JPH11293389A - Hot worked steel material excellent in machinability, product, and their production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve machinability and to provide high strength and toughness by subjecting a steel, in which chemical composition is specified and also Ni/Cu in the composition and graphitization index are respectively specified, to hot working and to cooling and precipitating graphite and forming a pearlitic structure. SOLUTION: This steel has a composition which consists of, by weight, 0.80-1.70% C, 0.70-2.50% Si, 0.01-2.0% Cu, 0.01-2.0% Ni, 0.0005-0.0100% Ca, 0.001-0.10% Al, ⊥.050% P, <0.050% S, <=0.0050% O, <=0.015% N, and the balance Fe and in which Ni/Cu>=0.2 is satisfied and also the graphitization index CE computed by CE=C+Si/3+Cu/9+Ni/9+Al/6 is regulated to >=1.30. This steel is hot worked and cooled to a room temperature. A product is produced by using this steel as a stock, and graphite of >=0.5 μm average grain size is precipitated by >=100 pieces/mm<2> and the metallic structure is composed essentially of pearlite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material such as a steel bar used as a material for parts of automobiles and industrial machines, such as a crankshaft and a differential gear, and a product such as the above-mentioned parts. It can be manufactured at low cost by using low-grade scrap with many impurities, and even without heat treatment for depositing graphite, it has fine graphite as it is hot worked, and due to the combined effect with Ca etc. The present invention relates to a hot-worked steel material and a product having extremely good machinability and having higher strength and toughness than conventional spheroidal graphite cast iron, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, a large amount of scrap from automobiles and electric products is mixed in scraps that have been marketed in the market. For example, a large amount of copper is used in an electric motor, and a stainless steel containing a large amount of Ni is used in an exhaust gas muffler, a catalyst, and the like, and these are inevitably mixed into scrap. Therefore, the quality of these scraps is reduced. In accordance with the lowering of the grade of the scrap, it is inevitable that the steel product contains a large amount of impurities in the electric furnace smelting steel using the scrap as a main raw material.

[0003] There is also a concern that the ductility of steel materials may be reduced due to lower grades of scrap. If these low grade scraps are not used and only high-grade scraps with less impurities are used, the recycling of steel scraps as an iron source in the future. The situation may worsen and low-grade scrap may be left on the market. Therefore, effective use of such low-grade scrap is strongly demanded.

[0004] Steel bars manufactured using scrap as an iron source are widely used as materials for parts of automobiles, construction machines, industrial machines and the like. For example, a piston rod of a construction machine is used by directly cutting the outer periphery of a rolled steel bar and then performing induction hardening, but the internal structure of the steel bar remains rolled. Therefore, it is necessary that the steel bar has not only excellent machinability but also desired strength and ductility as rolled. In the case of machining parts manufactured by hot forging from steel bars by cutting, for example, in the processing of connecting rods, crankshafts, camshafts, hypoid gears, and pinion gears that are parts around automobile engines, these parts are not machined before finishing. The forged product must have excellent machinability and have desired strength and ductility as forged or after heat treatment.

As described above, many parts are finished into a part shape by machining, but as for the machinability required for steel, it is important that the life of the cutting tool is long and the processing ability of chips is good. . Today's cutting is performed at a much higher speed than before in order to increase productivity, so tool wear is greater than before, and there is a need for free-cutting steel with excellent tool life.

In recent years, automatic machining is often carried out by unmanned machines, and when chips are long and entangled, it is necessary to stop the machine and perform extra work for removing the chips, which leads to a problem in production. Will decrease the performance. For this reason, chips are finely divided into appropriate sizes,
There is a need for free-cutting steel with excellent processability.

Further, connecting rods and crankshafts have several small diameter holes for supplying lubricating oil. However, since these holes are deep, in drilling, chips are finely divided. Therefore, it is necessary that the liquid is discharged from the drill hole without any trouble. That is, chips that are difficult to cut are not ejected from the holes, and the chips are clogged in the holes, causing breakage of the drill.

Accordingly, in machining the above-mentioned parts, in order to improve tool life and chip disposability, lead free-cutting steel containing 0.05 to 0.30 wt.% Of lead as a free-cutting element is used. Has been widely used. Because lead has a low melting point, it is easily melted by the heat of cutting, reducing the ductility of steel,
This extends the life of the tool and cuts the chips into appropriate sizes.

However, the chips of the lead free-cutting steel are curled into small pieces, and the cutting stress is concentrated on the cutting edge of the tool. As a result, the wear of the rake face increases, and the life of the cutting tool is not necessarily long.

[0010] Further, since lead is toxic, there has been a strong demand for lead-free free-cutting steel along with the recent trend of protecting the global environment. Pb is an element that improves machinability
In addition, elements such as S, Ca, Bi, Se, and Te are known, but these elements alone have disadvantages such as an effect of improving machinability that does not reach that of lead, being expensive, and toxic. Since it has at least one element, it cannot be a substitute element for lead.

[0011] On the other hand, graphite is an element that greatly improves machinability as seen in cast iron, but when carbon is added to steel, cementite precipitates, and it is not easy to obtain graphite in steel. 0.10 carbon in the conventional invention
In the case of steel having 1.5 wt.%, For example,
JP-A-07742 and JP-A-3-140411 disclose a steel material or a method of performing annealing at a temperature of 600 to 800 ° C. for a long time of several hours to 200 hours to precipitate graphite.

JP-A-49-67816 and JP-A-49-67817 disclose graphite free-cutting steels which are quenched at 750 to 950 ° C. and tempered at 600 to 750 ° C. to deposit graphite. I have.

[0013] Therefore, in each of the conventional disclosure examples, it is necessary to perform a graphitization heat treatment to obtain graphite, which results in an extremely high cost. In addition, the metal structure becomes ferrite by the graphitization heat treatment,
For this reason, the production is limited to the production of low-strength parts and small parts that can be produced by cold forging, and cannot be applied to the production of large forged parts such as crankshafts and connecting rods.

On the other hand, it is well known that cast iron and cast steel having a carbon content of about 3.8% by weight can easily obtain spheroidal graphite as cast by inoculation with Ca, Mg, etc., and have good machinability. . However, since cast iron and cast steel are used up to casting, although there is a degree of freedom in the shape of the steel product, there is a drawback that toughness such as elongation, drawing and impact value is low.

In recent years, the toughness has been improved by making the base structure bainite by austempering. For example, Japanese Patent Application Laid-Open No. Sho 61-243121 discloses a method of manufacturing a crankshaft which performs austempering on spheroidal graphite cast iron. A manufacturing method is disclosed. However, these cast products have a lower Young's modulus and are inferior in fatigue strength as compared with forged products of non-heat treated steel to which V of about 0.10 wt. Also, the toughness is still inferior to forged products. In addition, these castings have 0.1%.
Cast cavities of about 1 mm may be generated, which is a starting point of fatigue fracture, so that the reliability of the material is inferior. The drawback is that strict attention must be paid to the casting method and ultrasonic inspection of the product. .

[0016]

Any of the following problems has not been solved in the above various prior arts. Problem 1: The free-cutting elements used are toxic and have problems in environmental measures. Problem 2: Although carbon can be used as a non-toxic free-cutting element and precipitated in the form of graphite to exhibit a free-cutting effect, it requires a graphitizing heat treatment, which increases the cost. Problem 3: In the case of cast iron or cast steel having a high carbon content, free cutting properties are secured by precipitation of spheroidal graphite by inoculation, but the toughness is poor.

Problem 4: Even if the toughness is improved by heat treatment of free-cutting cast iron or free-cutting cast steel, sufficient toughness cannot be obtained, and there is a problem in the reliability of products due to defects in casting cavities. Problem 5: In the future, when a considerable amount of low-grade scrap in which Cu, Ni, etc. are mixed in a high concentration as a tramp element is used as an iron source, there is a concern that the ductility of the steel material may be reduced. Is missing. In the present invention, it is considered that solving this problem is extremely important.

