JPH11293387A - Hot worked steel excellent in machinability, and product, and their production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To precipitate fine and appropriate graphite in an as-hot-worked state without performing heat treatment and to produce a hot worked steel excellent in machinability and capable of avoiding the adverse effect of the contamination with Cu and Ni as tramp elements on manufacture and quality. SOLUTION: A steel stock, having 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, <=0.050% P, <=0.05% S, $0.0050% O, <=0.015% N, and the balance iron with inevitable impurities and in which Ni/Cu>=0.2 is satisfied and graphitization index is regulated to <=1.30, is heated up to the temperature lower than the solidus temperature of a Cu-Ni alloy with the above content ratio between Cu and Ni and ranging from 800 deg.C to (the solidus temperature of the steel stock) -50 deg.C, hot worked, and cooled down to room temperature. Graphite of >=0.5 μm average grain size is precipitated by >=100 pieces/mm<2> in the resultant hot worked steel, and a structure composed essentially of pearlite is provided. Further, graphite precipitation accelerating elements, pearlite refining elements, machinability improving elements, hardening accelerating elements, or the like, are properly added.

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 hypoid gear, and a product such as the above-mentioned parts. It can be manufactured inexpensively by using low-grade scrap, which has a lot of fine graphite without hot-working even without heat treatment for depositing graphite. Machinability is even better, and has higher strength and toughness than conventional spheroidal graphite cast iron, hot-worked steel materials and products,
Also, the present invention relates to a method for producing the same.

[0002]

2. Description of the Related Art In recent years, a large amount of waste materials from automobiles and electric products are mixed in scraps that are circulating 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, there is a strong demand for effective use of such low-grade scrap.

[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 performed unattended by an automatic lathe, and when chips are long and entangled, it becomes necessary to stop the machine and perform extra work for removing the chips. This will reduce productivity. Therefore, there is a demand for a free-cutting steel excellent in processability such that chips are finely divided into appropriate sizes.

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 discharged from the holes, and the chips are clogged in the holes, causing drill breakage.

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, since lead is toxic, there is a strong demand for lead-free free-cutting steel with the increasing momentum of global environmental protection. Elements such as S, Ca, Bi, Se, Te and the like other than Pb are known as elements for improving the machinability, but these elements alone are not effective in improving machinability, and are expensive. Since it has at least one disadvantage of being toxic, it cannot be a substitute for lead.

[0010] On the other hand, graphite is an element which greatly improves machinability as seen in cast iron, but when carbon is added to steel, cementite precipitates, so that it is not easy to obtain graphite in steel. 0.10 carbon in the conventional invention
In the case of steel having a concentration of about 1.5 wt.
JP-A-107742 and JP-A-3-140411 disclose a steel material or a method of performing graphite 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 precipitate graphite. I have.

[0012] 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 wt.% 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 a steel product, there is a drawback that toughness such as elongation, drawing, and impact value is low.

In recent years, the base structure has been made bainite by austempering to improve its toughness. 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 method for manufacturing a grod 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.% Is added with S48C as a basic component. Also, the toughness is still inferior to forged products. Also, 0.1%
mm may occur, which is a starting point of fatigue fracture, so that the reliability of the material is inferior. It is necessary to pay strict attention to the casting method and ultrasonic inspection of the product. .

[0015]

In the above various prior arts, any of the following problems has not been solved.

Problem 1: The free-cutting elements used are toxic and have problems in terms of 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 is ensured by the 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. No effective technology found. 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.

[0020]

SUMMARY OF THE INVENTION With the background of the prior art as described above, the present inventors have developed a lead-free free-cutting steel product that utilizes low-grade scrap and has machinability comparable to that of casting. As a result, as a result of intensive research, by combining the chemical components properly, it has good hot ductility,
It has been found that it is possible to obtain a hot-worked steel material and a product having excellent free-cutting properties and having fine graphite directly without hot-rolling, which can be hot-rolled without annealing.

The present invention has been made based on the above findings, and the gist thereof is as follows.
Relates to steel products and products according to the present invention. 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.%, P: 0.050 wt.% Or less, S:
0.050 wt.% Or less, O: 0.0050 wt.% Or less, and N: 0.015 wt.% Or less, with the balance being iron (Fe)
And unavoidable impurities, and Ni content and C
The wt.% ratio Ni / Cu with the u content satisfies the following formula (1): Ni / Cu ≧ 0.2 (1), and the following formula (2): CE = C + Si / 3 + Cu / 9 + Ni / 9 -------------- (2) However, each element symbol: the graphitization index CE calculated by the content (wt.%) Of each element is as follows: ) Formula: CE ≧ 1.30 (3) A hot-worked steel material which has a chemical composition satisfying the following formula, is hot-worked and then cooled to room temperature, and the hot-worked steel material The product is a raw material, and 100 particles / mm of graphite with an average particle size of 0.5 μm 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 the hot-worked steel material are as described above as to what is required in terms of the chemical composition. As described in the above, it means that the material has both requirements of being in the history of hot working as it is. And this invention product is such a hot-worked steel material and its product, and, as described above, specifies in what state graphite is required to be precipitated. It is.

