JP2015025162A - Ferrite pearlite type non-heat treated steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferrite pearlite type non-heat treated steel combining high mechanical strength with machinability, and furthermore excellent in producibility.SOLUTION: The ferrite pearlite type non-heat treated steel contains as essential additive elements, by mass%, C:0.20 to 0.60%, Si:0.20 to 2.00%, S:0.001 to 0.200%, Cr:0.40 to 2.00%, s-Al:0.010 to 0.050%, and contains as optional additional elements, by mass%, Mn:≤1.00%, P:≤0.200%, Cu:≤0.50%, Ni:≤0.50%, Mo:≤0.20%, V:≤0.50%, Ti:≤0.15% and N:≤0.030%, and the balance Fe with inevitable impurities and satisfies the predetermined relation in terms of mass% of each element of C, Si, Mn, P, Cu, Ni, Cr, Mo, V, Ti, N, S.

Description

  The present invention relates to a ferrite-pearlite type non-heat treated steel for machine parts, and more particularly to a ferrite-pearlite type non-heat treated steel having both high mechanical strength and machinability and excellent manufacturability.

  In non-heat treated steel used without heat treatment for tempering, in ferrite-pearlite type non-heat treated steel for machine parts, if bainite is included in the ferrite + pearlite metal structure, its mechanical strength And the machinability is deteriorated. Thus, steel types having a component composition that suppress the formation of bainite have been developed. In this regard, for example, Patent Documents 1 to 3 disclose that the upper limits of the Cr and Mn amounts that promote the formation of bainite are specified.

  In patent document 1, C: 0.15-0.25%, Si: 0.1-1.5%, Mn: 0.5-1.8%, S: 0.03-0. 15%, P: 0.03-0.15%, Cu: 0.01-0.5%, Ni: 0.01-0.5%, Cr: 0.01-1%, V: 0.1 -0.4%, s-Al: 0.001-0.01%, N: 0.005-0.035%, Ca: 0.0001-0.01%, O: 0.001-0.01 %, And the C, Si, Mn, Cu, Ni, Cr relational expressions together with C, Si, Mn, P, Cu, Ni, Cr, and V carbon equivalent formulas are within a predetermined range. Non-tempered steel is disclosed. While limiting the amount of Cr and Mn to suppress the formation of bainite, while increasing V, decreasing C, increasing S, adding P actively, adding Mn for MnS formation By adding Ca and O, fatigue strength and proof stress can be improved and machinability can also be improved.

  Moreover, in patent document 2, by mass%, C: 0.15-0.35%, Si: 0.5-2.0%, Mn: 0.5-1.5%, P: 0.03- 0.15%, S: 0.01 to 0.15%, Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%, Cr: 0.01 to 1.0%, s -Al: 0.001 to 0.01%, N: 0.005 to 0.035%, Ca: 0.0001 to 0.01%, and O: 0.001 to 0.01%. And similarly to Patent Document 1, a non-heat treated steel is disclosed in which the carbon equivalence formula, the relational expressions of the component compositions of C, Si, Mn, P, Cr, Cu, Ni, and Cr are in a predetermined range. . Here, similarly to Patent Document 1, the upper limits of the Cr and Mn amounts are defined to suppress the formation of bainite.

  Furthermore, in patent document 3, by mass%, C: 0.3-0.8%, Si: 0.1-2.0%, Mn: 0.5-1.5%, P: 0.01- 0.15%, Cr: 1.0%, V: 0.4% or less, Al: 0.05% or less, N: within a range of 0.005 to 0.03%, and P amount, Disclosed is a non-heat treated steel having a relational expression of hardness, C, Si, Mn, Cr, and V carbon equivalent formulas within a predetermined range. Here, as in Patent Documents 1 and 2, the upper limits of the Cr and Mn amounts are defined to suppress the formation of bainite.

  By the way, bainite is generated in the cooling process from austenite, like pearlite and martensite, and its generation can be controlled by the cooling rate. For example, Patent Documents 4 and 5 disclose that the formation of bainite is suppressed by controlling the cooling process together with the component composition of steel.

  In patent document 4, in mass%, C: 0.15-0.45%, Si: 0.1-1.5%, Mn: 0.5-1.5%, P: 0.03-0. 15%, Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%, Cr: 0.01 to 1%, V: 0.1 to 0.4%, N: 0.005 In non-tempered steel containing ˜0.035%, it is disclosed that the cooling rate is increased so as to increase the number density of V-based precipitates in the cooling process after forging, while the upper limit is set below the bainite generation critical cooling rate. doing. Such a bainite generation critical cooling rate is experimentally determined from the relationship among the contents of C, Si, Mn, Cu, Ni, and Cr.

Moreover, in patent document 5, by mass%, C: 0.16-0.35%, Si: 0.10-1.00%, Mn: 0.30-1.00%, P: 0.040- 0.070%, S: 0.08 to 0.13%, V: 0.10 to 0.35%, Ti: 0.08 to 0.15%, s-Al: 0.010 to 0.045% N: In non-heat treated steel containing 0.005 to 0.025% or less, C, Si, Mn, Cu together with an index for forging V and Ti and an index for sulfide formation of S, Ti, and N , Ni, Cr, and Mo bainite generation critical cooling rates are described in terms of experimentally obtained equations, respectively.

JP 2003-342671 A JP 2004-183094 A JP 2007-119819 A JP 2005-29825 A JP 2009-221590 A

  As described above, in machine parts made of non-heat-treated steel such as connecting rods and common rails for automobiles, the formation of bainite in the ferrite + pearlite metal structure during air cooling after hot working is suppressed, and the mechanical strength and coverage are reduced. Ensures machinability. In recent years, with such relatively small machine parts, further downsizing has progressed, and the cooling rate during air cooling after hot working has increased, and it is also required to increase the cooling rate in order to increase manufacturability. Even in that case, it is desirable not to produce bainite. That is, a steel type that can suppress the formation of bainite without greatly depending on the cooling rate is demanded.

  The present invention has been made in view of the above-described situation, and the object of the present invention is to provide a ferrite-pearlite type non-adjustment that combines high mechanical strength and machinability while also being excellent in manufacturability. To provide quality steel.

The ferritic pearlite type non-heat treated steel according to the present invention is mass%, and C: 0.20-0.60%, Si: 0.20-2.00%, Mn: ≦ 1.00 as essential additive elements. %, S: 0.001 to 0.200%, Cr: 0.40 to 2.00%, s-Al: 0.010 to 0.050%, and P: ≦ 0. 200%, Cu: ≦ 0.50%, Ni: ≦ 0.50%, Mo: ≦ 0.20%, V: ≦ 0.50%, Ti: ≦ 0.15%, N: ≦ 0.030% And comprising the balance Fe and impurities,
When the mass% of the element M is represented by [M],
(1) [V] + [Ti] ≧ 0.03,
(2) F1 = (836.2 × Ceq + 239.8) × F2 ≧ 500,
here,
Ceq = [C] + 0.03 × [Si] + 0.31 × [Mn] + 0.26 × [P] + 0.08 × [Cu] + 0.07 × [Ni] + 0.24 × [Cr] +0.17 * [Mo] + 1.2 * [V] +0.51 * ([Ti] -3.43 * [N])-0.7 * [S]
F2 = −0.18 × [C] + 0.061 × [Mn] −0.178 × [S] + 0.502 × [V] + 0.354 × [Ti] +0.629
And
(3) F3 = F4 / F5 ≧ 1.00,
here,
[S−Mn] = [Mn] −1.713 × [S] + 0.574 × [Ti] −1.963 × [N]
F4 = 429.86-310 × [C] + 25 × [Si] −130.86 [s-Mn] −14.5 × [Cu] −41.55 × [Ni] + 9.46 × [Cr] −49 .04 × [Mo] −51.09 × [V] + (218.97 × [C] +25.256) × [Ti]
F5 = 10 ^ (0.018 × [Si] + 0.774 × [s-Mn] + 0.261 × [Cu] + 0.177 × [Ni] + 0.608 × [Cr] + 4.823 × [Mo] − 0.948 × [C] + 1.196 × [V] + 5.257 × [Ti] +1.41)
It is characterized by being.

  According to such an invention, even if the cooling rate after hot working is increased, the generation of bainite can be suppressed without impairing the mechanical strength and machinability, and thus has both high mechanical strength and machinability. This is a ferritic / pearlite type non-tempered steel with excellent manufacturability.

  In the above-mentioned invention, as the essential additive element in mass%, Pb: 0.001 to 0.3%, Bi: 0.001 to 0.3%, Te: 0.001 to 0.3%, Ca: It may be characterized by further containing one or more of 0.001 to 0.01%. According to this invention, even if the cooling rate after hot working is increased, the formation of bainite can be suppressed and the machinability can be improved without impairing the mechanical strength. Therefore, higher mechanical strength and machinability can be achieved. Therefore, it becomes a ferritic / pearlite type non-heat treated steel with excellent manufacturability.

  In the above-described invention, the fastest average cooling rate that does not generate bainite among the average cooling rates in the temperature range of 700 to 400 ° C. may be 1.0 ° C./s or more. According to this invention, even a relatively small machine part that is required to increase the cooling rate after hot working can suppress the generation of bainite, while having high mechanical strength and machinability. This is a ferritic / pearlite type non-tempered steel with excellent manufacturability.

  In the above-described invention, the 0.2% proof stress may be 500 MPa or more. According to this invention, high mechanical strength is imparted to the ferrite-pearlite type non-heat treated steel having high machinability and excellent manufacturability.

It is a chart of a component composition of steel of an example and a comparative example. It is a list figure of the test result of an Example and a comparative example.

  In FIG. 1, the component composition of the ferrite pearlite type non-heat treated steel as an example of the present invention is shown. Moreover, in FIG. 2, the test result of the processing for master test, structure | tissue observation, the hardness test, and the tension test in the non-heat treated steel of this Example was shown. Similarly, as a comparative example, the component composition is shown in FIG. 1, and the test result is shown in FIG. Below, the preparation method of a test piece and each test method are demonstrated.

  First, test materials for cutting out various test pieces were prepared. That is, a 50D round bar forging material having the composition shown in FIG. 1 is forged into a 22D round bar at 1150 ° C., and cooled from 700 ° C. to 400 ° C. at a cooling rate of 1.0 ° C./sec. Got.

  In the processing for master test, the test piece is compressed and then cooled by changing the cooling rate, and among the average cooling rates in the temperature range from 700 ° C. to 400 ° C., the fastest average cooling rate that does not generate bainite, that is, the bainite formation criticality. The speed (B production critical speed) was determined. Specifically, a test piece of φ8 mm × 12 mm was cut out from the test material and compressed at 1150 ° C. to a height of 6 mm. Then, it cooled by changing a cooling rate from 700 degreeC to 400 degreeC, and also cooled to room temperature. The cross section was cut out and the structure was observed to confirm the presence or absence of bainite. FIG. 2 shows this critical bainite production rate (B production critical rate).

  In the hardness test, Rockwell hardness was measured for a test piece cut out from a test material. FIG. 2 shows the result.

  In the structure observation, the microstructure of the cross section of the test piece was observed using an optical microscope. FIG. 2 shows “F + P” in the case of the ferrite / pearlite structure, and “F + P + B” when the ferrite / pearlite structure is observed together with bainite.

  In the tensile test, a JIS No. 14A test piece (a proportional test piece having a parallel bar diameter of 5 mm and a gripping part M8) was cut out from the test material, and a tensile test was performed at room temperature to obtain 0.2% yield strength. FIG. 2 shows the result.

  Next, test results will be described.

  As shown in FIG. 2, in Examples 1 to 17, the bainite formation critical rates were 1.0 to 3.5 ° C./second and 1.0 ° C./second or more. Moreover, bainite was not observed in the structure observation. Further, the 0.2% proof stress was 531 to 788 MPa, and all were 500 MPa or more. At this time, the hardness was 21.6-44.3 HRC.

  As described above, according to the steels of Examples 1 to 17, the formation of bainite can be suppressed by air cooling (cooling) of at least about 1.0 ° C./second. Moreover, it was the hardness which can obtain 0.2% yield strength of 500 MPa or more which is equal to or higher than that of conventional non-tempered steel, and provides sufficient machinability. That is, it is excellent in manufacturability while having high mechanical strength and machinability.

By the way, for the series including Examples 1 to 17, regression calculation was performed for the contents of C, Si, Mn, P, Cu, Ni, Cr, Mo, V, Ti, N, and S and 0.2% proof stress. . Formula F1 for estimating this 0.2% proof stress is
F1 = (836.2 × Ceq + 239.8) × F2
It is. Here, when the mass% of the element M is [M], the carbon equivalents Ceq and F2 are
Ceq = [C] + 0.03 × [Si] + 0.31 × [Mn] + 0.26 × [P] + 0.08 × [Cu] + 0.07 × [Ni] + 0.24 × [Cr] +0.17 * [Mo] + 1.2 * [V] +0.51 * ([Ti] -3.43 * [N])-0.7 * [S]
F2 = −0.18 × [C] + 0.061 × [Mn] −0.178 × [S] + 0.502 × [V] + 0.354 × [Ti] +0.629
It is.

Then, 0.2% proof stress of 500 MPa or more can be obtained.
F1 = (836.2 × Ceq + 239.8) × F2 ≧ 500 (Formula 1)
This is the case.

In addition, for the series including Examples 1 to 17, regression calculation was performed on the content of C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, N, and S and the critical rate of bainite formation. Formula F3, which estimates the fastest cooling rate in the temperature range of 700 ° C. to 400 ° C. so as not to form this bainite,
F3 = F4 / F5
It is. Here, when the mass% of the element M is [M], F4 and F5 are
F4 = 429.86-310 × [C] + 25 × [Si] −130.86 [s-Mn] −14.5 × [Cu] −41.55 × [Ni] + 9.46 × [Cr] −49 .04 × [Mo] −51.09 × [V] + (218.97 × [C] +25.256) × [Ti]
F5 = 10 ^ (0.018 × [Si] + 0.774 × [s-Mn] + 0.261 × [Cu] + 0.177 × [Ni] + 0.608 × [Cr] + 4.823 × [Mo] − 0.948 × [C] + 1.196 × [V] + 5.257 × [Ti] +1.41)
It is. However,
[S-Mn] = [Mn] -1.713 × [S] + 0.574 × [Ti] −1.963 × [N]
It is.

Then, it is possible to obtain a bainite formation critical rate of 1.0 ° C./second or more.
F3 = F4 / F5 ≧ 1.0 (Formula 2)
This is the case.

  Subsequently, test results in Comparative Examples 1 to 14 will also be described.

  In Comparative Example 1, the 0.2% proof stress was 421 MPa, the bainite formation critical rate was 0.9 ° C./second, and bainite was observed by microstructure observation. This is probably because C was less than in the examples. In addition, F1 was 498 and did not satisfy Formula 1, and F3 was also 0.92 and did not satisfy Formula 2.

  In Comparative Example 2, the 0.2% yield strength was 478 MPa, the critical rate of bainite formation was 1.6 ° C./second, and no bainite was observed in the structure observation. This is probably because Si was less than in the examples. In addition, F1 was 489 and did not satisfy Formula 1, and F3 satisfied Formula 2 with 1.61.

  In Comparative Example 3, the 0.2% proof stress was 463 MPa, the critical rate of bainite formation was 0.9 ° C./second, and bainite was observed by microstructure observation. This is probably because Mn was greater than in the examples. Note that F1 is 561 and satisfies Expression 1, and F3 is 0.94 and does not satisfy Expression 2.

  In Comparative Example 4, the 0.2% yield strength was 482 MPa, the critical rate of bainite formation was 0.9 ° C./second, and bainite was observed by microstructure observation. It is thought that there was more Cu than the Example. Note that F1 was 545 and satisfied Expression 1, and F3 was 0.95 and did not satisfy Expression 2.

  In Comparative Example 5, the 0.2% proof stress was 484 MPa, the bainite formation critical rate was 0.9 ° C./second, and bainite was observed by microstructure observation. This is probably because Ni was more than in the examples. Note that F1 is 548 and satisfies Expression 1, and F3 is 0.94 and does not satisfy Expression 2.

  In Comparative Example 6, the 0.2% yield strength was 491 MPa, the bainite formation critical rate was 2.2 ° C./sec, and no bainite was observed in the structure observation. This is probably because Cr was less than in the examples. In addition, F1 was 482 and did not satisfy Formula 1, and F3 satisfied Formula 2 with 2.15.

  In Comparative Example 7, the 0.2% proof stress was 492 MPa, the bainite formation critical rate was 0.8 ° C./sec, and bainite was observed by microstructure observation. This is probably because there was more Cr than in the examples. Note that F1 was 595 and satisfied Expression 1, and F3 was 0.87 and did not satisfy Expression 2.

  In Comparative Example 8, the 0.2% proof stress was 467 MPa, the bainite formation critical rate was 0.8 ° C./second, and bainite was observed by microstructure observation. This is probably because Mo was more than in the examples. Note that F1 was 576 and satisfied Expression 1, and F3 was 0.78 and did not satisfy Expression 2.

  In Comparative Example 9, the 0.2% proof stress was 426 MPa, the bainite formation critical rate was 0.9 ° C./second, and bainite was observed by microstructure observation. This is probably because Ti was more than in the examples. Note that F1 is 532 and satisfies Expression 1, and F3 is 0.93 and does not satisfy Expression 2.

  In Comparative Examples 10 and 11, the 0.2% proof stress was 488 MPa and 497 MPa, respectively, and the bainite formation critical rates were 2.0 ° C./sec and 3.5 ° C./sec, respectively. Not observed. Although each composition component was in the same range as in the examples, it is considered that F1 was 494 and 490, respectively, and did not satisfy Formula 1. Note that F3 was 1.90 and 3.68, respectively, which satisfied the formula 2.

  In Comparative Examples 12 and 13, the 0.2% proof stress was 497 MPa and 446 MPa, respectively, and the bainite formation critical rates were 0.8 ° C./sec and 0.6 ° C./sec, respectively. Observed. Although the individual composition components were within the same range as in the examples, F3 was 0.84 and 0.68, respectively, which is considered to be because Equation 2 was not satisfied. In addition, F1 was 646 and 539, respectively, and satisfied Formula 1.

  In Comparative Example 14, forging cracks occurred and a test piece could not be produced. This is probably because Pb was larger than in the example.

  As described above, according to the present example, the mechanical strength equal to or higher than that of the conventional non-tempered steel, particularly 0.2% proof stress, is greatly dependent on the cooling rate in air cooling after forging. Thus, it is possible to obtain a ferritic / pearlite type non-heat treated steel that can suppress the formation of bainite.

  By the way, the range of the component composition considered as the ferrite-pearlite type non-heat treated steel described so far is determined by the following guidelines.

  C is necessary to ensure mechanical strength as a machine part to be obtained, in particular, yield strength. Moreover, the production | generation of a bainite can be suppressed. On the other hand, if contained excessively, the machinability of machine parts obtained by increasing the pearlite fraction in the ferrite-pearlite structure is deteriorated. Therefore, the content of C is mass% and is in the range of 0.20 to 0.60%, preferably in the range of 0.20 to 0.50%.

  Si is necessary for securing the yield strength as a machine part to be obtained. Moreover, the production | generation of a bainite can be suppressed. On the other hand, if excessively contained, the hot deformation resistance is excessively increased, and the life of the mold in the hot forging is reduced. Therefore, the Si content is in mass% and is in the range of 0.20 to 2.00%, preferably in the range of 0.20 to 1.00%.

  Mn is added in order to improve the yield strength of the resulting machine part. On the other hand, if contained excessively, the 0.2% yield strength and machinability of machine parts obtained by remarkably accelerating the formation of bainite are deteriorated. Therefore, the content of Mn is mass% and is in the range of 1.00% or less, preferably in the range of 0.10 to 0.90%.

  P is added in order to improve the yield strength of the resulting machine part. On the other hand, if it is excessively contained, castability is lowered. Therefore, the content of P is, by mass%, in the range of 0.200% or less, preferably in the range of 0.100% or less.

  S forms sulfides and enhances the machinability of the resulting machine parts. On the other hand, if it is excessively contained, the productivity is lowered. Therefore, the content of S is mass% and is in the range of 0.001 to 0.200%, preferably in the range of 0.010 to 0.120%.

  Cu and Ni can be included in the steel as impurities. When these elements are contained excessively, the formation of bainite is promoted, and the 0.2% yield strength and machinability of the resulting machine part are deteriorated. Therefore, the contents of Cu and Ni are each in the range of 0.50% or less, preferably 0.30% or less, in terms of mass%.

  Cr is necessary in order to ensure the proof stress of the machine part obtained. On the other hand, if contained excessively, the formation of bainite is promoted, and the 0.2% proof stress and machinability of the resulting machine part are deteriorated. Therefore, the Cr content is, in mass%, in the range of 0.40 to 2.00%, preferably in the range of 0.40 to 1.20%.

  Mo can be contained in steel as an impurity. On the other hand, if contained excessively, the 0.2% yield strength and machinability of machine parts obtained by remarkably accelerating the formation of bainite are deteriorated. Therefore, the Mo content is, by mass%, in the range of 0.20% or less, preferably in the range of 0.05% or less.

