JP3536770B2 - Non-heat treated steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a non-heat treated steel material. More specifically, the fatigue resistance at low cycles without hot-tempering tempering treatment after hot working, such as 1%
05 Fatigue limit ratio at ⁵ times of stress repetition, that is, “1”
05 Fatigue strength at ⁵ times of stress repetition "/" Tensile strength "
The present invention relates to a ferrite-pearlite type non-heat treated steel excellent in heat resistance.

[0002]

2. Description of the Related Art Parts for machine structures, especially suspension parts for automobiles such as wheel hubs, knuckles and arms,
After being formed by hot forging using carbon steel (S45C etc.) or alloy steel (SCM440 etc.) for the conventional machine structure,
Machine processing and tempering treatment were performed to secure the desired shape and performance.

However, the refining process consumes a lot of energy and cost. Furthermore, when medium and high carbon steels are quenched, quenching cracks often occur, and even if quenching does not occur, large transformation strain occurs. For this reason, parts correction and machining steps after heat treatment are required. Therefore, in recent years, non-heat-treated steels that can be used as they are in hot forging have been actively developed in order to meet the social demands for energy saving, while reducing costs and simplifying the manufacturing process.

As non-heat treated steels, bainite, martensite and ferrite / pearlite non-heat treated steels are well known. Of these, bainite-type and martensite-type non-heat treated steels provide high strength but low machinability, and furthermore, large transformation strains are required. I can't. Therefore, in terms of cost reduction, ferrite
There have been many proposals regarding pearlite type non-heat treated steel.

On the other hand, the wheel hub, knuckle,
For example, in recent years, anti-fatigue characteristics in a low cycle region, such as 10 mm, have been applied to a vehicle suspension part such as an arm in case an unsteady high load is applied such as running on a rough road, sudden start, sudden stop, and the like. ^five times equal to or greater than 0.60 greater fatigue limit ratio in the stress repetition number of times (hereinafter, for example, referred to as "fatigue limit ratio of 10 ⁵ cycles,""10 ⁵ times fatigue limit ratio in the stress the number of repetitions of", etc.) In some cases, it is required.

[0006] In general, the fatigue characteristics of steel materials at 10 ⁷ cycles have been discussed, and studies have been made to improve the fatigue resistance characteristics in this case. When a steady load is applied to the steel material (part), it is significant to increase the fatigue resistance characteristics in the above 10 ⁷ cycles, and this can be reflected in the safety design of the part. However, when said non-stationary high load is applied is to the components as, but it is of course to increase the fatigue resistance at 10 ⁷ cycles, more to enhance the fatigue resistance of the low-cycle region It becomes important.

As a technique relating to a ferrite / pearlite type non-heat treated steel, for example, Japanese Patent Application Laid-Open No. 63-199848 is disclosed.
JP, JP-A-7-70698, JP-A-7-10
No. 2340 is disclosed.

[0008] Of these, Japanese Patent Application Laid-Open No. 63-199848 discloses that the ferrite is strengthened by the nitride of V and the solute N by limiting the Al content to less than 0.020%. Non-heat treated steel for hot forging having excellent workability and machinability ”is disclosed. However, since the non-heat treated steel proposed in this publication has a low Al content, the effect of AlN on grain refinement may not be obtained. In particular, a large fatigue limit ratio of 0.60 or more in 10 ⁵ cycles cannot be stably and reliably secured.

Japanese Patent Application Laid-Open No. 7-70698 discloses "high fatigue strength free-cutting non-heat treated steel" having a specific chemical composition. However, the technology proposed in this publication is 900
It is an object of the present invention to provide a non-heat treated steel excellent in machinability having a tensile strength of not less than MPa and a fatigue limit ratio of 0.5 or more. Although satisfying, no consideration has been given to raising the fatigue limit ratio in the low cycle region, especially in the 10 ⁵ cycle, so that a large fatigue limit ratio of 0.60 or more in the 10 ⁵ cycle can always be ensured stably. is not.