The hot-worked steel material and the product excellent in free-cutting property according to claim 2 are the same as those in 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: The content (wt.%) Of each element is used, and the above steel material and the above 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.%.

The hot-worked steel material and the product excellent in free-cutting property according to claim 3 are 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) However, each element symbol: each Using the content (wt.%) Of the element, and the chemical composition of the steel and the product, the following 5
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 five kinds of chemical component compositions are A
l: 0.01 to 0.50 wt.%, Ti: 0.01 to 0.5
0 wt.%, Zr: 0.01 to 0.50 wt.%, V: 0.0
1 to 0.30 wt.% And Nb: 0.01 to 0.30 w
Refers to t.%.

A hot-worked steel material and a product excellent in free-cutting property according to claim 4 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) However, each element symbol: each The content of the element (wt.%) Is used, and at least one selected from the group consisting of the following three chemical components is further added to the chemical components of the steel and the product. Is characterized by the fact that Here, the above three types of chemical component compositions are C
a: 0.0010 to 0.0100 wt.%, Mg: 0.00
10 to 0.10 wt.% And REM: 0.0010
0.10 wt.%.

The hot-worked steel material and the product excellent in free-cutting property according to claim 5 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.%, P: 0.050 wt.%
Hereinafter, S: 0.050 wt.% Or less, O: 0.0050 wt.%
Or less, and N: 0.015 wt.% Or less.
Al: more than 0.10 to 0.50 wt.%, Ti: 0.01 to
0.50 wt.%, Zr: 0.01 to 0.50 wt.%, V:
0.01-0.30 wt.% And Nb: 0.01-0.
30 wt.%. At least one selected from the group consisting of
Ca: 0.0010 to 0.0100 wt.%, Mg: 0.0
010 to 0.10 wt.% And REM: 0.0010
At least one selected from the group consisting of 0.10 wt.%, The balance being iron (Fe) and unavoidable impurities, and a wt.% Ratio Ni / C of Ni content and Cu content.
u is the following formula (1): Ni / Cu ≧ 0.2 ---------------------------------- -(1) is satisfied, and the following formula (6) is satisfied: CE = C + Si / 3 + Cu / 9 + Ni / 9 + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 -(6) 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) It has a chemical composition that satisfies and is hot worked And then cooled to room temperature, wherein the steel material and the product made of the steel material are 1% graphite having an average particle size of 0.5 μm or more.
It is characterized in that it precipitates at least 00 grains / mm ² and that the main constituent of the metal structure is pearlite.

[0027] The hot-worked steel material and product excellent in free-cutting property according to claim 6 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.%, P: 0.050 wt.%
Hereinafter, S: 0.050 wt.% Or less, O: 0.0050 wt.%
Or less, and N: 0.015 wt.% Or less.
Mn: 0.01 to 1.0 wt.%, Cr: 0.01 to 1.0
wt.%, Mo: 0.01 to 0.50 wt.%, and B:
0.0005-0.010 wt.%. At least one selected from the group consisting of: Al: more than 0.10 to 0.50 w
t.%, Ti: 0.01 to 0.50 wt.%, Zr: 0.01
0.50 wt.%, V: 0.01 to 0.30 wt.%, And Nb: 0.01 to 0.30 wt.%. At least one selected from the group consisting of: Ca: 0.0010 to 0.0
100 wt.%, Mg: 0.0010 to 0.10 wt.%, And REM: at least one selected from the group consisting of 0.0010 to 0.10 wt.%, With the balance being iron (Fe).
And unavoidable impurities, and Ni content and C
The Ni / Cu ratio with respect to the u content is expressed by the following formula (1): Ni / Cu ≧ 0.2 ------------- (1) is satisfied, and the following formula (5) is satisfied: 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) , Each element symbol: the graphitization index CE calculated from the content (wt.%) Of each element is expressed by the following equation (3): CE ≧ 1.30 -------------- -------------------------- Having a chemical composition that satisfies (3), and after being hot worked, cooled to room temperature A steel product and a product made from the steel material, wherein the graphite having an average particle size of 0.5 μm or more is 1
It is characterized in that it precipitates at least 00 grains / mm ² and that the main metal structure is pearlite.

Hereinafter, claims 7 to 12 relate to a method for producing a steel material and a product according to the present invention. The method for producing a hot-worked steel material and a product excellent in free-cutting property according to claim 7 is as follows: C: 0.80 to 1.70 wt.%, Si: 0.70
-2.50 wt.%, Cu: 0.01-2.0 wt.%, Ni:
0.01 to 2.0 wt.%, P: 0.050 wt.% Or less, S:
0.050 wt.% Or less, O: 0.0050 wt.% Or less, and N: 0.015 wt.% Or less, with the balance being iron (Fe)
And unavoidable impurities, and Ni content and C
The Ni / Cu ratio with respect to the u content is expressed by the following formula (1): Ni / Cu ≧ 0.2 ------------- Satisfies (1) and the following equation (2): CE = C + Si / 3 + Cu / 9 + Ni / 9 -(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 billet or steel material having a chemical composition satisfying
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.

[0029] The method for producing a hot-worked steel material and a product excellent in free-cutting property according to claim 8 is the invention according to claim 7, wherein the graphitization index CE is calculated by the following formula (4).
Formula: CE = C + Si / 3 + Cu / 9 + Ni / 9-Mn / 12-Cr / 9-Mo / 9-B ------------------------- ------- (4) However, each element symbol: using the content (wt.%) Of each element, and as the steel slab or the steel material, from the group consisting of the following four chemical component compositions The present invention is characterized in that at least one selected one is additionally used. 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 the ninth aspect of the present invention is the invention according to the seventh or the eighth aspect, wherein 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 The steel material is characterized in that at least one selected from the group consisting of the following five chemical component compositions is further added and contained. Here, the above five chemical component compositions are
Al: 0.01-0.50 wt.%, Ti: 0.01-0.
50 wt.%, Zr: 0.01 to 0.50 wt.%, V: 0.
01 to 0.30 wt.% And Nb: 0.01 to 0.30
Refers to wt.%.

According to a tenth 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 according to the seventh or eighth aspect, wherein the graphitization 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 present invention is characterized in that a steel slab or the steel material further includes at least one selected from the group consisting of the following three chemical component compositions. Here, the above five chemical component compositions are
Ca: 0.0010 to 0.0100 wt.%, Mg:
0.0010 to 0.10 wt.% And REM: 0.00
10 to 0.10 wt.%.

The method for producing a hot-worked steel material and a product excellent in free-cutting property according to claim 11 is as follows: C: 0.80 to 1.70 w
t.%, Si: 0.70 to 2.50 wt.%, Cu: 0.01
-2.0 wt.%, Ni: 0.01-2.0 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, and Al: more than 0.10 to 0.50 wt.%, Ti:
0.01 to 0.50 wt.%, Zr: 0.01 to 0.50 w
t.%, V: 0.01 to 0.30 wt.%, and Nb:
0.01 to 0.30 wt.%. At least one selected from the group consisting of: Ca: 0.0010 to 0.0100 wt.
%, Mg: 0.0010 to 0.10 wt.%, And RE
M: at least one selected from the group consisting of 0.0010 to 0.10 wt.%, The balance being composed of iron (Fe) and unavoidable impurities, and the wt.% Of the Ni content and the Cu content. .% Ratio Ni / Cu is the following formula (1): Ni / Cu ≧ 0.2 --- (1) is satisfied, and the following formula (6) is satisfied: CE = C + Si / 3 + Cu / 9 + Ni / 9 + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 ------- (6) 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 billet having a chemical composition that satisfies (3) or Replace the steel material with the 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.

The method for producing a hot-worked steel material and a product excellent in free-cutting property according to claim 12 is as follows: C: 0.80 to 1.70 w
t.%, Si: 0.70 to 2.50 wt.%, Cu: 0.01
-2.0 wt.%, Ni: 0.01-2.0 wt.%, P: 0.
050 wt.% Or less, S: 0.050 wt.% Or less, O: 0.0
050 wt.% Or less, N: 0.015 wt.% Or less, Mn: 0.01 to 1.0 wt.%, Cr: 0.0
1 to 1.0 wt.%, Mo: 0.01 to 0.50 wt.%, And B: 0.0005 to 0.010 wt.%. And at least one selected from the group consisting of: Al: more than 0.10
0.50 wt.%, Ti: 0.01 to 0.50 wt.%, Zr:
0.01 to 0.50 wt.%, V: 0.01 to 0.30 w
t.% and Nb: 0.01 to 0.30 wt.%. At least one selected from the group consisting of: Ca: 0.0010
0.010 wt.%, Mg: 0.0010 to 0.10 w
%, and at least one selected from the group consisting of REM: 0.0010 to 0.10 wt.%, the balance being iron (Fe) and unavoidable impurities. The wt.% Ratio Ni / Cu with the Cu content is as follows (1)
Formula: Ni / Cu ≧ 0.2 ------------------------------------ (1) is satisfied And 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: calculated by the content (wt.%) Of each element The graphitization index CE is calculated by the following equation (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. The main feature of the metal structure is that of pearlite.

