel part according to the present invention.

Continuation of the front page (56) References JP-A-49-6817 (JP, A) JP-A-6-279849 (JP, A) JP-A-11-2933387 (JP, A) JP-A-11-293388 (JP) JP-A-11-293389 (JP, A) JP-A-11-350066 (JP, A) JP-A-11-350067 (JP, A) JP-A-11-350068 (JP, A) 3-146618 (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 30, 1966, p. 438 (58) Field surveyed (Int.Cl. ⁷ , DB name) C21D 6/00 C21D 8/06-8/08 C22C 38/00-38/60

Claims (7)

(57) [Claims] 1. In 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 composition composed of a balance of iron (Fe) and unavoidable impurities. A slab or a slab having a graphitization index CE of 1.30 or more determined by the equation 1) was used for 850
After heating to a temperature in the range of １1150 ° C., hot rolling and cooling to room temperature, the hot rolled steel thus obtained is heated at a temperature in the range of 600 to 1000 ° C. for a time of 3 hours or less. , Air-cooled, average particle size of 1.0
It is necessary to precipitate 100 μm / mm ² or more of graphite having a size of 70 μm or more, and to make the metal structure consist of 70% or more of ferrite and the balance of pearlite, or only ferrite, and have a Brinell hardness of 200 or less. Characterized by the method for manufacturing super-cutting steel rods and wires. CE = C + Si / 3-Mn / 12 --- (1) However, the symbol of the element in the above formula represents the weight% of each element. 2. The cast slab or the steel slab further contains at least one element selected from the group consisting of the following elements, and the graphitization index CE is calculated by the following formula (2). The method according to claim 1, wherein the calculated value of the formula (1) is 1.30 or more, and the calculated value is 1.30 or more. 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%. Note that 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. 3. The cast slab or the steel slab further contains one or more elements selected from the group consisting of the following elements, and the graphitization index CE is calculated as follows: 3) The method for manufacturing a super-cuttable steel rod or rod according to claim 1 or 2, wherein a material having a chemical component composition whose calculated value is equal to or greater than 1.30 when using equation (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% CE is calculated 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. 4. The cast slab or the steel slab further contains at least one element selected from the group consisting of the following elements, and the graphitization index CE is calculated as follows: 4) The method for producing a super-cuttable steel rod or wire according to claim 1, 2 or 3, wherein a material having a chemical component composition whose calculated value is 1.30 or more when equation (4) is used is used. . Ca: 0.0010-0.0100%, Mg: 0.0010-0.10%, and REM: 0.0010-0.10% by weight. Note that 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. 5. An ultra-free-cutting steel rod or wire manufactured by the method according to any one of claims 1 to 4, and directly or after cold-working the manufactured super-cutting steel rod or wire. A method for manufacturing a super-free-cutting steel part, characterized by performing processing. 6. An ultra-free-cutting steel rod or wire made by the method according to any one of claims 1 to 4. 7. A super free-cutting steel part manufactured by the method according to claim 5. Description: