JPH09310152A - Non-heat treated steel for hot forging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a non-heat treated steel for hot forging having high proof stress and fatigue characteristics equal to those of a V-added steel and furthermore good in machinability without adding expensive precipitation hardening type elements by allowing specified amounts of C, Si, Mn, Al, N, P, S, Cr, Cu, Ni and Fe to be contaied in a steel and regulating its tensile strength to specified value. SOLUTION: This steel is the one having a compsn. contg., by mass, 0.3 to 0.8% C, 0.1 to 2.5% Si, 0.3 to 2% Mn, 0.001 to 0.06% Al, 0.005 to 0.1% N, <=0.3% P, <=0.12% S, <=1% Cr, <=0.3% Cu, <=0.3% Ni, and the balance substantial Fe. The tensile strength of the steel is regulated to 600 to 900N/mm<2> . Furthermore, the compsn. satisfies the inequalities. The [elements] in the inequalities denote the content and mass % of each element. Moreover, the steel is preferably incorporated with one or more kinds among <=0.3% (including no zero %) Pb, <=0.2% (including no zero %) Zr, <=0.01% (including no zero %) Ca, <=0.1% (including no zero %) Te and <=0.1% (including no zero %) Bi.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-heat treated steel for machine structures used for engine parts and undercarriage parts of automobiles and construction machines, and particularly to quenching and tempering carried out after hot forging. The present invention relates to an inexpensive non-tempered steel for hot forging that can obtain high strength even without refining by refining.

[0002]

2. Description of the Related Art Machine structural parts used in automobiles and construction machinery are usually made of carbon steel for machine structural use or alloy steel for machine structural use, and are hardened after hot forging in order to secure necessary strength and toughness. It has been manufactured by carrying out a tempering process. However, in recent years, for the purpose of saving the energy required for the above-described heat treatment and reducing the cost of work-in-process, for example, carbon steel for machine structure specified in JIS G 4051 or machine structure specified in JIS G 4106. Non-heat treated steels made by adding precipitation hardening elements such as V and Nb to manganese steel have been developed and applied to automobile engine parts, undercarriage parts, construction machine parts and the like.

[0003] These non-heat treated steels have a target strength by cooling after hot forging to form a ferrite-pearlite mixed structure and precipitating carbides or nitrides such as V and Nb in the ferrite part. If such a non-heat treated steel is used, quenching and tempering treatment after hot forging can be omitted, and further, the heat treatment strain generated during quenching can be reduced and the subsequent straightening process can be simplified. In addition to the advantage of being hardened, quench cracking is less likely to occur, the rate of defective products due to quench cracking is reduced, and it is possible to significantly reduce the component manufacturing cost. For these reasons, recently, a non-heat treated steel has been developed which can be applied to mechanical structural parts that require high strength such as tensile strength of 90 kgf / mm ² or more (for example, JP-A-63-19984).
No. 8, etc.).

By the way, in each of the above-mentioned non-heat treated steels, charcoal / nitride forming elements such as V are added, and the precipitation and hardening of these charcoal / nitride enhances the strength characteristics such as tensile strength and proof stress. Therefore, in order to obtain a particularly high-strength non-heat treated steel, a large amount of elements such as V must be added. However, since these elements are expensive, the raw steel material cost increases, and the advantage of cost reduction due to non-refining cannot be effectively utilized.

Further, as a steel material having a tensile strength of 90 kgf / mm ² level or more, a high strength / high toughness type non-heat treated steel having a Charpy impact value of ² mm U notch of 6 kgf · m / cm ² or more has been proposed. (For example, JP-A-6
No. 1-238941), this high-strength, high-toughness non-heat treated steel has the C content reduced as much as possible in order to obtain high toughness. Therefore, in order to obtain high strength in the state where it is left to cool after hot forging, it is necessary to add a large amount of strengthening elements such as Mn and Cr corresponding to the reduction of the amount of C, which also increases the cost of raw steel materials. Is inevitable. In addition, a method of performing rapid cooling such as water quenching after hot forging has been attempted in order to reduce the amount of these strengthening elements, but this method causes a big problem of causing heat treatment distortion and quench cracking due to quenching. . Further, since the non-heat treated steel as described above is mainly composed of a ferrite / pearlite structure transformed from a coarse austenite structure, a problem of insufficient toughness is pointed out.