  V can improve the yield strength of the resulting machine part. On the other hand, when it adds excessively, cost can be increased. Therefore, the content of V is, in mass%, in the range of 0.50% or less, preferably in the range of 0.30% or less.

  Ti can improve the yield strength of the resulting machine part. On the other hand, if contained excessively, the 0.2% yield strength and machinability of machine parts obtained by remarkably accelerating the formation of bainite are deteriorated. Therefore, the Ti content is in mass% and is in the range of 0.15% or less.

  In addition, since V and Ti can improve the yield strength of steel substantially equivalently, the total content of V and Ti is within a range of 0.03% or more by mass%. That is, when the mass% of the element M is [M], [V] + [Ti] ≧ 0.03.

  N may be included in the steel as an impurity. On the other hand, excessively reducing or increasing beyond a certain range can increase refining costs. Therefore, the N content is in mass% and is in the range of 0.030% or less.

  s-Al has a deoxidizing action of molten steel. Further, it is necessary to form AlN that combines with N to refine crystal grains, improve the toughness of the resulting machine part, and particularly to suppress the formation of bainite. On the other hand, if it is contained excessively, the machinability of the machine part is lowered. Then, content of s-Al is a mass%, and exists in the range of 0.010-0.050%.

  Pb, Bi, Te and Ca improve the machinability of the resulting machine part. On the other hand, if contained excessively, the proof stress and hot workability are lowered. Therefore, in the case where one or more of Pb, Bi, Te and Ca are added, their contents are in mass%, 0.001-0.3%, 0.001-0.3%, 0.001 respectively. It is in the range of ˜0.3% and 0.001 to 0.01%.

  So far, representative examples and modified examples based on the examples have been described, but the present invention is not necessarily limited thereto. Those skilled in the art will recognize a variety of alternative embodiments without departing from the scope of the appended claims.

Claims (4)

As an essential additive element by mass%, C: 0.20 to 0.60%, Si: 0.20 to 2.00%, S: 0.001 to 0.200%, Cr: 0.40 to 2. 00%, s-Al: 0.010 to 0.050%,
As optional additional elements, Mn: ≦ 1.00%, P: ≦ 0.200%, Cu: ≦ 0.50%, Ni: ≦ 0.50%, Mo: ≦ 0.20%, V: ≦ 0.0. 50%, Ti: ≦ 0.15%, N: ≦ 0.030%, which consists of the balance Fe and impurities,
When the mass% of the element M is represented by [M],
(1) [V] + [Ti] ≧ 0.03,
(2) F1 = (836.2 × Ceq + 239.8) × F2 ≧ 500,
here,
Ceq = [C] + 0.03 × [Si] + 0.31 × [Mn] + 0.26 × [P] + 0.08 × [Cu] + 0.07 × [Ni] + 0.24 × [Cr] +0.17 * [Mo] + 1.2 * [V] +0.51 * ([Ti] -3.43 * [N])-0.7 * [S]
F2 = −0.18 × [C] + 0.061 × [Mn] −0.178 × [S] + 0.502 × [V] + 0.354 × [Ti] +0.629
And
(3) F3 = F4 / F5 ≧ 1.00,
here,
[S−Mn] = [Mn] −1.713 × [S] + 0.574 × [Ti] −1.963 × [N]
F4 = 429.86-310 × [C] + 25 × [Si] −130.86 [s-Mn] −14.5 × [Cu] −41.55 × [Ni] + 9.46 × [Cr] −49 .04 × [Mo] −51.09 × [V] + (218.97 × [C] +25.256) × [Ti]
F5 = 10 ^ (0.018 × [Si] + 0.774 × [s-Mn] + 0.261 × [Cu] + 0.177 × [Ni] + 0.608 × [Cr] + 4.823 × [Mo] − 0.948 × [C] + 1.196 × [V] + 5.257 × [Ti] +1.41)
Ferritic pearlite type non-tempered steel characterized by   As the essential additive element in mass%, Pb: 0.001-0.3%, Bi: 0.001-0.3%, Te: 0.001-0.3%, Ca: 0.001-0 The ferritic pearlite type non-heat treated steel according to claim 1, further comprising at least one of 0.01%.   The fastest average cooling rate that does not produce bainite among the average cooling rates in a temperature range of 700 to 400 ° C is 1.0 ° C / s or more. Tempered steel. The ferritic pearlite type non-heat treated steel according to any one of claims 1 to 3, wherein 0.2% proof stress is 500 MPa or more.

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