According to the present invention, the above problems are solved.
For hot-worked steel bars used as materials for parts of automobiles and industrial machines, and for hot-working the steel bars, cutting and finishing to produce products, and manufacturing the above components without heat treatment In addition, the machinability is good, even when using scrap containing a high concentration of tramp elements,
An object of the present invention is to develop a technology which is excellent in strength and toughness, suppresses surface flaws, and can be manufactured at low cost without environmental protection problems. The present inventors, in order to achieve the above object, in particular, to precipitate fine and appropriate graphite while hot working without heat treatment, and to reduce the production and quality of mixed Cu and Ni of the Trump element. The main task was to avoid adverse effects.

[0019]

SUMMARY OF THE INVENTION With the background of the prior art as described above, the present inventors have developed a lead-free ultra-free-cutting steel product utilizing low-grade scrap and having machinability comparable to that of casting. As a result of diligent research for the purpose, by appropriately combining the chemical components, it has good hot ductility, hot-rolling is possible, and it is possible to directly fine It has been found that a hot-worked steel material and a product having excellent graphite and excellent free-cutting properties can be obtained.

The present invention has been made based on the above findings, and the gist is as follows. The hot-worked steel material and product excellent in free-cutting property according to claim 1 are: C: 0.80 to 1.70 wt.%, Si: 0.70 to
2.50 wt.%, Cu: 0.01 to 2.0 wt.%, Ni:
0.01 to 2.0 wt.%, Ca: 0.0005 to 0.01
00 wt.%, Al: 0.001 to 0.10 wt.%, P: 0.
050 wt.% Or less, S: 0.050 wt.% Or less, O: 0.0
050 wt.% Or less and N: 0.015 wt.% Or less, the balance consisting of iron (Fe) and unavoidable impurities, and the Ni / Cu ratio of Ni / Cu of Ni / Cu ,
The following formula (1) is satisfied: Ni / Cu ≧ 0.2 (1), and the following formula (2) is satisfied: CE = C + Si / 3 + Cu / 9 + Ni / 9 + Al / 6 -(2) However, each element symbol: the graphitization index CE calculated by the content (wt.%) Of each element is expressed by the following equation (3): CE ≧ 1.30 ------ (3 A hot-worked steel material having a chemical composition that satisfies the following and cooled to room temperature after hot-working, and a product made of the hot-worked steel material, wherein the average particle size is 0. 100 µm / mm graphite of 5μm or more
It is characterized in that ^two or more are precipitated and the main constituent of the metal structure is pearlite. That is, in the above, the hot-worked steel material and the product made of this hot-worked steel material are, as described above,
What is required in terms of chemical composition,
And, as described above, it means that it has both requirements that it is under the history of hot working as it is. And this invention is such a hot-worked steel material and its product,
As described above, it has been specified in what state the graphite is required to be precipitated. A hot-worked steel material and a product excellent in free-cutting property according to claim 2 is the invention according to claim 1, wherein the graphing index CE is calculated by the following equation (4): CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9-B (4) However, each element symbol: The content (wt.%) Of each element is used, and the steel material and the product are used. The chemical composition of the following 4
At least one selected from the group consisting of species of chemical components
It is characterized in that the seed is further included. Here, the four kinds of chemical component compositions are defined as M
n: 0.01 to 1.0 wt.%, Cr: 0.01 to 1.0 w
t.%, Mo: 0.01 to 0.50 wt.%, and B:
0.0005 to 0.010 wt.%.

A hot-worked steel material and a product excellent in free-cutting property according to claim 3 is the invention according to claim 1 or 2, wherein the graphitization index CE is calculated by the following equation (5): CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9-B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 ------ (5) where each element symbol : Using the content (wt.%) Of each element, and adding the following 4
At least one selected from the group consisting of species of chemical components
It is characterized in that the seed is further included. Here, the above four chemical component compositions are T
i: 0.001 to 0.10 wt.%, Zr: 0.001
0.10 wt.%, V: 0.005 to 0.30 wt.%, And Nb: 0.005 to 0.30 wt.%.

The hot-worked steel material and the product excellent in free-cutting property according to claim 4 are the same as the formula (1), (2) or (3), wherein the graphitization index CE is calculated as follows: 5) Formula: CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9-B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 (5) However, each element symbol: the content (wt.%) Of each element is used, and at least one selected from the group consisting of the following two chemical component compositions is used as the chemical component composition of the steel material and the product. It is characterized by being additionally included. Here, the two kinds of chemical component compositions are Mg
: 0.0010 to 0.10 wt.% And REM: 0.
0010 to 0.10 wt.%.

The method for producing a hot-worked steel material and a product excellent in free-cutting property according to claim 5 is as follows: C: 0.80 to 1.70 wt.
%, Si: 0.70 to 2.50 wt.%, Cu: 0.01 to
2.0 wt.%, Ni: 0.01 to 2.0 wt.%, Ca: 0.
0005-0.0100 wt.%, Al: 0.001-0.
10 wt.%, P: 0.050 wt.% Or less, S: 0.050 w
t.% or less, O: 0.0050 wt.% or less, and N: 0.
015 wt.% Or less, the balance consisting of iron (Fe) and unavoidable impurities, and the difference between the Ni content and the Cu content.
When the wt.% ratio Ni / Cu is expressed by the following formula (1): Ni / Cu ≧ 0.
2 ---- Satisfies (1) and the following formula (2): CE = C + Si / 3 + Cu / 9 + Ni / 9 + Al / 6 (2) However, each element symbol: each A slab or a steel material having a chemical component composition having a graphitization index CE calculated from the element content (wt.%) Satisfying the following formula (3): CE ≧ 1.30 And the above C
Cu having a composition equal to the ratio between the u content and the Ni content.
The temperature is lower than the solidus temperature of the alloy of Ni and Ni, and 50 ° C lower than the solidus temperature of the steel slab or the steel material, with the upper limit being 800 ° C. After hot working and cooling to room temperature, the hot-worked steel material thus obtained is precipitated with graphite having an average particle size of 0.5 μm or more in an amount of 100 / mm ² or more. The main feature of the metal structure is that of pearlite.

According to the sixth aspect of the present invention, there is provided a method for producing a hot-worked steel material and a product excellent in free-cutting property, wherein the graphitization index CE is calculated by the following equation (4): CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9-B (4) However, each element symbol: The content (wt.%) Of each element is used, and The steel slab or the steel material is characterized by using at least one selected from the group consisting of the following four chemical component compositions in addition thereto. Here, the above four chemical component compositions are
Mn: 0.01 to 1.0 wt.%, Cr: 0.01 to 1.0
wt.%, Mo: 0.01 to 0.50 wt.%, and B:
0.0005 to 0.010 wt.%.

The method for producing a hot-worked steel material and a product excellent in free-cutting property according to claim 7 is a method according to claim 5 or claim 6, wherein the graphitization index CE is calculated by the following formula (5). Formula: CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9-B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 (5) However, the symbol of each element: the content (wt.%) Of each element is used, and at least one selected from the group consisting of the following four chemical components is further added as the steel slab or the steel material. It is characterized by using what is included. Here, the above four chemical component compositions are
Ti: 0.001 to 0.10 wt.%, Zr: 0.001 to
0.10 wt.%, V: 0.005 to 0.30 wt.%, And Nb: 0.005 to 0.30 wt.%.