[0006]

The present invention has been made in view of the above circumstances, and its purpose is to add a precipitation hardening element such as V due to the formation of expensive carbon / nitride. It is an object of the present invention to provide a non-heat treated steel for hot forging, which has a high yield strength and fatigue characteristics similar to those of V-added steel without deterioration, and has good machinability.

[0007]

The non-heat treated steel for hot forging according to the present invention, which was able to solve the above-mentioned problems, is C: 0.30 to 0.80% Si: 0.1 to 2.5. % Mn: 0.30 to 2.0% Al: 0.001 to 0.06% N: 0.005 to 0.10% P: 0.30% or less (including 0%) S: 0.12% Satisfies the following requirements (including 0%): Cr: 1.0% or less (including 0%) Cu: 0.3% or less (including 0%) Ni: 0.3% or less (including 0%) In addition, the balance is Fe and unavoidable impurities, satisfies the relationship of the following formulas (1) and (2), and has a tensile strength of 600 to 900 N / mm ² . A = [Si] +3.4 ・ [Mn] +19.5 ・ [P] -13.4 ・ [S] +2.7 ・ [Cr] ≧ 3.5 …… (1) B = [C] +1.1 ・ [Mn] -1.9 ・[S] +1.5 ・ [Cu] +1.8 ・ [Ni] +0.6 ・ [Cr] ≦ 2.6 (2) (In the formula, [element] represents the content rate of each element:% by mass.) In non-heat treated steel, Pb: 0.3% or less (not including 0%) Zr: 0.2% or less (not including 0%) Ca: 0.010% or less (0%) Te: 0.10% or less (0% is not included) Bi: 0.1% or less (0% is not included) One or more types can be added to further improve machinability. In addition, Ti: 0.05% or less (0
%), And a precipitate having an average particle size of 10 nm or more, which is composed of a carbide, nitride, sulfide of Ti, or a composite compound thereof of Ti in any cross section, is 1 μm.
^If 3 or more are present per ^{2, the} non-heat treated steel has good toughness in addition to the above properties.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors can omit quenching and tempering treatments, and have strength characteristics and fatigue characteristics comparable to those of conventional non-heat treated steels containing expensive precipitation hardening elements such as V and Nb. In order to develop an inexpensive non-heat treated steel for hot forging, which has a structure after hot forging and cooling is ferrite / pearlite, it affects strength properties such as proof stress and fatigue properties, as well as machinability. We have conducted intensive research into the effects of various alloying elements.

As a result, if the content of each element contained in the steel material is properly adjusted so as to satisfy the requirement of the above formula (1), the yield strength, which is an important characteristic as the component design strength, can be effectively increased. It is possible to suppress an excessive increase in tensile strength (hardness) that causes a decrease in machinability, that is, to increase the yield ratio, which is the ratio of proof stress to tensile strength, and to improve machinability. I found him.

That is, the fatigue strength of ferritic-pearlite steel depends on the strength of the proeutectoid ferrite portion, which is structurally weak, and the proof stress, which is the deformation resistance when a small strain is applied, also shows the slip resistance Since the fatigue strength of non-heat treated steel has a higher correlation with the proof stress than the tensile strength (hardness), it is possible to improve the fatigue strength by increasing the proof stress. Then, if the components are adjusted so as to satisfy the requirement of the above formula (1), the deterioration of machinability can be suppressed as much as possible, the yield strength can be increased, and a non-heat treated steel having a high level of fatigue characteristics can be obtained. Can be done.

By the way, in the case of the steel material having the value of the above-mentioned formula (1) less than 3.5, a satisfactory yield ratio cannot be obtained as will be apparent from the examples described later, and the purpose of improving the fatigue characteristics cannot be fulfilled. A more preferable value defined by the formula (1) is 3.8 or more.

The requirement defined by the above formula (2) is defined as a requirement for preventing the formation of bainite, which is a supercooled structure generated during cooling after hot forging, and suppressing a decrease in yield strength and fatigue strength. If this value exceeds 2.6, not only the proof stress and fatigue strength are lowered, but also the machinability is lowered, as will be apparent from the examples described later.

Hereinafter, the reason for defining the composition of the steel material in the present invention will be mainly described in detail. First of all, the reasons for defining the chemical composition of steel materials will be clarified.