Japanese Patent Application Laid-Open No. 7-102340 discloses that a steel material having a specific chemical composition of a ferrite + pearlite structure in which 90% or more of the structure cooled after hot forging is 20%.
A "method for producing a non-heat treated steel having excellent fatigue properties" in which aging treatment is performed at 0 to 700C is disclosed. However, in the technique proposed in this publication, quenching of the refining treatment can be omitted, but heat treatment for aging similar to tempering needs to be performed, so that the energy cost increases.

Japanese Unexamined Patent Publication (Kokai) No. 6-287677 discloses a "high-strength non-heat treated steel for hot forging" of a ferrite / pearlite type having a specific chemical composition. However, non-heat treated steel proposed in this publication also, low-cycle region, so is not made consideration for increasing the fatigue limit ratio at among others 10 ⁵ cycles, necessarily 10 ⁵ cycles 0.
A large fatigue limit ratio of 60 or more cannot be ensured stably.

Japanese Unexamined Patent Publication (Kokai) No. 7-62488 discloses a "non-heat-treated steel for hot forging" having a specific chemical composition and defining a pearlite structure ratio. However, the non-heat treated steel proposed in this publication also has a low cycle region,
Since no consideration has been given to increasing the fatigue limit ratio in the 0 ⁵ cycle, a large fatigue limit ratio of 0.60 or more in the 10 ⁵ cycle cannot always be stably and reliably secured.

[0013]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and has been made under ordinary hot working and cooling conditions.
It is also a non-heat treated ferrite / pearlite type non-heat treated steel which is excellent in low cycle fatigue resistance without heat treatment including aging treatment after hot working without heat treatment. Among them, tensile strength is 750 MPa. above, the fatigue limit ratio at 10 ⁵ cycles at 0.60 or more, further an object to provide a non-heat treated steel excellent ferrite-pearlite type in machinability at a low cost.

[0014]

The gist of the present invention resides in a non-heat treated steel shown in the following (1) and (2).

(1) In mass%, C: 0.20 to 0.45
%, Si: 0.40 to 1.00%, Mn: 0.80 to
1.40%, P: 0.010 to 0.20%, S: 0.0
40 to 0.080%, Cr: 0.45 to 0.90%,
V: 0.10 to 0.25%, Ti: 0.005 to 0.0
5%, Al: 0.01 to 0.10%, N: 0.008 to
It contains 0.030%, the value of fn1 represented by the following formula (1) is 0.70 to 1.15, and the value of fn2 represented by the following formula (2) is
Is from 60.0 to 140.0, f expressed by the following formula (3).
The value of n3 satisfies 0.40 to 1.20, the balance is a chemical composition consisting of Fe and impurities, and the structure is ferrite.
The ratio of the ferrite phase to the structure in pearlite and the area ratio is 0.15 to 0.65, and the ferrite grain size is JI.
A non-heat treated steel material having an S particle size number of 7.0 or more.

Fn1 = C + 0.11Si + 0.19Mn-0.7S + 0.23Cr + 1.65V (1) fn2 = 75.2 × α + 15Si-19.4Mn + 133P + 153S + 84. 2V + 52.3Cr + 523N (2) fn3 = (1.5Si + 2.4V) × α (3) Here, the element symbol in each formula indicates the content by mass% of the element, and α is the structure Shows the ratio of the ferrite phase in the above.

(2) In addition to the components described in the above (1),
Further, it contains 0.001 to 0.10% by mass of Zr, and the value of fn1 represented by the formula (1) is 0.70 to 1.
15. The value of fn2 represented by the formula (2) is 60.0 to 1
40.0, the value of fn3 represented by the above equation (3) is 0.40
１．２1.20, the balance being Fe and impurities, the structure of which is ferrite / pearlite, and the ratio of the ferrite phase to the structure in the area ratio is 0.15.
0.65, ferrite particle size is 7.0 according to JIS particle size number
The non-heat treated steel material above.

Note that the ferrite-pearlite structure refers to a mixed structure of a ferrite phase and a pearlite phase, and the area ratio of the structure refers to the structure ratio when observed with a microscope. Less than,
In the present specification, the “phase” in the structure is not shown, and for example, the “ferrite phase” is simply referred to as “ferrite”.

The area ratio of 0.15 to 0.65 means that the ratio in the organization when the whole is 1 is 0.15 to 0.6.
5, that is, 15 to 65 of the organization
%.

The JIS particle size number of ferrite is JI
It refers to a value measured based on “Test method for ferrite crystal grain size of steel” described in SG 0552 (1998).

The present inventors have conducted various studies on the chemical composition and structure of the non-heat treated steel to achieve the above-mentioned object. As a result, the following findings were obtained.

(A) The fatigue strength of a steel material at 10 ⁷ cycles has a correlation with the tensile strength, but a low cycle region of 10 ⁵ cycles or less (hereinafter, “low cycle” in the present invention means “10 ⁵ cycles or less”). shall refer), the fatigue strength at Among these ¹⁰⁵ cycles without merely improving only increases the tensile strength and therefore also not improved fatigue ratio. In order to improve the fatigue limit ratio in the low cycle region, it is important to increase the tensile strength of the steel material and optimize the structure.

(B) Fatigue limit ratio of ferrite / pearlite type non-heat treated steel material in a low cycle region, especially 10
^In order to increase the fatigue limit ratio in ^five cycles, it is sufficient to refine the structure, increase the proportion of ferrite in the structure by area ratio, and strengthen the ferrite.

(C) Ferrite / pearlite type non-heat-treated steel having a specific chemical composition, in particular, ferrite / pearlite type non-heat-treated steel in which the ratio of ferrite to the structure by area ratio is 0.15 to 0.65. The tensile strength of
(1) It can be arranged by fn1 expressed by the equation, and the value of fn1 is 0.
When it is 70 or more, a tensile strength of 750 MPa or more can be stably obtained.

(D) containing an appropriate amount of Mn and S
If S is generated, this MnS becomes a ferrite precipitation site in the cooling process after hot working, so that the structure can be refined and the proportion of ferrite in the structure can be increased by the area ratio.

(E) If Ti is added to precipitate Ti carbonitride in the steel, it is possible to prevent austenite grains from being coarsened during heating for hot working. Becomes fine.

(F) In order to increase the ratio of ferrite in the structure by the area ratio and to strengthen the ferrite, the C content should be reduced to contain Si and V.

(G) Fatigue limit ratio in the low cycle region of ferrite / pearlite type non-heat treated steel, especially 10
The fatigue limit ratio in ^five cycles can also be achieved by increasing the ductility (drawing) of the steel.

(H) If Cr is contained, the lamellar spacing of pearlite is reduced, and the drawing of steel can be increased.

(I) P has the effect of solid solution strengthening ferrite to increase the fatigue limit ratio, particularly in the low cycle region.

[0031] (j) the fatigue limit ratio in the low-cycle area of the ferrite-pearlite type non-heat treated steel having a specific chemical composition and structure, the fatigue limit ratio at among others 10 ⁵ cycles, tables in the (2) Fn2 can be arranged, fn2
Is 60.0 to 140.0, a value of 0.60 or more can be stably obtained. (K) The machinability of a ferritic / pearlite type non-heat treated steel material having a specific chemical composition is expressed by fn3
And good machinability is obtained when the value of fn3 is 0.40 or more.

The present invention has been completed based on the above findings.

[0033]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each requirement of the present invention will be described in detail below. In addition, "%" of the content of each element means "% by mass". (A) Chemical composition C: 0.20 to 0.45% C is an element effective for increasing the strength of a steel material having a ferrite-pearlite structure. To obtain the effect, a content of 0.20% or more is required. However, 0.
When the content exceeds 45%, the fatigue limit ratio in the low-cycle region percentage of ferrite occupying the tissue area ratio is low, the fatigue limit ratio at among others 10 ⁵ cycles decreases. Therefore, the content of C is set to 0.20.
０．４0.45%. In addition, C content is 0.25-0.
Preferably it is 35%.