[0034]

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 securing 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. increases the deformation resistance of the material and places an excessive load on rolling mills and tools, so that 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 as a substitute for Mn, and promotes precipitation of graphite and improves 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 into 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 and improves the hardenability similarly to Cu. 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 0.01 to 2.0 watts.
between t.%. Here, when Cu and Ni are used together,
The reason why the weight wt.% Ratio of Ni / Cu is 0.2 or more is as follows.

As described above, when Cu is alloyed with Ni, the solidus temperature rises. The heating temperature of the steel
If the temperature is higher than the solidus temperature of the u-Ni alloy, hot ductility decreases, and surface flaws and cracks occur. Therefore, the upper limit of the heating temperature needs to be lower than the solidus temperature of the Cu-Ni alloy. is there. On the other hand, as described later, the heating temperature of the steel material of the present invention can reduce the optimum processing temperature to about 1165 ° C. determined from the solidus temperature of the steel material, and is lower than the processing temperature of the conventional steel for machine structural use. It can be reduced by about 200 ° C. Therefore, in order to prevent the formation of a Cu melt during heating of the steel material and to reduce the above-mentioned optimum working temperature, Cu (melting point: 1083 ° C.) is alloyed with Cu—Ni to form a solid. It is necessary to increase the phase line 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 the generation of surface defects. 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) Oxygen (O) O is an element that lowers the cleanliness of the steel material and inhibits the graphitization, and therefore should be kept as low as possible.
However, from this viewpoint, up to 0.0050 wt.% Is permissible, so the upper limit is made 0.0050 wt.%.

(8) 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 set to 0.015 wt.% Or less.

(9) 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.

(10) 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.%.

(11) 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.%.

(12) Boron (B) B is an element that enhances hardenability in a trace amount. N in steel
Is fixed as BN to reduce the graphitization inhibiting effect of N.
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 set to 0.0005 to 0.5.
It is desirable that the content be between 010 wt.%.

(13) Aluminum (Al) Al 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 0.01 wt.% Or more. However, if it is added in excess of 0.50 wt.%, The amount of oxide-based inclusions increases, which reduces the cleanliness of the steel and causes cracking during hot working. Therefore, it is desirable to contain Al in the range of 0.01 to 0.50 wt.%.

(14) 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 0.01 wt.%, The effect is small. On the other hand, if the amount of Ti exceeds 0.50 wt.%, Hard TiN and TiC are generated in large amounts to promote tool wear. Therefore, Ti is set to 0.01 to 0.50 w
%.

(15) Zirconium (Zr) Like Zr, Zr also precipitates nitrides and carbides, refines crystal grains, and promotes the precipitation of graphite. If the Zr content is less than 0.01 wt.%, The effect is small, while if it exceeds 0.50 wt.%, Tool wear is accelerated. Therefore, it is desirable that Zr be contained between 0.01 and 0.50 wt.%.

(16) 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 01 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,
V is desirably contained in the range of 0.01 to 0.30 wt.%.

(17) 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. If the amount of Nb is less than 0.01 wt.%, The effect is small, while if it 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 in a range of 0.01 to 0.30 wt.%.

(18) Calcium (Ca) Ca is used as an inoculant in cast iron and used to promote graphitization. The mechanism of promoting graphitization by Ca is thought to be that the vapor pressure of Ca is high, and the vapor of Ca forms small cavities in iron during casting, which serves as nuclei for graphite precipitation to precipitate spheroidal graphite. As with cast iron, it facilitates graphite precipitation after hot working in steel. Also Ca
Is a desirable element in free-cutting steel, because when it exists as an oxide-based inclusion, it forms a belag in carbide tool cutting and prolongs tool life and is expensive. For this purpose, Ca needs to be added in an amount of 0.0010 wt.% Or more, but the effect is saturated even if it exceeds 0.010 wt.%. Therefore, it is desirable that Ca be contained between 0.0010 and 0.010 wt.%.