The method for producing a hot-worked steel material and a product excellent in free-cutting property according to claim 8 is a method for calculating the graphitization index CE according to claim 5, 6, or 7. The following formula (5) is used: CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9-B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 --------------------------- (5) However, each element symbol: Use the content rate (wt.%) Of each element, and The steel slab or the steel material is characterized by using at least one selected from the group consisting of the following two chemical components in addition thereto. Here, the two types of chemical component compositions are as follows:
Mg: 0.0010 to 0.10 wt.%, And REM:
0.0010 to 0.10 wt.%.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the chemical composition, the graphite precipitation state and the metal structure of the billet, steel material and products manufactured from the steel material according to the present invention, and the heating of the billet and the steel material The reason why the conditions are limited as described above will be described.

(1) Carbon (C) Carbon is an important element for precipitating graphite and ensuring strength. In order to deposit graphite while hot working, 0.80 wt.% Or more is required. However, if the carbon content exceeds 1.70 wt.%, The decrease in hot ductility is large,
The occurrence of surface flaws increases during bar rolling. In addition, graphite grains precipitated after hot working become coarse, and the toughness is reduced.
Therefore, the carbon content is between 0.80 and 1.70 wt.%.

(2) Silicon (Si) Si is an element that plays an important role in the present invention.
That is, Si is an element which promotes the graphitization of cementite, and its effect is small when it is less than 0.70 wt.%. But,
If Si exceeds 2.50 wt.%, Nonmetallic inclusions increase and not only decrease in toughness, but also increase decarburization during heating during hot working. Therefore, the Si content is 0.70
２．５2.50 wt.%.

(3) Copper (Cu) The Cu content is gradually increasing today during scrap. On the other hand, when the steel is heated at a high temperature, Fe is first selectively oxidized, and Cu in the steel is concentrated on the material surface. Cu
Has a low melting point and thus melts, and a melt of Cu penetrates into crystal grain boundaries with many gaps. This lowers hot ductility and causes surface flaws and cracks. Therefore, Cu is rarely used positively in automobiles, industrial machine parts and the like. However, since the melting point of Cu is about 1083 ° C., if hot working is performed at a temperature lower than this melting point, it is possible to prevent Cu from penetrating into grain boundaries and reducing hot ductility.

However, hot working at a low temperature of less than 1083 ° C. involves an increase in the deformation resistance of the material, and an excessive load is applied to the rolling mill and tools. Therefore, the conventional steel has not been put to practical use. It is necessary to develop steel with low deformation resistance.

On the other hand, Cu forms Cu ₂ S and CuS and renders the harmful effect of S harmless. Therefore, Cu is a useful element that can substitute for Mn, promotes the precipitation of graphite, and improves the hardenability. Element that causes Therefore, for this purpose C
When using u, it is necessary to add 0.01 wt.% or more. However, if Cu exceeds 2.0 wt.%, The solid solubility limit in steel will be exceeded, and undissolved Cu will remain, reducing hot ductility and promoting the generation of surface flaws.
The content is between 0.01 and 2.0 wt.%.

(4) Nickel (Ni) Ni is mixed in the scrap to some extent.
Forms a solid solution with Cu, so that it is well mixed. N
The melting point of i is about 1453 ° C. and Cu (melting point: 1083)
° C) to raise the temperature at which it begins to melt (solidus temperature). That is, as the Ni concentration in Cu increases, an alloy of Cu and Ni (Cu-Ni alloy)
Has a higher solidus temperature T _S. Therefore, Cu in the surface layer of the steel material becomes an alloy of Cu and Ni due to the addition of Ni, and hardly melts. Therefore, penetration into crystal grain boundaries is suppressed, and generation of surface defects is suppressed.

Further, Ni is a useful element that promotes the precipitation of graphite as well as Cu and improves the hardenability. When added for these purposes, Ni is added in an amount of 0.1.
Requires addition of more than 01 wt.%. However, Ni was added to 2.
Addition of more than 0 wt.% Not only saturates the effect, but also increases the deformation resistance. Therefore, the Ni content is between 0.01 and 2.0 wt.%.

Here, when Cu and Ni are used together, Ni
The reason why the weight wt.% Ratio of / Cu is 0.2 or more is as follows. As described above, when Cu is alloyed with Ni, the solidus temperature increases. The heating temperature of the steel material is
If the temperature is higher than the solidus temperature of the i-alloy, hot ductility decreases,
Since surface flaws and cracks occur, the upper limit of the heating temperature is Cu
-It must be lower than the solidus temperature of the Ni alloy. on the other hand,
The heating temperature of the steel material of the present invention is, as described later, about 1165 ° C., which determines the optimum processing temperature from the solidus temperature of the steel material.
And can be lowered by about 200 ° C. from the processing temperature of the conventional steel for machine structural use. Therefore, in order to prevent generation of a Cu melt during heating of the steel material and to be able to carry out the reduction of the above-mentioned optimum processing temperature, Cu
(Melting point: 1083 ° C.) must be alloyed with Cu—Ni to increase the solidus temperature.

As a result of repeated experiments, the present inventors have found that in order to prevent the occurrence of surface flaws, the Ni / Cu wt.
It was determined that this was necessary. Ni / Cu
When the wt.% ratio is 0.2, the melting point of this alloy is about 1145 ° C.
It was found that the melting point of Cu could be increased by about 60 ° C.

(5) Phosphorus (P) P is an element that promotes graphitization, but segregates at the grain boundaries to reduce hot ductility and promotes surface flaws. Therefore,
The P content is limited to 0.050 wt.% Or less.

(6) Sulfur (S) S is an element that greatly inhibits graphitization. If the S content exceeds 0.050 wt.%, It is necessary to add a large amount of a graphitization promoting element such as Si. And causes a reduction in hot ductility. Therefore, the S content is 0.050 W to suppress the above-mentioned adverse effects.
Limited to t.% or less. Desirably, the content is 0.030 wt.% Or less.

(7) Calcium (Ca) Ca is mixed with Si—Al—O-based inclusions to form Ca—S
A low melting point inclusion having a melting point of about 1200 ° C. of i-Al—O system is formed. This low-melting inclusions melt with the temperature rise during high-speed cutting, and thinly adhere to the flank and rake surfaces of the tool.
A so-called bellag is formed to suppress the progress of tool wear and extend tool life.

Ca is used as an inoculant in cast iron and promotes graphitization. This is because the vapor pressure of Ca is high and the vapor of Ca forms small cavities in the iron during casting, which is considered to be the nucleus of graphite precipitation and precipitate spheroidal graphite. Facilitates graphite deposition after hot working. For this purpose, Ca is 0.000
It is necessary to add 5% or more, but if it exceeds 0.010%, the effect is saturated. Therefore, the range of the Ca content is between 0.0005 and 0.010 wt.%.

(8) Aluminum (Al) Al forms Ca-Si-Al-O-based inclusions together with Ca and Si. It is an element that promotes graphitization like Si. For these purposes, it is necessary to add Al at least 0.001% or more. But 0.10
%, The Al in the Ca-Si-Al-O-based inclusions
And the melting point rises, making it difficult to form belag. Therefore, the range of the Al content is 0.00
It should be between 1 and 0.10 wt.%.

(9) Oxygen (O) O is Ca—Si—O between Si, Al and Ca in steel.
Form oxide-based inclusions. However, O is an element that inhibits graphitization, and if its content exceeds 0.0050 wt.%, It is necessary to add a large amount of an element that promotes graphitization. Therefore, the O content is limited to 0.0050 wt.% Or less.

(10) Nitrogen (N) When N alone exists in steel, it inhibits graphitization. If the N content exceeds 0.015 wt.%, Precipitation of graphite becomes difficult, and a large number of blowholes are formed by nitrogen gas, which causes surface defects after rolling. Therefore, the N content is limited to 0.015 wt.% Or less.

(11) Manganese (Mn) Mn is an element that enhances hardenability, refines pearlite, and strengthens steel. When used for this purpose, 0.01 wt.% Or more of Mn is required, but Mn is also an element that greatly inhibits the precipitation of graphite. Therefore, it is desirable to contain Mn in a range of 1.0 wt.% Or less.