Si: 0.40 to 1.00% Si promotes deoxidation and acts as a solid solution in ferrite to strengthen ferrite to increase the fatigue limit ratio, especially the fatigue limit ratio in a low cycle region. Having. Further, Si also has the effect of embrittlement of ferrite to enhance machinability. In order to sufficiently exhibit the above effects, it is necessary to use Si
Is required to be 0.40% or more. However, when Si is excessively added, the above effect is not only saturated, but also the heating for hot working promotes decarburization of the surface of the steel material to lower the surface strength. In particular, when the content of Si exceeds 1.00%, decarburization on the surface of the steel material becomes remarkable. Therefore, the content of Si is set to 0.40 to 1.00%. Note that the content of Si is preferably set to 0.70 to 0.90%.

Mn: 0.80 to 1.40% Mn has a deoxidizing effect and an effect of increasing strength. In order to secure such an effect, a content of 0.80% or more is required. However, even if it is contained in excess of 1.40%, the effect is saturated, and the hardenability becomes too high to form bainite and island-like martensite, thereby reducing machinability. Furthermore, the ductility is lowered, the fatigue limit ratio in the low-cycle region, fatigue limit ratio at among others 10 ⁵ cycles is reduced. Therefore, the content of Mn is set to 0.8
0 to 1.40%. The Mn content is 1.00.
It is preferable to set it to 1.30%.

P: 0.010% to 0.20% P has an effect of increasing the machinability and the fatigue limit ratio. However, if the content is 0.010% or less, it is difficult to obtain the effect. On the other hand, if the content exceeds 0.20%, the toughness is significantly reduced. Therefore, the content of P is set to 0.010
０．２0.20%. The content of P is 0.015
Preferably, it is 0.10%.

S: 0.040% to 0.080% S enhances the machinability of steel and combines M with Mn.
nS acts as a precipitation site of ferrite in the cooling process after hot working, and contributes to the refinement of the structure and the increase in the proportion of ferrite in the structure in terms of the area ratio. However, if the content is less than 0.040%, the effect of addition is poor. On the other hand, even if the content exceeds 0.080%, the above effect is saturated. Therefore, the content of S is set to 0.040 to 0.0
80%. The content of S is 0.050 to 0.0
Preferably, it is 70%.

Cr: 0.45 to 0.90% Cr, as a solid solution strengthening element, increases the strength, reduces the pearlite lamella spacing, increases ductility (drawing), and increases the fatigue limit ratio in the low cycle region. 10 ⁵
It has the effect of increasing the fatigue limit ratio in cycles. However, if the content is less than 0.45%, the effect of addition is poor. On the other hand, if the content exceeds 0.90%, the effect is saturated and the cost increases. Furthermore, since the hardenability becomes too high, bainite and island-like martensite are formed, so that the machinability decreases. Therefore, the content of Cr is set to 0.45 to
0.90%. The content of Cr is 0.50 to 0.50.
It is preferably set to 0.70%.

[0039] V: 0.10 to 0.25% V is an precipitation strengthening element, the fatigue limit ratio in the low-cycle region to strengthen the ferrite has an effect of improving the fatigue limit ratio at among others 10 ⁵ cycles . V also has the effect of embrittle ferrite to enhance machinability. However, if the content is less than 0.10%, it is difficult to obtain the above effects. On the other hand, if the content exceeds 0.25%, the effect is saturated, and only the cost is increased, which impairs the economic efficiency. Therefore, the content of V is 0.10 to 0.25%
And Note that the V content is preferably 0.15 to 0.20%.

Ti: 0.005 to 0.05% Ti combines with C and N and precipitates as Ti carbonitride, thereby preventing austenite crystal grains from being coarsened. When cooled after processing, it becomes a precipitation site of ferrite, which contributes to refinement of the structure and an increase in the ratio of ferrite in the structure in terms of the area ratio. However, if the content is less than 0.005%, the desired effect cannot be obtained. On the other hand, even if the content of Ti exceeds 0.05%, the above effect is saturated and the cost is increased. Therefore, the Ti content is set to 0.005 to 0.05%.

Al: 0.01 to 0.10% Al has a deoxidizing effect. Further, Al is A together with N
to form a lN preventing coarsening of austenite grains, having tissue is miniaturized fatigue ratio in the low-cycle region, the effect of improving the fatigue limit ratio at among others 10 ⁵ cycles. However, if its content is less than 0.01%, the effect of addition is poor, and if it exceeds 0.10%, oxide-based inclusions increase, leading to a reduction in tool life during cutting and fatigue due to subepidermal inclusions. This leads to deterioration of characteristics. Therefore, the content of Al is set to 0.01 to 0.10%.

N: 0.008 to 0.030% N combines with V and Ti together with C to form V carbonitride or Ti
It acts as a carbonitride to increase the strength. or,
The above-mentioned AlN combined with Ti carbonitride and Al prevents the austenite grains from being coarsened, so that it is also effective in refining the structure. However, if the content is less than 0.008%, it is difficult to obtain the above effects. On the other hand, if the content exceeds 0.030%, the effect is not only saturated, but also causes deterioration in hot workability. Therefore, the content of N is set to 0.
008 to 0.030%. The content of N is 0.1.
It is preferable to be 012 to 0.030%.

Zr: 0.001 to 0.10% Zr disperses inclusions very uniformly and finely to refine the structure after hot working, and achieves a fatigue limit ratio in a low cycle region, especially 10 ⁵ cycles. Has the effect of increasing the fatigue limit ratio of steel, especially in the low cycle region (especially 10 ⁵
Cycle)), the effect is hardly obtained when the content is 0.001%, but the effect is saturated even when the content exceeds 0.10%. Only the cost increases. Therefore, Z
When adding r, the content is preferably 0.001 to 0.10%.

Fn1: 0.70 to 1.15 Ferrite / pearlite type non-heat-treated steel material in which the content of each element is in the range described above, in which the ratio of ferrite to the structure by area is 0.15 to 0%. The tensile strength of the ferritic pearlite type non-heat treated steel material,
It can be arranged by fn1 represented by the formula, and when this value is 0.70 or more, a tensile strength of 750 MPa or more can be stably obtained. On the other hand, if the value of fn1 exceeds 1.15, the tensile strength becomes too large, leading to a reduction in machinability. Therefore, f
The value of n1 was set to 0.70 to 1.15. If the lower limit of fn1 is 0.80, a high tensile strength of 850 MPa or more can be stably secured.

Fn2: 60.0 to 140.0 Ferrite / pearlite type non-heat-treated steel material in which the content of each element is in the above-mentioned range. fatigue ratio in the low-cycle area of the ferrite-pearlite type non-heat treated steel is 0.65, the fatigue limit ratio at among others 10 ⁵ cycles, the (2) can be organized in fn2 of the formula, this value In the case of 60.0 to 140.0, a large value of 0.60 or more can be stably obtained.

FIG. 1 shows various ferrites in which the content of each element is in the range already described, and the ratio of ferrite to the structure by area ratio is in the range of 0.15 to 0.65 described later. - pearlite type non-heat treated steels a relationship between fatigue limit ratio by the value of fn2 and 10 ⁵ cycles shows an example obtained in the same test method as in examples below. It is also apparent from this figure that when the value of fn2 is 60.0 to 140.0, a fatigue limit ratio of 0.60 or more can be stably obtained.