(19) Magnesium (Mg) Mg, like Ca, is used as an inoculant in cast iron and promotes graphitization, and also facilitates the 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.%.

The 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 The 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. However, unless CE is 1.30 or more, graphite such as annealing is deposited. Unless heat treatment is performed, it becomes difficult to precipitate graphite under practical conditions. Therefore,
CE is 1.30 or more.

(22) Structure of Worked Products Hot rolled steel bars, hot forged crankshafts, and other products contain fine graphite in order 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 ℃.

On the other hand, when the heating temperature of the steel material is the 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, the solidus temperature (the temperature at which a 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 and Si represent carbon and silicon contents (wt.%), And are calculated as 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.

Here, 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 in the case of containing 2 wt.% Si. 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. 7 exemplifies a Cu—Ni binary equilibrium diagram, which shows the range of the ratio of the content ratio (wt.%) Of Cu and Ni in the steel material according to the present invention, as shown in the following (6) and (7). ) 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 of the steel of the present invention is approximately in the range of 1145 to 1451 ° C. From the above, the difference between the (T _S -50) ° C. of the steel material and the solidus temperature of the Cu—Ni alloy can be easily calculated, and even if the solidus temperature of the Cu—Ni alloy is lower, the difference can be calculated. It can be clearly seen that is reduced. When the solidus temperature of the Cu—Ni alloy is higher than (T _S -50) ° C. of the steel material, the Cu—Ni alloy does not melt during heating and holding. No intrusion.

Next, if the material temperature at the time of working 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.

[0080]

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.

[0082]

[Table 1]

[0083]

[Table 2]

[0084]

[Table 3]

[0085]

[Table 4]

Steel types Nos. 1 to 23 are within the scope of the present invention with respect to the chemical composition, and the numbers of the corresponding claims 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, Comparative Examples Nos. 21 to 23 are respectively provided.

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, steel type No. 46 is conventional spheroidal graphite cast iron, steel type N.
o.47 is S48C with V: 0.10 wt.%, Pb: 0.22
The conventional non-heat treated steel to which wt.% is added, steel type No. 48 is conventional S50C, and steel type No. 49 is conventional SCM822. And steel grade No.24-49
The production conditions of the test using the test include various conditions outside and within the scope of the present invention, but all of the tests correspond to comparative examples which are test examples outside the scope of the present invention. Therefore, these were designated as Comparative Examples 24 to 49, respectively.

Here, the graphitization index CE is important to promote the precipitation of graphite in the production process of steel products, 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 1180 ° C., and hot rolled into a bar having a diameter of 26 mm or 93 mm.

After the hot rolling, the steel bars were allowed to cool or the cover was gradually cooled to precipitate graphite. 800 of 26mm steel bar
The average cooling rate from 500C to 600C is about 1.5C / sec.
c, at 0.4 ° C./sec, 93
The cooling rate at the time of standing cooling of the mm steel bar was 0.25 ° C./sec, and that at the time of slow cooling of the cover was 0.08 ° C./sec.

The surface of the steel bar was visually judged for flaws, and the state of graphite and the metal structure were examined with an optical microscope. In addition, 26
The steel bar of mm was machined by cutting into a piston rod of the shock absorber, and the steel bar of 93 mm 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 using a carbide P20 cutting tool at a cutting speed of 200 m / min for 20 minutes.

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.
The spheroidal graphite cast iron of Comparative Example No. 46 was 88 mm
An ingot directly cast on a φ sand mold was used as a comparison material.

Embodiment Nos. 1 to 20 which are embodiments of the present invention
Is appropriate for both the chemical composition and the rolling heating temperature, and there is no crack in the rolled product. In addition, the size of the graphite particles is 0.5 to
25 μm, and the number of graphite particles is 100 / m
m ² or more is sufficient. For this reason, all the chips had a good shape divided into two or less pieces. Further, the metal structure was a structure of pearlite single phase or pearlite-based ferrite + pearlite.

FIG. 3 shows the microscopic structure of the etched microscopic surface of Example No. 1. It is a structure composed of graphite and pearlite. FIG. 4 shows the state of graphite deposition by a microscope on the speculum surface of Example No. 4 with no etching. Graphite exists mainly at grain boundaries. FIG. 5 shows the microscopic structure of the etched microscopic surface. The structure is grain boundary ferrite + pearlite.

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

Comparative Example No. 22 had a heating temperature of T _S -50 ° C.
Although lower than the melting point of the Cu-Ni alloy, 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 reduced the hot ductility.