(12) Chromium (Cr) Like Cr, Cr is an element which greatly improves the hardenability and makes pearlite fine. When Cr is used for this purpose, it must be added in an amount of 0.01 wt.% Or more. However, since Cr also has a strong effect of inhibiting graphitization like Mn, if it exceeds 1.0 wt.%, A large amount of graphitization promoting element is required, resulting in an increase in cost. Therefore, if Cr is 0.01 to
It is desirable that the content be between 1.0 wt.%.

(13) Molybdenum (Mo) Mo is also an element that enhances the hardenability of steel and makes pearlite fine. When used for this purpose, 0.01 wt.% Or more must be added. However, Mo is also an element that inhibits graphitization like Mn and Cr. If it exceeds 0.50 wt.%, A large amount of graphitization promoting element is required. Therefore, Mo
Is desirably contained between 0.01 and 0.50 wt.%.

(14) Boron (B) B is an element that enhances hardenability in a trace amount. Further, N in the steel is fixed as BN, and the graphitization inhibiting effect of N is reduced. When B is used for this purpose, 0.0005 wt.% Or more must be added. However, if B is added in excess of 0.010 wt.%, The effect is not only saturated, but also the hot ductility is reduced. Therefore, the B content is 0.0005 to
It is desirable that the content be between 0.010 wt.%.

(15) Titanium (Ti) Ti precipitates TiN and TiC and refines crystal grains. In addition, these precipitates act as nuclei for graphite deposition and promote graphite deposition. If the amount of Ti is less than 0.001 wt.%, The effect is small. On the other hand, if the amount of Ti exceeds 0.10 wt.%, Hard TiN and TiC are generated in a large amount to promote wear of the tool. Therefore, Ti is set to 0.001 to 0.1.
It is desirable to contain it between 0 wt.%.

(16) Zirconium (Zr) Like Zr, Zr also precipitates nitrides and carbides, refines crystal grains, and promotes precipitation of graphite. If the Zr content is less than 0.001 wt.%, The effect is small, while if it exceeds 0.10 wt.%, Tool wear is accelerated. Therefore, it is desirable that Zr be contained between 0.001 and 0.10 wt.%.

(17) Vanadium (V) V also precipitates nitrides and carbides and refines crystal grains.
Further, since V precipitates are fine, they increase the yield stress of the steel and improve the fatigue limit stress. However, when the amount of V added is 0.1.
If it is less than 005 wt.%, The effect is small. On the other hand, V is an element that inhibits the precipitation of graphite, and if added in excess of 0.30 wt.%, A large amount of the graphitization promoting element is required. Therefore, it is desirable that V be contained between 0.005 and 0.30 wt.%.

(18) Niobium (Nb) Nb also precipitates nitrides and carbides, refines crystal grains, and increases the yield stress. Nb carbonitride is 115
Even at a high temperature of 0 ° C., it does not form a solid solution in the steel, prevents coarsening of austenite grains, refines grains after forging, and improves toughness. When the amount of Nb is less than 0.005 wt.%, The effect is small. On the other hand, when the amount exceeds 0.30 wt.%, The precipitation of graphite is inhibited, and a large amount of the graphitization promoting element is required. Therefore, it is desirable to contain Nb between 0.005 and 0.30 wt.%.

(19) Magnesium (Mg) Mg, like Ca, is used as an inoculant in cast iron and promotes graphitization, and also facilitates precipitation of graphite after working in steel. If the amount is less than 0.0010 wt.%, The effect is small, while if it exceeds 0.10 wt.%, The effect is saturated. Therefore, 0.0010 to 0.10w of Mg
%. (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 wt.%, The effect is small,
On the other hand, the effect is saturated even if it exceeds 0.10 wt.%. Therefore, it is desirable to contain REM between 0.0010 and 0.10 wt.%.

[0053] Steel contains elements inevitably mixed, such as Sn and As, in addition to the usual elements. If the environmental problem is small, it is also possible to supplementarily add a small amount of free-cutting elements such as Bi, Se, and Te.

(21) Graphitization Index Graphitization index CE is important for accelerating the precipitation of graphite. This CE is represented by the following formula (5) for the main element. That is, CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9-B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 --------------------- (5) However, each element symbol represents the content (wt.%) Of each element.
Then, when other conditions are constant, the precipitation of graphite is promoted as the graphitization index CE increases. The precipitation of graphite depends on the heating temperature, the working degree and the cooling rate, and is not uniquely determined by CE.
If it is not more than 30, it is difficult to deposit graphite under practical conditions unless heat treatment for depositing graphite such as annealing is performed. Therefore, CE is set to 1.30 or more.

(22) Structure of Worked Products Products such as hot-rolled steel bars and hot-forged crankshafts contain fine graphite to ensure free-cutting properties, and the main metal structure is toughness. Must be pearlite in order to ensure In addition to pearlite, partly grain boundary ferrite, ferrite generated around graphite grains,
Bainite may be present alone or in combination.

(23) Heating temperature The hot working temperature is an important factor for promoting the precipitation of graphite. This results in the precipitation of fine graphite while the steel is held at a high temperature, if the heating temperature during processing is appropriate. Further, by leaving a large amount of lattice defects introduced by processing, precipitation of graphite during subsequent cooling is facilitated. However, if it is held at an excessively high temperature for a long time, the graphite once precipitated during the high-temperature holding will again form a solid solution, and the number of graphite particles obtained after processing will decrease.

In the present invention, the upper limit of the heating temperature of the steel material is set to an alloy of Cu and Ni having a composition equal to the ratio of the content ratio (wt.%) Of Cu and Ni in the steel material (“Cu-Ni alloy”). 50) from the solidus temperature of the steel material
Compare with the temperature lower by only ° C and adopt the lower temperature.
However, when the solidus temperature of the former Cu—Ni alloy is lower, the upper limit of the heating temperature is lower than the solidus temperature of the Cu—Ni alloy. The reasons for the above limitation are as follows.

First, the heating temperature of the steel material is lower than the solidus temperature T _{S of the} steel material by 50 ° C., that is, (T _S -50)
When the temperature exceeds ℃, the hot ductility of the steel material is sharply reduced, and a surface flaw is generated in a hot-rolled steel bar, and a crack is generated in a hot forged product. Therefore, the heating temperature is (T _S
-50) It must be below ℃.

[0059] On the other hand, the heating temperature of the steel material described above (T _S -5
0) Even when the temperature is lower than 0 ° C., Fe in the surface layer portion is selectively oxidized, and the remaining and concentrated Cu and Ni are alloyed, and the melt of the Cu—Ni alloy thus generated enters the austenite grain boundary. Therefore, it is necessary to prevent the hot ductility from being lowered and to prevent surface flaws and cracks from being generated. Therefore, the steel material heating temperature is determined by the content of Cu and Ni (wt.
%) Must be lower than the solidus temperature of a Cu-Ni alloy having a composition equal to the ratio of (%).

From the above, the upper limit of the steel material heating temperature is (Cu content (wt.%)) / (Ni content (wt.%)) In the steel material.
Should be lower than the solidus temperature of the Cu-Ni alloy having a composition equal to the ratio of and below the solidus temperature of the steel material by 50 ° C. The upper limit of the heating temperature of the steel slab is the same as the upper limit of the heating temperature of the steel for exactly the same reason as in the case of the steel.