Fn3: 0.40 to 1.20 The machinability of the ferrite / pearlite type non-heat treated steel material in which the content of each element is in the range described above is summarized by fn3 represented by the above equation (3). When the value of fn3 is 0.40 or more, good machinability is obtained. However, when the value of fn3 is 1.
If it exceeds 20, the toughness is significantly reduced. Therefore, the value of fn3 was set to 0.40 to 1.20. (B) Microstructure α of steel material: 0.15 to 0.65 When α, which is the area ratio of ferrite in the ferrite-pearlite structure, is less than 0.15, a low cycle region;
In Of these 10 ⁵ cycles, it is difficult to ensure the 0.60 or more fatigue ratio. On the other hand, if α exceeds 0.65, on the contrary, the fatigue limit ratio in the low cycle region, especially in the 10 ⁵ cycle, decreases, and the machinability also deteriorates. Therefore, the area ratio (α) of ferrite in the structure is 0.1%.
It was set to 5 to 0.65. The area ratio (α) of the ferrite may be obtained by corroding a mirror-polished surface with, for example, nital, and observing the corroded surface as a test surface with an optical microscope.

Ferrite grain size: 7.0 in JIS grain size number
As described above, by refining the structure, in particular, by refining the ferrite, the fatigue limit ratio in the low cycle region,
The fatigue limit ratio in ⁵ cycles can be increased, and when the ferrite grain size is 7.0 or more in JIS grain size number, 10 ⁵
A fatigue limit ratio of 0.60 or more can be stably obtained in a cycle. Incidentally, 8.0 fatigue limit ratio at 10 ⁵ cycles If you want more elevated, the ferrite grain size in JIS grain size number
It is better to do the above. As already described, the JIS grain size number of ferrite refers to a value measured based on “Method of testing ferrite grain size of steel” described in JIS G 0552 (1998). After melting the steel having the chemical composition described in the above item (A) by an ordinary method,
It is obtained by heating to １1300 ° C., processing at 1200 to 800 ° C. to form a predetermined shape, and then cooling at a cooling rate of 0.1 to 5.0 ° C./sec.

[0049]

EXAMPLES Steel having the chemical composition shown in Tables 1 to 5 was melted by a conventional method using a 180 kg vacuum melting furnace. Tables 1-5
2 also shows values of fn1 calculated from the chemical composition and values of fn2 and fn3 calculated from α obtained by observing the chemical composition and the structure.

Steels 1 to 46 in Tables 1 to 3 are examples of the present invention in which the chemical composition is within the range specified in the present invention. on the other hand,
Steels 47 to 72 in Tables 4 and 5 are comparative examples out of the conditions specified in the present invention.

[0051]

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[Table 5] After turning these steels into billets by the usual method,
After heating to 0 to 1300 ° C., it was hot forged into a round bar having a diameter of 50 mm at a finishing temperature of 1000 ° C. or higher. Then
The above-mentioned round bar hot forged to a diameter of 50 mm has a length of 1000
mm and further heated to 1200 ° C. with a high-frequency heating device, and then a round bar having a diameter of 30 mm was obtained using a hot forging press. That is, according to the composition of the steel, formed into a round bar having a diameter of 30 mm at a finishing temperature of 1100 to 900 ° C.,
Then, it cooled to normal temperature (room temperature) at the cooling rate of 0.5-2.0 degreeC / second. From the center of the round bar thus obtained
14A test piece described in JIS Z 2201 (diameter: 10 mm)
Was cut out and subjected to a tensile test at room temperature to yield strength (Y
S), tensile strength (TS) and draw (RA) were measured.

An Ono-type rotary bending fatigue test piece having a parallel part diameter of 8 mm was cut out from the center of the round bar, and was cut at room temperature (room temperature).
Rotational speed of 600 rpm to reproduce unsteady state in the atmosphere
perform fatigue test under the conditions of m, it was measured fatigue strength at 10 ⁵ cycles.

A test piece having a diameter of 30 mm and a thickness of 20 mm was cut out and cut along a plane parallel to the longitudinal direction including the center, and the structure of the center was observed with an optical microscope. The tissue examination method is to collect a light microscopic observation specimen,
It corroded with 5% nital and observed at 200 times. Also, JI
The ferrite crystal grain size was also measured based on “Test method for ferrite grain size of steel” described in SG 0552 (1998).