In Comparative Example No. 23, the heating temperature was 800 ° C.
Because of this, 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 thus no graphite deposition was observed.

On the contrary, in Comparative Example No. 25, C was high outside the range of the present invention, the hot ductility was insufficient, and the steel bar cracked. In Comparative Example No. 26, Si was lower than that of the present invention, so that the graphitization index CE was small, no graphite was observed, and the chips were long. For this reason, it was necessary to stop the machine and remove the chips.

In Comparative Example No. 27, Si 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, Cu was higher than that of the present invention, the hot ductility was insufficient, and the steel bar cracked.

In Comparative Example No. 29, Ni was higher than that of the present invention,
Due to insufficient ductility, the steel bar cracked. Comparative Example No. 30 has a Ni / Cu ratio lower than the range of the present invention and a heating temperature of Cu.
-Cracking occurred because the temperature was higher than the solidus temperature of the Ni alloy.

In Comparative Example No. 31, P was higher than that of the present invention, and cracks occurred in the rolled steel bars. Comparative Example No. 32 has Mn and S
Was higher than that of the present invention, so that graphite deposition did not occur.

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

In Comparative Example No. 35, B and N were higher than those of the present invention, and a large amount of BN was precipitated to cause cracking due to insufficient ductility.
Comparative Example No. 36 was made of Ti, Comparative Example No. 37 was made of Zr, Comparative Example No. 38 was made of V, Comparative Example No. 39 was made of Al, Comparative Example No. 40 was made of Nb. Higher than the range, which caused the bar to crack due to insufficient ductility.

In Comparative Example No. 41, Ca was used, and in Comparative Example No. 41, Ca was used.
42 was higher in Mg and Comparative Example No. 43 was higher in REM than in the present invention. For this reason, a large amount of oxide-based inclusions was involved, which caused rolling flaws and cracked the steel bar.

In Comparative Examples No. 44 and Comparative Example No. 45, the chemical composition was within the range of the present invention, but no graphite was precipitated because the graphitization index CE was lower than the range of the present invention. Comparative Example No. 46 is an example of conventional spheroidal graphite cast iron,
Contains Mg as 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. Although the tensile strength was appropriate, the elongation was 4%, which was poor in ductility.

Comparative Example No. 47 is an example of a conventional non-refined steel. However, it has no particular problem in various properties and, since it contains Pb, has good chip controllability. However, from the viewpoint of environmental protection, it is required to manufacture components in a direction not using Pb.

Comparative Example No. 48 is an example of the conventional S50C and does not contain Pb, so that the chip controllability is inferior. In addition, the tensile strength is slightly insufficient at about 700 N / mm ² ,
It was necessary to increase the tensile strength by quenching and tempering.

Comparative Example No. 49 is a SCM822 for gears.
, Which is described in Comparative Example N of Test 3 described later.
This will be described in o.49D. As described above, according to the embodiment within the scope of the present invention, it is possible to produce a lead-free non-heat treated free-cutting steel bar having strength and ductility comparable to conventional non-heat treated steel bars.

[Test 2] Group A of steel types Nos. 3 and 17 in which the components shown in Table 1 are within the scope of the present invention, and
The following tests were performed on steels of Group B of steel types Nos. 47, 48 and 46 which are outside the scope of the present invention.
The tests in Group A are within the scope of the present invention, and are referred to as Examples Nos. 3A and 17A, respectively. The tests in Group B are out of the scope of the present invention, and are in Comparative Examples No. 4 and 4, respectively.
7B, 48B, 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, 88 mmφ steel bars 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, the 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 to the outer periphery, 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.

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.

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-refined free-cutting steel part excellent in machinability, and the machinability is lead free-cutting steel or spheroidal graphite cast iron. The characteristics are higher than those of conventional spheroidal graphite cast iron, and have high fatigue strength equivalent to a quenched and tempered material.

[Test 3] For the C groups of steel types Nos. 1 and 5 having the components shown in Table 1 within the scope of the present invention, and the D group of steel types Nos. 49 and 47 out of 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 47D, respectively.
Call it.

Example Nos. 1C and 5C and the conventional S
In Comparative Example 49D according to CM822, the billet was 100 mm
It was rolled into a steel bar, hot forged into a differential drive gear having an outer diameter of 220 mm, and allowed to cool as it was. In Comparative Example No. 47D, the 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, the tool had little wear, and the cutting surface was smooth, and the cutting condition was good. 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 material of the spheroidal graphite cast iron of Comparative Example No. 47D was 32.
It was as low as 0 N / mm ² .