C: 0.30 to 0.80% C is an element indispensable for increasing the amount of pearlite in the metal structure of the forged product after hot forging and cooling to secure the necessary strength. For that purpose, at least 0.30% or more must be contained. However, if the amount of C is too large, the toughness is reduced and the machinability is also significantly reduced, so it must be suppressed to 0.80% or less. Considering strength, toughness and machinability, a more preferable C content is in the range of 0.35 to 0.60%.

Si: 0.1 to 2.5% Si effectively acts on deoxidation when steel is melted, and is solid-solved in a ferrite material of steel to strengthen a forged product after hot forging and cooling. It is an effective element for improving the yield strength and fatigue strength of the forged product. At least 0.1% or more must be contained in order to effectively exhibit such an action, but if too much, machinability will be adversely affected, so 2.5% is made the upper limit. The more preferable range of Si is 0.15 to 1.5%.

Mn: 0.3-2.0% Mn is an element effective as a deoxidizing / desulfurizing element when steel is melted, and also enhances the pearlite hardenability of the forged product to increase the amount of pearlite and pearlite. The lamellar spacing in the inside is made fine to contribute to the strength increase. In order to exert such effects effectively, at least 0.3% or more must be contained, but if it is too large, bainite is generated in the metal structure after hot forging and cooling, which adversely affects machinability. Therefore, it must be kept to 2.0% or less. The more preferable Mn content is in the range of 0.5 to 1.6%.

Al: 0.001 to 0.06% Al effectively acts as a deoxidizing element during melting of steel materials, and also contributes to the improvement of toughness by refining austenite crystal grains due to the formation of nitrides. Therefore, in order to effectively exhibit these effects, the content must be 0.001% or more. However, if it is too large, the austenite crystal grains are rather coarsened and adversely affect the toughness. Therefore, it must be suppressed to 0.06% or less. Considering these advantages and disadvantages, the more preferable range of Al is 0.00
It is 3 to 0.05%.

N: 0.005-0.10% N has a function of strengthening a forged product after hot forging and cooling by forming a solid solution in a ferrite material of a steel material, whereby hardness and tensile strength are increased. In addition to increasing the strength, it also works effectively in combination with a nitride forming element such as Al or Ti to refine the austenite crystal grains and increase the toughness and fatigue strength. This effect is effectively exhibited at 0.005% or more, but if it is too large, it adversely affects the toughness, and also hinders the hot workability to easily cause cracking during casting or hot working. Machinability deteriorates due to hardening of steel, so 0.10
% Must be kept below. The N content is more preferably 0.007 to 0.07% in consideration of the above advantages and disadvantages.

P: 0.30% or less (including 0%) P is effective in strengthening the forged product after hot forging / cooling by forming a solid solution in the ferrite material of the steel material and increasing the proof stress and fatigue strength. However, if the amount is too large, not only the toughness is significantly deteriorated, but also the hot workability is adversely affected and cracks are likely to occur during casting or hot working.
Must be kept below.

Components other than the above components of the steel material used in the present invention are Fe and unavoidable impurities, but by further containing appropriate amounts of the following elements as other elements, the characteristics as non-heat treated steel are further improved. It can be modified.

S: 0.12% or Less (Including 0%) S has an action of forming MnS to enhance machinability, and also produces intragranular ferrite having MnS as a nucleus to refine the structure. It also contributes to the improvement of toughness. In order to positively utilize such effects, it is desirable to contain about 0.035% or more, but if it is too much, the deformability during hot work will be significantly reduced, so keep it below 0.12%. There must be.

Cr: 1.0% or Less (Including 0%) Cr is an element that increases pearlite hardenability and contributes to strength improvement, similar to Mn. It is desirable to contain about 0.2% or more. However, if it is too large, it becomes too hard and bainite is easily generated, which adversely affects the machinability, so it must be suppressed to 1.0% or less.

Cu: 0.3% or Less (Including 0%) Cu acts as a solid solution in the ferrite material of steel to strengthen the forged product after hot forging and cooling, but is too much. In addition to degrading the toughness, it also adversely affects the hot workability and easily causes cracks during casting or hot working.
It must be kept below 0.3%.

Ni: 0.3% or Less (Including 0%) Ni has the effect of enhancing the bainite hardenability, but if it is too much, it not only makes the strength characteristics unstable, but also improves machinability. Also has an adverse effect, so it must be suppressed to 0.3% or less.

Pb: 0.3% or less, Zr: 0.2% or less, Ca: 0.010%, Te: 0.10% or less, B
i: one or more selected from 0.1% or less These elements are elements that act on the improvement of machinability like S, and Zr, Ca, Te and Bi are MnS granulated and forged. It also acts to improve the anisotropy of the product.
However, if the content of each of the above elements is too large, the toughness and corrosion resistance will be adversely affected, so the content must be suppressed to the upper limit or less.

Ti: 0.003 to 0.05% Ti forms precipitates composed of carbides, nitrides, sulfides, or composite compounds thereof to refine austenite crystal grains, and toughness, proof stress, fatigue characteristics ( It is an effective element that contributes to the improvement of the fatigue limit ratio, and its effect is 0.003
%, It is effectively exhibited. However, if the Ti content is too large, the machinability deteriorates due to an excessive increase in the number of hard precipitates.
It must be kept below 5%. Considering these advantages and disadvantages, a more preferable content ratio of Ti is 0.005 to 0.
It is in the range of 03%.

When Ti is contained as described above, Ti
Carbides, nitrides, sulfides or their compound compounds
Fine precipitates are formed and the austenite crystal grains are refined.
Various physical properties are enhanced by this, but such a modification effect
Varies depending on the size and amount of the deposits.
Etc. confirmed that it was caused by the precipitate
The above-mentioned action and effect are obtained when the average particle size is 10 nm or more.
Gem is 1 μm^Two Effectively issued when there are 3 or more per
Be vaporized. Extremely fine with an average particle size of less than 10 nm
The effect is effectively exhibited even with precipitates, but it is industrially evaluated.
It is difficult to evaluate the average particle size of 10 nm or more
The number of samples was evaluated. However, if the average particle size of the precipitate is too large
If it gets too big, the machinability will be adversely affected.
Therefore, the average particle size of the precipitates should be kept below about 10 μm.
Is desirable. Regarding the number of the precipitates, hard inclusions
Since the machinability will be adversely affected if the amount of
1 μm ^Two It is desirable to keep the number below 500 or less
Yes.

The size and number of the precipitates depend on rolled products.
Measured at the desired cross section, for example, cutting out a sample
And a magnification of 1 with a transmission electron microscope using the extraction replica method.
Photographed at 100,000 times and 10 fields of view,
Calculate the size and number, 1 μm ^Two Convert to the number of pieces
It can be confirmed by the method.

A = [Si] + 3.4 · [Mn] + 19.5 · [P] -13.4 · [S] + 2.7 · [Cr] ≧ 3.5 (1) This requirement does not deteriorate machinability. As described above, this is an essential requirement for securing a yield ratio comparable to that of the conventional non-heat treated steel to which a precipitation hardening element such as V is added, and obtaining excellent fatigue properties. These values are 3 If it is less than 0.5, the yield ratio is significantly reduced, and the yield strength and fatigue limit for the same tensile strength are poor and the object of the present invention cannot be achieved.

B = [C] +1.1 ・ [Mn] -1.9 ・ [S] +1.5 ・ [Cu] +1.8 ・ [Ni] +0.6 ・ [Cr] ≦ 2.6 (2) This requirement is first As described above, it is an important requirement to suppress the formation of bainite structure that adversely affects the yield strength and fatigue strength by making the structure ferrite / pearlite in the cooling process after hot forging, and if this value exceeds 2.6. However, bainite is generated in the internal structure of the forged product, and not only the yield strength and fatigue strength deteriorate, but also the machinability deteriorates, and the object of the present invention cannot be achieved.

Tensile strength: 600 to 900 N / mm ^{2 The} mechanical structural steel requires a tensile strength of 600 N / mm ² or more from the viewpoint of strength design such as resistance to plastic deformation and fatigue strength, but 900 N / mm ^{If the} tensile strength exceeds ² , the machinability will deteriorate significantly.

[0032]

EXAMPLES Next, the structure and operation and effect of the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples, and can be adapted to the spirit of the preceding and following examples. Of course, the present invention can be implemented with modifications within the scope, and all of them are included in the technical scope of the present invention.

Example 1 Steel materials having chemical compositions shown in Tables 1 and 2 were melted in a vacuum furnace or a converter, and then forged into a round bar having a diameter of 35 mm by hot forging or hot rolling, and then a predetermined length. Disconnect.
Next, the steel types 1 to 32 were heated to 1250 ° C., forged into a round bar having a diameter of 25 mm, and then air-cooled. The steel type 32 is a conventional V-added non-heat treated steel. Steel type 33 is steel equivalent to JIS S45C,
About this, after forging into a round bar with a diameter of 25 mm,
After heating at 0 ° C for 1 hour and oil quenching, 50
0 ° C x 2 hours / water-cooled tempering treatment (conditioning treatment)
This is an example of tempered steel.