Tables 6 to 8 summarize the test results.

[0055]

[Table 6]

[Table 7]

[Table 8] From Table 6-8, in the case of Test No. 1 to 46 of the present invention embodiment, in both the ferrite-pearlite structure, and since which satisfies the condition defined in the present invention, the above tensile strength and 10 ⁵ cycles 750MPa It can be seen that a fatigue limit ratio of 0.60 or more was obtained. In contrast, in the case of Test No. 47 to 72 of the comparative example, since the out of the range regulated by the present invention, the fatigue limit ratio at 10 ⁵ cycles does not reach the 0.60.

In the case of Test Nos. 1 to 46 of the examples of the present invention, the round bar having a diameter of 30 mm was used as a test piece, and the diameter was 8 m.
m high speed drill, no lubrication, cutting speed 50m
As a result of performing a drilling test under the conditions of m / min and feed of 1.0 mm / rev, it was confirmed that there was no problem in machinability.

[0057]

Non-heat treated steel material of the present invention, according to the present invention is, because it has a 0.60 or more of the fatigue limit ratio or more of tensile strength and 10 ⁵ cycles 750MPa, wheel hub as a foot around automobile parts, knuckle, arm, etc. Can be used.

[Brief description of the drawings]

FIG. 1 fn2 of ferrite / pearlite type non-heat treated steel
It is a diagram illustrating an example of the values and the relationship between the fatigue limit ratio at 10 ⁵ cycles.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-10-265891 (JP, A) JP-A-11-269599 (JP, A) JP-A-62-219975 (JP, A) JP-A-9-99 291339 (JP, A) JP-A-8-253842 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] C .: 0.20 to 0.45% by mass, S
i: 0.40 to 1.00%, Mn: 0.80 to 1.40
%, P: 0.010-0.20%, S: 0.040-
0.080%, Cr: 0.45 to 0.90%, V: 0.
10 to 0.25%, Ti: 0.005 to 0.05%, A
l: 0.01 to 0.10%, N: 0.008 to 0.03
0%, and the value of fn1 represented by the following formula (1) is 0.1%.
70 to 1.15, the value of fn2 represented by the following equation (2) is 6
0.0 to 140.0, the value of fn3 represented by the following formula (3) satisfies 0.40 to 1.20, the balance is a chemical composition comprising Fe and impurities, and the structure is ferrite / pearlite. A non-heat treated steel material in which the ratio of the ferrite phase to the structure in the area ratio is 0.15 to 0.65 and the ferrite grain size is 7.0 or more in JIS grain size number. fn1 = C + 0.11Si + 0.19Mn-0.7S + 0.23Cr + 1.65V (1) fn2 = 75.2 × α + 15Si-19.4Mn + 133P + 153S + 84. 2V + 52.3Cr + 523N (2) fn3 = (1.5Si + 2.4V) × α (3) Here, the element symbol in each formula indicates the content by mass% of the element, and α is the structure Shows the ratio of the ferrite phase in the above. 2. The composition according to claim 1, which further contains 0.001 to 0.10% by mass of Zr in addition to the component described in claim 1.
The value of fn1 represented by the equation (1) is 0.70 to 1.15, and the value of fn2 represented by the following equation (2) is 60.0 to 140.
0, the value of fn3 represented by the following formula (3) is 0.40-1.
20, the balance being Fe and impurities, the structure of which is ferrite / pearlite, and the ratio of the ferrite phase to the structure in the area ratio of 0.15 to 0.
65, a non-heat treated steel material having a ferrite grain size of 7.0 or more in JIS grain size number. fn1 = C + 0.11Si + 0.19Mn-0.7S + 0.23Cr + 1.65V (1) fn2 = 75.2 × α + 15Si-19.4Mn + 133P + 153S + 84. 2V + 52.3Cr + 523N (2) fn3 = (1.5Si + 2.4V) × α (3) Here, the element symbol in each formula indicates the content by mass% of the element, and α is the structure Shows the ratio of the ferrite phase in the above.