In the above, the upper limit of the heating temperature of the steel material is
A temperature (T _S -5) lower by 50 ° C. than the solidus temperature of the steel material (the temperature at which the liquid phase starts to form when the steel is heated).
The operation and effect when 0) ° C. is permitted will be described. For example, for 1.2 wt.% C-1.5 wt.% Si steel,
Considering the upper limit of the heating temperature, it is as follows. First,
Solidus temperature (temperature at which liquid phase starts to appear when heated) T _S
Depends on the composition of the steel material, for example, the following approximate formula: T _S (° C.) = 1420-250 (C-0.5) -20Si, where C, Si: carbon and silicon contents (wt.%) Is calculated to be 1215 ° C. Therefore, the heating upper limit temperature is (solidus temperature T _S -50) ° C. = 1215-5.
0 = 1165 ° C. The eutectic temperature of this steel material is about 1140 ° C., and the solidus temperature T _S does not fall below the eutectic temperature. In general, since the solidus temperature T _S does not fall below the eutectic temperature, the calculated value of T _{S in} the above equation is 1140 ° C.
, The actual solidus temperature is 1140 ° C. or higher.

As an example of the composition of the steel material according to the present invention, for example, the above-mentioned 1.2 wt.% C-1.5 wt.% Si steel, the solidus temperature T _S is 1215 ° C. About 20% higher than the solidus temperature (T _S = 1420 ° C.) of medium carbon steel of 0.5 wt.
It will be 0 ° C lower. This suggests that if the steel material of the present invention is used, even if hot working is performed at a heating temperature lower by about 200 ° C. than that of the conventional steel material, the steel material has the same deformation resistance and deformability as the conventional steel material. Therefore, it can be said that the steel material is preferable.

FIG. 1 shows an Fe—C phase diagram when 2 wt.% Si is contained. In the figure, the temperature at point S is A ₁
The temperature at point E indicates the eutectic temperature, and the HE line indicates the solidus temperature. Since this figure is a Fe-C binary phase diagram, the solidus temperature of the steel of the present invention when the Si content is 2.0 wt.% Cannot be accurately estimated. Therefore, although the solidus temperature of the steel material of the present invention cannot be determined accurately, the effect of the C content on the decrease of the solidus temperature of the steel material, and the solidus temperature T of the slab or steel material in the present invention, _A temperature 50 ° C. lower than _S ((T _S -50) ° C.) is useful for practical estimation. In the figure, the temperature range of 800 ° C. or more and (T _S -50) ° C. or less at C = 0.80 to 1.70 wt.% Is indicated by oblique lines.

For example, the above 1.2 wt.% C-1.5 wt.% S
(solidus temperature T _S -50) ° C. is 1165
The temperature level of 0 ° C. is very close to the melting point of 1083 ° C. of Cu, and in the present invention, the alloying of Ni with Cu raises the solidus temperature of the Cu—Ni alloy. _S- 50) C = 1165C is Cu-
The temperature becomes closer to the solidus temperature of the Ni alloy, enabling low-temperature processing near the solidus temperature of the Cu-Ni alloy. Therefore, it can be said that 1.2 wt.% C-1.5 wt.% Si steel is a steel having low deformation resistance.

FIG. 2 exemplifies a Cu—Ni binary equilibrium diagram, in which the steel material according to the present invention has the following ranges (6) and (7), which are the ranges of the ratios of the contents of Cu and Ni (wt.%). ) And formula (1): Cu: 0.01 to 2.0 wt.% ----------- (6) Ni: 0.01 to 2.0 wt.% ------ ------ (7) Ni / Cu ≧ 0.2 ------------ Under the condition of (1), the content of Ni in the Cu-Ni alloy (wt. %) (This is represented by “C _Ni ”): C _Ni = [Ni (wt.%) / ｛Cu (wt.%) + Ni (wt.
%)｝] × 100 (wt.%), 16.7 wt.% ≦ C _Ni ≦ 99.5 wt.% --- (8) is obtained. The range that C _Ni can take is indicated by the arrow range in FIG. The content ratio of Ni and Cu is Ni / C
As long as u = 0.2 is satisfied, the Ni content C in the Cu—Ni alloy is always constant regardless of the Cu and Ni content.
_Ni becomes C _Ni = 16.7 wt.%. As can be seen, the solidus temperature of the Cu-Ni alloy produced during heating the steel of the present invention is approximately in the range of 1145 to 1451C. According to the above, (T _S -50) ° C. of the steel material
It is easily understood that the difference between the solidus temperature of the Cu—Ni alloy and the solidus temperature of the Cu—Ni alloy is reduced, and the difference is reduced even if the solidus temperature of the Cu—Ni alloy is lower. When the solidus temperature of the Cu—Ni alloy is higher than the (T _S -50) ° C. of the steel material, the Cu—Ni alloy does not melt during the heating and holding, so that it enters the austenite grain boundary. I will not do it.

Next, if the material temperature during the processing of the steel material is not higher than 800 ° C., the deformation resistance increases and the life of the forging tool is shortened. In addition, since the deformability is insufficient and it causes forging cracks,
It is necessary to maintain the temperature at 800 ° C. or higher.

As described above, the heating temperature of the steel material is Cu-N
The solidus temperature of steel less than the melting point of i-alloy and -5
The temperature is between 0 ° C. or less and 800 ° C. or more. (24) Particle Size of Graphite If the average particle size of graphite precipitated in a granular form is less than 0.5 μm,
The effect of breaking small chips during cutting is small, and the contribution to cutting properties is small. Therefore, the average particle size of graphite is 0.5
μm or more. Although the upper limit is not particularly limited, precipitation of a large amount of graphite exceeding 30 μm causes a decrease in toughness. Therefore, the particle size of graphite is preferably 30 μm or less.

The shape of graphite in the present invention is generally expressed as a lump, but it may be spherical or granular, and there is no particular problem if the thickness / length ratio is 5 or less. (25) Number of Graphite Increasing the number of graphite per unit area is important for dividing chips into small pieces. 100 /
If it is less than ² mm, the effect of improving the chip disposability is small.
The number of graphite is 100 / mm ² or more. The number of graphite depends on the size of the graphite, the smaller the larger the grain,
The smaller the number, the more. In the present invention, when graphite having a diameter of 10 to 25 μm is precipitated, the number thereof is approximately 100 to 1 μm.
Although it is between 000, in the case of graphite having a diameter of 0.5 to 5 μm, it reaches approximately 3000 to 50,000.

[0069]

Next, the present invention will be described in more detail with reference to examples. Here, Test 1 to Test 3 were performed.

[Test 1] Tables 1 and 2 show the chemical composition of the test materials used in the tests and the graphitization index C described later.
E and the solidus temperature T _S are shown. In Tables 3 and 4,
The main manufacturing conditions and test results are shown.

[0071]

[Table 1]

[0072]

[Table 2]

[0073]

[Table 3]

[0074]

[Table 4]

Steel types Nos. 1 to 23 are within the scope of the present invention with respect to the chemical composition, and the corresponding claim numbers are also described. Of these, the test using steel types Nos. 1 to 20 corresponds to the examples which are test examples within the scope of the present invention, since the production conditions are also within the scope of the present invention. Therefore, these were designated as Examples Nos. 1 to 20, respectively. However, steel grade No. 21-
The test using No. 23 corresponds to a comparative example which is a test example out of the scope of the present invention because the heating temperature of the steel slab described later is out of the scope of the present invention among the manufacturing conditions. Therefore, they are referred to as Comparative Examples Nos. 21 to 23, respectively.

Steel types Nos. 24 to 49 have a chemical composition outside the scope of the present invention. Among them, steel types Nos. 24 to 45 are comparative examples, and steel type No. 46 is a conventional spheroidal graphite cast iron.
For steel type No. 47, V: 0.10 wt.%, Pb:
Conventional non-heat treated steel with 0.22 wt.% Added, steel grade No. 48
Is a conventional S50C sulfur-added steel, and steel type No. 49 is a conventional SCM822, which is a conventional component example. The production conditions of the tests using steel types Nos. 24 to 49 include various types outside and within the scope of the present invention. Applicable. Therefore, these are respectively referred to as Comparative Examples Nos. 24 to 49.