Since the deformation of the gears after the heat treatment causes noise when the gears mesh with each other, the amount of deformation of the pressure angle on the drive side of each gear was measured. 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 at 3 minutes, but was large at 21 minutes after the number of times of fatigue of 1,000 times. 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.

With respect to the gear materials of Examples Nos. 1C and 5C, the average particle size of graphite and the number of graphite particles were measured. 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.

[0123]

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 form classification of chips in cutting.

FIG. 3 is a view showing a microscopic structure of an etched speculum surface of Example No. 1;

FIG. 4 is a view showing a state of graphite deposition by a microscope on a microscopic surface of Example No. 4 with no etching.

FIG. 5 shows the microstructure of the etched speculum surface of Example No. 4.

FIG. 6 is a schematic longitudinal sectional view illustrating distortion of a gear.

FIG. 7 is a diagram showing a composition range of a Cu—Ni alloy generated in a surface layer portion in a heating step of a steel material of the present invention and a solidus temperature range of the Cu—Ni alloy.

[Explanation of symbols]

 1 angular displacement of pressure angle 2 gear

Claims (12)

[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.% %, P: 0.050 wt.% Or less, S: 0.050 wt.% Or less, O: 0.0050 wt.% Or less, and N: 0.015 wt.% Or less, with the balance of iron (Fe) and It consists of unavoidable impurities, and the Ni / C ratio of Ni / Cu content is Ni / C.
u is the following formula (1): Ni / Cu ≧ 0.2 ---------------------------------- -(1) is satisfied, and the following formula (2) is satisfied: CE = C + Si / 3 + Cu / 9 + Ni / 9 (2) where each element symbol is: The graphitization index CE calculated from the content (wt.%) Of each element is expressed by the following equation (3): CE ≧ 1.30 --------------------- (3) A steel material having a chemical composition that satisfies (3) and which has been hot-worked and then cooled to room temperature. A product made of the steel material, wherein graphite having an average particle size of 0.5 μm or more is 1
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 5
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. Al: 0.01 to 0.50 wt.%, Ti: 0.01 to 0.50 wt.%, Zr: 0.01 to 0.50 wt.%, V: 0.01 to 0.30 wt.%, And Nb: 0.01 to 0.30 wt.%. 4. The invention according to claim 1, 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: The content (wt.%) Of each element is used, and the steel material and the product are used. The chemical composition of the following 3
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. Ca: 0.0010 to 0.0100 wt.%, 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.% .%, P: 0.050 wt.% Or less, S: 0.050 wt.% Or less, O: 0.0050 wt.% Or less, and N: 0.015 wt.% Or less. Over 10 to 0.50 wt.%, Ti: 0.01 to 0.50 wt.%, Zr: 0.01 to 0.50 wt.%, V: 0.01 to 0.30 wt.%, And Nb: 0 0.01-1.30 wt.%. At least one selected from the group consisting of: Ca: 0.0010 to 0.0100 wt.%, Mg: 0.0010 to 0.10 wt.%, And REM: 0.0010 to 0.10 wt.% At least one member selected from the group consisting of iron (Fe) and unavoidable impurities, and N
The wt.% ratio Ni / Cu between the i content and the Cu content is expressed by the following formula (1): Ni / Cu ≧ 0.2 ----------------- (1) is satisfied, and the following formula (6) is satisfied: CE = C + Si / 3 + Cu / 9 + Ni / 9 + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 (6) However, each element symbol: The graphitization index CE calculated by the content (wt.%) Of each element is as follows: Formula: CE ≧ 1.30 ---------------------------------------- (3) A steel material and a product made of the steel material having a chemical composition that satisfies the following and cooled to room temperature after hot working, wherein graphite having an average particle diameter of 0.5 μm or more is 1
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. 6. 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.% %, P: 0.050 wt.% Or less, S: 0.050 wt.% Or less, O: 0.0050 wt.% Or less, and N: 0.015 wt.% Or less. 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.%. At least one selected from the group consisting of: Al: more than 0.10 to 0.50 wt.%, Ti: 0.01 to 0.50 wt.%, Zr: 0.01 to 0.50 wt.%, V : 0.01 to 0.30 wt.%, And Nb: 0.01 to 0.30 wt.%. At least one selected from the group consisting of: Ca: 0.0010 to 0.0100 wt.%, Mg: 0.0010 to 0.10 wt.%, And REM: 0.0010 to 0.10 wt.% At least one member selected from the group consisting of iron (Fe) and unavoidable impurities, and N