The HB hardness of the cross section of each round bar thus obtained was measured, and the diameter of the parallel part from the round bar was 10 mm.
A tensile test piece was prepared and the diameter of the parallel part was 8m.
An Ono-type rotary bending test piece of m was prepared and subjected to a tensile test and a fatigue test. The results are shown in Tables 3 and 4.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

[Table 4]

As is clear from Tables 1 to 4, steel type 1
No. 14 to No. 14 are Examples satisfying all the requirements of the present invention,
Both have tensile strengths of 600 to 900 N / mm ² , exhibiting high values for both the yield ratio and the fatigue limit ratio, and are similar to V-added non-heat treated steel of grade 32 and tempered steel of grade 33. It has equivalent strength and fatigue properties.

On the other hand, the steel types 15 and 16 have a low C content, and the steel type 19 has a low Mn content, so that the strength is low. Further, since the steel type 19 has an A value of less than 3.5, the yield ratio is Both fatigue limit ratios are low. On the other hand, steel type 17 has a C content
Steel type 18 has an amount of Si, steel type 20 has an amount of Mn, steel type 22 has an amount of N, steel type 23 has an amount of Cr, steel type 24 has an amount of Cu, and steel type 25 has an amount of Ni. The hardness and tensile strength become excessively high, and the machinability deteriorates significantly. Steel type 21 had a P content exceeding the specified range and cracked during forging, which was not suitable for practical use.

The steel types 26 to 28 satisfy the prescribed requirements for the content of each element, but the yield ratio and the fatigue limit ratio are low because the value of A is less than 3.5, and the steel types 29 to 31.
In addition, since the value of B exceeds 2.6, not only is it clear that a bainite structure is generated inside and it becomes excessively hard, which adversely affects the machinability, but also the yield ratio and fatigue limit ratio. It's getting low.

Example 2 Steel types 1, 4, 10, 1 among those shown in Tables 1 and 2 above
Select 8, 30, 32, melt in a vacuum furnace or converter, then hot forge or hot roll the diameter 80
mm forged into a round bar, cut to length and then 1
It heated at 100 degreeC, hold | maintained for 20 minutes, and cooled with air.

The HB hardness of the cross section of each round bar thus obtained was measured, and a tool life test during drilling was performed. Cutting conditions are SKH9 as tool, diameter 10mm
Using a straight drill, feed 0.5 mm / rev
The tool life was defined as the time when melting or breakage occurred (dry type). Table 5 shows the hardness measurement results and the evaluation results of the drill life [L20] at the cutting speed of 20 m / min.

[0044]

[Table 5]

As is clear from Table 5, steel types 1,
The steels of the present invention Nos. 4 and 10 each have a long drill life [L20] and have machinability equivalent to that of the V-type non-heat treated steel of the steel type 32. On the other hand, the steel types 18 and 30 of the comparative examples have extremely high hardness and thus have a very poor drill life [L20].

Example 3 Steel types 34 and 36 shown in Table 6 were placed in a 150 kg experimental furnace.
Steel type 35 is a 3 ton electric furnace, steel type 33 is melted in a production furnace, and after casting, steel types 34 and 36 are hot forged.
Further, the steel types 33 and 35 were hot-rolled into a round bar having a diameter of 35 mm or a diameter of 80 mm, and cut into a predetermined length. The steel type 33 is steel corresponding to JIS S45C.

Among the obtained round rods having a diameter of 35 mm, steel types 34 to 36 were heated to 1250 ° C., forged into round rods having a diameter of 25 mm, and then air-cooled. Also steel type 33
Was forged into a round bar with a diameter of 25 mm, heat-treated at 870 ° C for 1 hour, and then oil-quenched.
A tempering treatment was performed under the conditions of 00 ° C. × 2 hours / water cooling.
For each round bar having a diameter of 25 mm, the HB hardness of the cross section was measured, and a tensile test piece having a parallel part with a diameter of 10 mm was used.
Ono type rotary bending fatigue test pieces and JIS No. 3 impact test pieces each having a diameter of a parallel portion of 8 mm were prepared, and a tensile test, a rotary bending fatigue test and a Charpy impact test were performed.