Here, the graphitization index CE is important to promote the precipitation of graphite in the process of producing a steel material, and when other conditions are the same, the larger the CE, the more the precipitation of graphite is promoted. . This CE has the following formula (5): CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9-B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 ----------------------------- (5) However, each element symbol: expressed by the content of each element (wt.%) It is. The precipitation of graphite is not uniquely determined by CE since it depends on the heating temperature, the working ratio, and the cooling rate. However, in all steel types Nos. 1 to 23 in Table 1, it is 1.30 or more. The components were adjusted to be as follows.

The test materials having the chemical component compositions shown in Tables 1 and 2
After being melted in a 30-ton electric furnace, it was made into a slab by continuous casting or ingot-making method. The slab was slab-rolled into a 160 mm square steel slab, then heated in a slab heating furnace to a temperature of 820 to 1160 ° C., and hot rolled into a bar having a diameter of 26 mm or 93 mm.

After the hot rolling, the steel bar was allowed to cool or the cover was gradually cooled to precipitate graphite. 80 of 26mmφ steel bar as it is
The average cooling rate from 0 ° C to 600 ° C is about 1.5 ° C / s
ec, that in the cover slow cooling is 0.4 ° C./sec, 9
Cooling rate of 3mmφ steel bar at the time of cooling is 0.25 ° C / sec.
c, The rate of slow cooling of the cover was 0.08 ° C./sec.

The surface of the steel bar was visually inspected for flaws, and the state of graphite and the metal structure were examined with an optical microscope. In addition, 26
The steel bar of mmφ was machined by cutting into a piston rod of the shock absorber, and the steel bar of 93 mmφ was machined by cutting into the piston rod of a construction machine, and the chip controllability was determined.

As shown in FIG.
If the chips were separated by less than the number of windings, they were ranked as good, and if the chips were separated by 3 to 6 volumes, they were rated as normal. If they were connected to more than 8 volumes, they were ranked as inferior. The cutting was performed for 20 minutes at a cutting speed of 200 m / min using a carbide P20 cutting tool.

Further, a JIS No. 4 tensile test piece was sampled from a steel bar and subjected to a tensile test to determine tensile strength and elongation.
In addition, only the spheroidal graphite cast iron of Comparative Example No. 46 was 93 mm
An ingot directly cast on a φ sand mold was used as a comparison material. In Examples Nos. 1 to 20, which are examples of the present invention, the chemical composition and the rolling heating temperature are appropriate, and the rolled product does not crack. The size of the graphite particles is 0.5 to 25 μm.
And the number of graphite particles is 100 particles / mm ² or more, which is sufficiently large. In addition, an appropriate amount of Ca-Al-Si-O-based low-melting oxide was also produced. For this reason, Belag adhered to the rake face, which suppressed the progress of wear, so that the wear depth of each rake face was 5 μm or less. In addition, the chips all had a good shape that was cut into small pieces of 2 or less due to the chip cutting effect of graphite. Further, the metal structure was a structure of pearlite single phase or pearlite-based ferrite + pearlite.

FIG. 4 shows a metallographic structure of the microscopic surface of the microscopic surface of Example No. 1 after the nital etching. The structure is pearlite on which graphite is precipitated. In FIG.
The precipitation state of graphite of Example No. 5 is shown. Graphite exists mainly at grain boundaries. The metal structure by the microscope is shown. The structure is grain boundary ferrite + pearlite.

In the examples, the tensile strengths were all 800.
N / mm ² or more, and elongation was 15% or more, which was sufficient for a piston rod to have sufficient strength and ductility. In contrast to the above example, Comparative Example No. 21 had a heating temperature of Cu-Ni.
Although the melting point was lower than the melting point of the alloy, it was higher than the T _S -50 ° C. of the steel material, so that the hot ductility was insufficient and the bar was cracked.

In Comparative Example No. 22, the heating temperature was T _S -50.
Although it was lower than C, it was higher than the melting point of the Cu-Ni alloy, so that the melt of the Cu-Ni alloy penetrated into the grain boundaries and lowered the hot ductility, so that the steel bar was cracked.

In Comparative Example No. 23, the heating temperature was 800
Since the temperature was too low at less than 0 ° C, the hot ductility was also insufficient, and the steel bar was cracked. In Comparative Example No. 24, the C content was low outside the range of the present invention, and therefore the graphitization index CE was low outside the range of the present invention, no graphite was observed, and the chips were long. Was. For this reason, it was necessary to stop the machine and remove the chips.

In Comparative Example No. 25, on the contrary, the C content was high outside the range of the present invention, the hot ductility was insufficient, and cracks occurred in the steel bars. Comparative Example No. 26 had a low Si content outside the range of the present invention, and therefore had a low graphitization index CE, no graphite precipitation, and poor chip controllability.

In Comparative Example No. 27, the Si content was high outside the range of the present invention, and as a result, the hot ductility was insufficient and the steel bar was cracked. In Comparative Example No. 28, the Cu content was higher than the range of the present invention, the hot ductility was insufficient, and the steel bar cracked.

In Comparative Example No. 29, the Ni content was higher than the solder of the present invention, the ductility was insufficient, and cracks occurred in the steel bars.
In Comparative Example No. 30, the Ni / Cu content ratio was lower than the range of the present invention, and the heating temperature was higher than the solidus temperature of the Cu-Ni alloy. It was concentrated to form a melt, penetrated into grain boundaries, and cracked the rolled steel bar.

In Comparative Example No. 31, the P content was higher than the range of the present invention, and cracking occurred due to insufficient ductility. Comparative Example No. 3
In No. 2, the Mn and S contents were higher than the range of the present invention, and cracking occurred due to insufficient ductility.

In Comparative Example No. 33, the Cr content was higher than the range of the present invention, so that the hot ductility was insufficient and the steel bar cracked. In Comparative Example No. 34, the Mo content was higher than the range of the present invention, and the steel bar also cracked.

In Comparative Example No. 35, the B and N contents were higher than the range of the present invention, a large amount of BN was precipitated, and cracks occurred due to insufficient ductility. Comparative Example No. 36 is composed of Ti and Nb.
The content ratio of Comparative Example No. 37 was Zr content, and that of Comparative Example No. 37.
In No. 38, the V content was higher than the range of the present invention, so that the steel bar cracked due to insufficient ductility.

In Comparative Example No. 39, since the Al content was higher than the range of the present invention, the Al content in the oxide-based inclusions was increased, the melting point of the inclusions was increased, and the inclusions were not melted during cutting. As a result, the wear depth of the rake face was increased to 88 μm.

In Comparative Example No. 40, the Ca content was low outside the range of the present invention, so that a low melting point oxide could not be formed, and the wear depth of the rake face also became deep.
In Comparative Example No. 41, since the Ca content was higher than the range of the present invention, cracks caused by a large amount of oxides occurred.

Comparative Example No. 42 had a Mg content of Comparative Example N.
In the case of O.43, the REM content was higher than the range of the present invention, so that a large amount of oxide-based inclusions was involved, which caused rolling flaws and cracked the steel bar.

In Comparative Example No. 44 and Comparative Example No. 45, although the individual values of the chemical composition were within the range of the present invention, the graphitization index CE was low outside the range of the present invention. No precipitation occurred. For this reason, the chip disposability was poor.

Comparative Example No. 46 is an example of conventional spheroidal graphite cast iron and contains Mg as an inoculant. Several small holes of about 0.10 mm were present on the surface of the casting, which was not a preferable state as a mechanical part. The rake face wear depth is much deeper than the 5 μm result of the example of the present invention. The tensile strength is appropriate, but the elongation is 4
% And poor ductility.