The wt.% ratio Ni / Cu between the i content and the Cu content is expressed by the following formula (1): Ni / Cu ≧ 0.2 ----------------- (1) is satisfied, and the following formula (5) is satisfied: 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 graphitization index CE calculated from the content (wt.%) Of each element is expressed by the following equation (3): CE ≧ 1.30 ------------------------------ Having a chemical composition that satisfies (3) and after hot working A steel material cooled to room temperature and a product made of the steel material, wherein graphite having an average particle size of 0.5 μm or more is 1
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. 7. 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.% %, P: 0.050 wt.% Or less, S: 0.050 wt.% Or less, O: 0.0050 wt.% Or less, and N: 0.015 wt.% Or less, with the balance of iron (Fe) and It consists of unavoidable impurities, and the Ni / C ratio of Ni / Cu content is Ni / C.
u is the following formula (1): Ni / Cu ≧ 0.2 ---------------------------------- -(1) is satisfied, and the following formula (2) is satisfied: CE = C + Si / 3 + Cu / 9 + Ni / 9 (2) where each element symbol is: The graphitization index CE calculated from the content (wt.%) Of each element is expressed by the following equation (3): CE ≧ 1.30 --------------------- (3) A slab or a steel material having a chemical composition that satisfies
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. 8. The invention according to claim 7, 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 as the billet or the steel material, the following 4
At least one selected from the group consisting of species of chemical components
A method for producing a hot-worked steel material and a product excellent in free-cutting property, characterized by using a material further containing a seed. 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.%. 9. The invention according to claim 7, 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 As a steel material, a hot-worked steel material excellent in free-cutting property, characterized by using at least one selected from the group consisting of the following five chemical component compositions additionally added thereto: Product manufacturing method. Al: 0.01 to 0.50 wt.%, Ti: 0.01 to 0.50 wt.%, Zr: 0.01 to 0.50 wt.%, V: 0.01 to 0.30 wt.%, And Nb: 0.01 to 0.30 wt.%. 10. The invention according to claim 7, 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 three types of chemical components additionally added thereto: Product manufacturing method. Ca: 0.0010 to 0.0100 wt.%, Mg: 0.0010 to 0.10 wt.%, And REM: 0.0010 to 0.10 wt.%. 11. 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.% .%, P: 0.050 wt.% Or less, S: 0.050 wt.% Or less, O: 0.0050 wt.% Or less, and N: 0.015 wt.% Or less. Over 10 to 0.50 wt.%, Ti: 0.01 to 0.50 wt.%, Zr: 0.01 to 0.50 wt.%, V: 0.01 to 0.30 wt.%, And Nb: 0 0.01-1.30 wt.%. At least one selected from the group consisting of: Ca: 0.0010 to 0.0100 wt.%, Mg: 0.0010 to 0.10 wt.%, And REM: 0.0010 to 0.10 wt.% At least one member selected from the group consisting of iron (Fe) and unavoidable impurities, and N
The wt.% ratio Ni / Cu between the i content and the Cu content is expressed by the following formula (1): Ni / Cu ≧ 0.2 ----------------- (1) is satisfied, and the following formula (6) is satisfied: CE = C + Si / 3 + Cu / 9 + Ni / 9 + Al / 6 + Ti / 3 + Zr / 3-V / 3-Nb / 3 (6) However, each element symbol: The graphitization index CE calculated by the content (wt.%) Of each element is as follows: Formula: CE ≧ 1.30 ---------------------------------------- (3) A slab or a steel material having a chemical composition that satisfies
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. 12. 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.% %, P: 0.050 wt.% Or less, S: 0.050 wt.% Or less, O: 0.0050 wt.% Or less, and N: 0.015 wt.% Or less. 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.%. At least one selected from the group consisting of: Al: more than 0.10 to 0.50 wt.%, Ti: 0.01 to 0.50 wt.%, Zr: 0.01 to 0.50 wt.%, V : 0.01 to 0.30 wt.%, And Nb: 0.01 to 0.30 wt.%. At least one selected from the group consisting of: Ca: 0.0010 to 0.0100 wt.%, Mg: 0.0010 to 0.10 wt.%, And REM: 0.0010 to 0.10 wt.% At least one member selected from the group consisting of iron (Fe) and unavoidable impurities, and N
The wt.% ratio Ni / Cu between the i content and the Cu content is expressed by the following formula (1): Ni / Cu ≧ 0.2 ----------------- (1) is satisfied, and the following formula (5) is satisfied: 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 graphitization index CE calculated from the content (wt.%) Of each element is expressed by the following equation (3): CE ≧ 1.30 ------------------------------ (3) A slab or a steel material having a chemical composition satisfying
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.