The diameter of the Ti-carbide, nitride, sulfide or composite compound thereof is 3
A sample is cut out from the position of 1 / 4.D (D is the diameter of the round bar) of the cross section of the 5 mm round bar, and a photograph of 10 fields of view is taken with a transmission electron microscope at a magnification of 100,000 by the extraction replica method.
The number of precipitates whose major axis is 10 nm or more is measured, and 1
It was calculated by converting to the number within μm ² .

Regarding the round bar having a diameter of 80 mm, the steel types 34 to 36 were heated to 1100 ° C., held for 20 minutes, and then air-cooled. Steel type 33 was heated at 870 ° C. for 1 hour, oil-quenched, and then tempered at 470 ° C. for 2 hours / water cooling. The HB hardness of the cross section of a round bar having a diameter of 80 mm was measured, and a tool life test during drilling was performed in the same manner as in Example 1.

Table 7 shows hardness measurement results, tensile test results, fatigue test results, impact test results, number density measurement results of precipitates composed of Ti carbide / nitride / sulfide and their compound compounds, and drill life test results. Shown in.

[0051]

[Table 6]

[0052]

[Table 7]

As is clear from Tables 6 and 7, steel type 3
What was air-cooled after hot forging using the steel of the present invention No. 4 of
The tensile strength satisfies 600 to 900 N / mm ² , and the yield ratio, the fatigue limit ratio, and the impact value are high.
It has the same strength characteristics as the heat-treated steel.

On the other hand, since the steel type 35 has a Ti-based precipitate number density of less than 3 pieces / μm ² , the yield ratio and the fatigue limit ratio of the steel type 32 are the V addition type non-use of the steel type 32 shown in Table 2 above. Although it shows the same value as the tempered steel, it can be seen that it lacks impact properties. Since the ingot was cast using a mold of about 3 tons, the Ti-based precipitates were excessively coarsened and the number density was decreased due to the decrease in the cooling rate during casting, and the austenite crystal grains were formed by the precipitates. It is considered that the miniaturization effect was not effectively exhibited. The steel grade 36 is
Since the amount of Ti exceeds the prescribed requirement of the present invention, T
The coarsening of i-based precipitates has progressed, and the drill life [L20] has decreased due to the decrease in machinability.

[0055]

The present invention is configured as described above, and the composition of the steel material is specified and the above formulas (1) and (2) are used.
By specifying the values of A and B specified in, the high yield strength equivalent to V-added steel without adding refining hardening elements such as V due to the formation of expensive charcoal and nitrides Therefore, it has become possible to provide a non-heat treated steel for hot forging which has fatigue characteristics and good machinability.

Claims (3)

[Claims] 1. C: 0.30 to 0.80% (hereinafter referred to as mass% unless otherwise specified) Si: 0.1 to 2.5% Mn: 0.30 to 2.0% Al: 0 0.001 to 0.06% N: 0.005 to 0.10% P: 0.30% or less (including 0%) S: 0.12% or less (including 0%) Cr: 1.0% or less (Including 0%) Cu: 0.3% or less (inclusive of 0%) Ni: 0.3% or less (inclusive of 0%), the balance consists of Fe and inevitable impurities, and A non-heat treated steel for hot forging, which satisfies the relations of the expressions (1) and (2) and has a tensile strength of 600 to 900 N / mm ² . A = [Si] +3.4 ・ [Mn] +19.5 ・ [P] -13.4 ・ [S] +2.7 ・ [Cr] ≧ 3.5 …… (1) B = [C] +1.1 ・ [Mn] -1.9 ・[S] +1.5 ・ [Cu] +1.8 ・ [Ni] +0.6 ・ [Cr] ≦ 2.6 (2) (In the formula, [element] represents the content of each element:% by mass) 2. As other elements, Pb: 0.3% or less (0% is not included) Zr: 0.2% or less (0% is not included) Ca: 0.010% or less (0% is included) The non-temperature according to claim 1, which contains one or more of Te: 0.10% or less (not including 0%) Bi: 0.1% or less (not including 0%). steel. 3. Ti: 0.05% or less (not including 0%) as another element, and Ti carbide, nitride, sulfide or a composite compound thereof in any cross section. 1 precipitates with an average particle size of 10 nm or more
The non-heat treated steel according to claim 1 or 2, which is present in an amount of 3 or more per μm ² .