Comparative Example No. 47, which is a conventional non-refined example, had a large rake face wear because the tool cutting edge was subjected to intensive wear due to the small curl radius of the chips. In addition, since Pb was contained, the chip controllability was good. However, from the viewpoint of environmental protection, it is required to manufacture components in a direction not using Pb in the future.

Comparative Example No. 48 is an example of the conventional S50C. Since it does not contain Pb, the chip processing performance is poor, but the rake face wear depth is shallower than that of Pb-added steel. Further, the tensile strength was slightly insufficient at about 700 N / mm ^2, and it was necessary to perform quenching and tempering to increase the tensile strength. Comparative Example No. 49 is an example of SCM822 for gears, which will be described in Comparative Example No. 49D of Test 3 described later.

FIG. 7 is a longitudinal sectional view for explaining the rake face wear depth of the cutting tool. In the figure, 2 is a wear depth, 1 is a cutting tool, 3 is a chip, and 4 is a work material. FIG. 8 shows progress curves of the rake face wear depth of Example 1, Comparative Example No. 40, and Comparative Examples No. 47 and 48 using conventional steel. In Example 1, it can be seen that the progress of wear is slow.

As described above, according to the embodiments within the scope of the present invention, it is possible to manufacture a lead-free ultra-free-cut non-heat treated steel bar having strength and ductility comparable to conventional non-heat treated steel bars. . [Test 2] Group A of steel types No. 3 and 17 whose components shown in Table 1 are within the scope of the present invention, and Group B of steel types No. 47, 48 and 46 outside the scope of the present invention The steel was tested as follows. Group A tests are within the scope of the present invention, and
Nos. 3A and 17A, the tests of Group B were out of the scope of the present invention, and Comparative Examples 47B and 48, respectively.
B, 46B.

In Examples Nos. 3A and 17A, 93 mm
After heating to 1000 ° C. using a φ bar, the crankshaft was hot forged and air-cooled with a fan. In addition, a 93 mmφ steel bar of Comparative Example No. 47B, which is a conventional non-heat-treated steel, and Comparative Example No. 48B, which is a conventional SC material, were manufactured in the same process as in Test 1. In this case, deformation resistance was large. It was heated to a high temperature of 1250 ° C., hot forged into a crankshaft having the same shape, and air-cooled by a fan. Further, for comparison, a conventional spheroidal graphite cast iron of Comparative Example No. 46B was directly cast on a crankshaft having the same shape and solidified.

As a machinability test, these forged products or cast products were cut at their outer circumferences, and then oil holes were drilled with a small diameter deep hole drill to a diameter of 3 mm. The form of the chips at that time was determined in Examples Nos. 3A and 17A and Comparative Example No. 46.
B and 47B were fine chips of less than 2 turns, which were finely divided. However, in Comparative Example No. 48B containing no Pb, the chips were extended to 10 turns or more, resulting in frequent drill breakage. Although wear of the tool during outer peripheral cutting was hardly observed in Examples Nos. 3A and 17A, 50 to 150 μm was observed in Comparative Examples Nos. 46B, 47B and 48B.
Wear of m depth occurred.

As a fatigue test, the manufactured crankshaft was subjected to a bending fatigue test. The fatigue strength of Example No. 3A was 500 N / mm ² , the fatigue strength of Example No. 17A was 530 N / mm ² , and the fatigue strength of Comparative Example No. 47B was 50.
It had a good strength of 0 N / mm ² . On the other hand, the spheroidal graphite cast iron of Comparative Example No. 46B had a fatigue strength of only 420 N / mm ² . This is presumably because cast iron has a low Young's modulus and small bubbles serve as a starting point of fatigue, thereby lowering the fatigue limit. Further, the fatigue strength of S50C of Comparative Example No. 48B was only about 430 N / mm ² . Therefore, when quenching was performed at 580 ° C. after quenching at 870 ° C., the fatigue strength could be improved to 520 N / mm ² .

The average particle size of graphite and the number of graphite particles were measured for the crankshafts of Examples Nos. 3A and 17A. As a result, all of them satisfied the requirements of the present invention. As described above, according to Example 3A and Example 17A, it is possible to manufacture a lead-free non-heat-treated ultra-free-cutting steel part excellent in machinability, and the machinability is lead free-cutting steel or spheroidal graphite. It surpasses cast iron and its properties are superior to conventional spheroidal graphite cast iron, and has high fatigue strength equivalent to a quenched and tempered material.

[Test 3] Group C of steel types Nos. 1 and 5 having the components shown in Table 1 within the scope of the present invention and group D of steel types No. 49 and 46 outside the scope of the present invention The steel was tested as follows. The tests in group C are within the scope of the present invention and are referred to as Examples Nos. 1C and 5C, respectively, and the tests in group D are outside the scope of the present invention and are comparative examples Nos. 49D and 46D, respectively.
Call it.

Embodiments Nos. 1C and 5C and the conventional S
In Comparative Example 49D according to CM822, the billet was 130 mm
It was rolled into a steel bar, hot forged into a differential drive gear having an outer diameter of 320 mm, and allowed to cool as it was. In Comparative Example No. 46D, conventional spheroidal graphite cast iron was directly cast into a gear sand mold having the same shape.

The gear materials of Examples Nos. 1C and 5C were directly cut into gears by a hobbing machine, and then subjected to gas soft nitriding at 570 ° C. for 5 hours to harden the surface. SCM82
In Comparative Example No. 49D according to No. 2, the as-forged structure was bainite, and it was difficult to cut it as it was because it was hard. Then 920 ℃ × 2.5 hours → 650 ℃ ×
After being softened by one-hour cycle annealing, cutting was performed. Thereafter, in order to harden the surface, carburizing and quenching treatment was performed at 920 ° C. × 5 hours → 840 ° C. × 40 minutes to harden the surface.

In Comparative Example No. 47D, the spheroidal graphite cast iron was taken out of the mold, directly cut, and then subjected to 900 mm.
An austempering treatment of immersion in a salt bath was performed at 240 ° C. for 2 hours.

In the hobbing process, all showed good chip controllability, little wear of the tool, little cutting surface, and good cutting condition. The gears subjected to each heat treatment were subjected to a fatigue test. The root softening fatigue strength of the gas nitrocarburized gears of Examples No. 1C and 5C using the steel of the present invention was 440 N / mm ² , and Comparative Example No. 49D
Fatigue strength of carburizing and quenching gear of SCM822 is 440N
/ Mm ² . However, the fatigue strength of the austempered spheroidal graphite cast iron of Comparative Example No. 46D was 32.
It was as low as 0 N / mm ² .

Since the deformation of the gears after the heat treatment causes noise at the time of gear engagement, the amount of deformation of the pressure-angle on the drive side of each gear was measured. FIG. 9 is a diagram illustrating the distortion of the gear. In the drawing, reference numeral 5 denotes an angular displacement (shift) of the angle, and reference numeral 6 denotes a tooth tip of the gear. The angle deviation of the carburized and quenched material was 14 minutes (1 minute was 1/60 of 1 °), but the nitrocarburized material was hardly deformed at 1 minute. The deformation of the austempered material immediately after the heat treatment was relatively small, ie, 3 minutes.
When the number of times of fatigue was exceeded, the deformation was as large as 21 minutes. This is presumably because the retained austenite retained in the structure was transformed into martensite by the austempering treatment, so that the deformation amount was increased.

The average particle size of graphite and the number of graphite particles were measured for the gear materials of Examples No. 1C and 5C. As a result, all of them satisfied the requirements of the present invention. As described above, according to Example 1C and Example 5C, even if the gear was not subjected to soft annealing, the machinability was good, the fatigue strength was higher than that of the spheroidal graphite cast iron, and the conventional SCM steel was carburized and quenched. It was confirmed that it had comparable high strength, small distortion, and low noise generation.

[0113]

As described above, according to the present invention,
The production of hot-work products with excellent machinability, fatigue strength, and elongation characteristics using low-grade scraps with a lot of impurities such as Cu and Ni as raw materials without using toxic Pb. It is possible to manufacture a non-refined free-cutting steel part and a gear having low strain and high fatigue strength. Such a hot-worked steel material and a product excellent in free-cutting property and a method for producing the same can be provided, and an industrially useful effect is obtained.

[Brief description of the drawings]

FIG. 1 is an Fe—C phase diagram when 2.0 wt.% Si is contained.

FIG. 2 is a diagram showing a composition range of a Cu—Ni alloy generated in a surface layer portion in a steel material heating step of the present invention and a solidus temperature range of the Cu—Ni alloy.

FIG. 3 is a diagram showing a form classification of chips in cutting.

FIG. 4 is a diagram showing a metallographic structure of a microscope surface of Example No. 1 which has been subjected to a nital-etching by a microscope.

FIG. 5 is a diagram showing the results of microscopic observation of the state of graphite deposition on the speculum surface after no etching in Example No. 5;

FIG. 6 is a diagram showing a metallographic structure of a microscope surface of Example No. 5 which has been subjected to nital etching and observed by a microscope.

FIG. 7 is a schematic longitudinal sectional view for explaining the rake face wear depth of the cutting tool.

FIG. 8 is a graph showing an example of a progress curve of a rake face wear depth of a cutting tool in Examples and Comparative Examples.

FIG. 9 is a schematic longitudinal sectional view illustrating distortion of a gear.

[Explanation of symbols]

 Reference Signs List 1 cutting tool 2 wear depth 3 chip 4 work material 5 angular displacement of pressure angle 6 gear tip

Claims (8)

[Claims] 1. C: 0.80 to 1.70 wt.%, Si: 0.70 to 2.50 wt.%, Cu: 0.01 to 2.0 wt.%, Ni: 0.01 to 2.0 wt.% .%, Ca: 0.0005 to 0.0100 wt.%, Al: 0.001 to 0.10 wt.%, P: 0.050 wt.% Or less, S: 0.050 wt.% Or less, O: 0.0050 wt.% % And N: 0.015 wt.% Or less, and the balance is composed of iron (Fe) and unavoidable impurities, and the Ni / C ratio between the Ni content and the Cu content is Ni / C.
u is the following formula (1): Ni / Cu ≧ 0.2 ---------------------------------- -(1) is satisfied, and the following equation (2) is satisfied: CE = C + Si / 3 + Cu / 9 + Ni / 9 + Al / 6 (2) where each element symbol: the content of each element (wt. %) Is calculated by the following formula (3): CE ≧ 1.30 --------------------------- ------------- (3) A steel material and a product made from the steel material that has a chemical composition that satisfies (3), and that has been hot worked and then cooled to room temperature. There is one graphite having an average particle size of 0.5 μm or more.
A hot-worked steel material and a product excellent in free-cutting properties, characterized in that at least 00 pieces / mm ² are precipitated and the main metal structure is pearlite. 2. The invention according to claim 1, wherein the graphitization index CE is calculated by the following equation (4): CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9 −B -------------------------------- (4) However, each element symbol: content rate of each element (wt % Of the steel material and the product,
At least one selected from the group consisting of species of chemical components
A seed is further included and contained,
Hot-worked steel products and products with excellent free-cutting properties. Mn: 0.01 to 1.0 wt.%, Cr: 0.01 to 1.0 wt.%, Mo: 0.01 to 0.50 wt.%, And B: 0.0005 to 0.010 wt.%. 3. The invention according to claim 1, wherein the formula for calculating the graphitization index CE is as follows:
Formula: CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9-B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 ----------- --------------------- (5) However, each element symbol: The content (wt.%) Of each element is used, and the steel material and the product are used. The chemical composition of the following 4
At least one selected from the group consisting of species of chemical components
A seed is further included and contained,
Hot-worked steel products and products with excellent free-cutting properties. Ti: 0.001 to 0.10 wt.%, Zr: 0.001 to 0.10 wt.%, V: 0.005 to 0.30 wt.%, And Nb: 0.005 to 0.30 wt.%. 4. The invention according to claim 1, 2 or 3, wherein the graphing index CE is calculated by:
The following formula (5): CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9-B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 ------------------------- (5) However, each element symbol: Use the content (wt.%) Of each element, and The chemical composition of steel products and the above products, the following 2
At least one selected from the group consisting of species of chemical components
A seed is further included and contained,
Hot-worked steel products and products with excellent free-cutting properties. Mg: 0.0010 to 0.10 wt.%, And REM: 0.0010 to 0.10 wt.%. 5. C: 0.80 to 1.70 wt.%, Si: 0.70 to 2.50 wt.%, Cu: 0.01 to 2.0 wt.%, Ni: 0.01 to 2.0 wt.% .%, Ca: 0.0005 to 0.0100 wt.%, Al: 0.001 to 0.10 wt.%, P: 0.050 wt.% Or less, S: 0.050 wt.% Or less, O: 0.0050 wt.% % And N: 0.015 wt.% Or less, and the balance is composed of iron (Fe) and unavoidable impurities, and the Ni / C ratio between the Ni content and the Cu content is Ni / C.
u is the following formula (1): Ni / Cu ≧ 0.2 ---------------------------------- -(1) is satisfied, and the following equation (2) is satisfied: CE = C + Si / 3 + Cu / 9 + Ni / 9 + Al / 6 (2) where each element symbol: the content of each element (wt. %) Is calculated by the following formula (3): CE ≧ 1.30 --------------------------- ------------- A slab or a steel material having a chemical composition satisfying (3)
Cu having a composition equal to the ratio between the u content and the Ni content
The temperature is lower than the solidus temperature of the alloy of Ni and Ni, and 50 ° C lower than the solidus temperature of the steel slab or the steel material, with the upper limit being 800 ° C. After hot working and cooling to room temperature, the hot-worked steel material thus obtained is precipitated with graphite having an average particle size of 0.5 μm or more in an amount of 100 / mm ² or more. A method for producing a hot-worked steel material and a product excellent in free-cutting property, characterized in that a main component of the metal structure is pearlite. 6. The invention according to claim 5, wherein the graphing index CE is calculated by the following equation (4): CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9 −B -------------------------------- (4) However, each element symbol: content rate of each element (wt %), And the steel slab or the steel material further contains at least one selected from the group consisting of the following four chemical component compositions. To produce hot-worked steel products and products with excellent free-cutting properties. Mn: 0.01 to 1.0 wt.%, Cr: 0.01 to 1.0 wt.%, Mo: 0.01 to 0.50 wt.%, And B: 0.0005 to 0.010 wt.%. 7. The invention according to claim 5, wherein the graphing index CE is calculated by the following formula (5).
Formula: CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9-B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 --------------------- (5) However, each element symbol: Use the content (wt.%) Of each element, and A hot-working steel excellent in free-cutting property, characterized by using, as the steel, at least one selected from the group consisting of the following four types of chemical components additionally added thereto; Product manufacturing method. Ti: 0.001 to 0.10 wt.%, Zr: 0.001 to 0.10 wt.%, V: 0.005 to 0.30 wt.%, And Nb: 0.005 to 0.30 wt.%. 8. The invention according to claim 5, 6, or 7, wherein a formula for calculating the graphitization index CE is:
The following formula (5): CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9-B + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 ------------------------- (5) However, each element symbol: Use the content (wt.%) Of each element, and A heat-excellent free-cutting material characterized in that at least one selected from the group consisting of the following two chemical components is further added and contained as a billet or the steel material. Method of manufacturing cold worked steel products and products. Mg: 0.0010 to 0.10 wt.%, And REM: 0.0010 to 0.10 wt.%.