JP2015001018A - Steel for machine component and machine component for non-heat treated steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel which has excellent machinability and can enhance mechanical strength in a required part and a machine component using the same.SOLUTION: A steel for a machine component for a non-heat treated steel which is composed of ferrite and pearlite non-heat treated steel and has a reinforcement part by strain aging treatment comprises, by mass%, C:0.20 to 0.60%, Si:≤1.5%, Mn:0.30 to 1.50%, P:0.01 to 0.20%, S:0.01 to 0.20%, Cu:≤1.0%, Ni:≤1.0%, Cr:≤1.0%, V:≤0.10%, Ti:≤0.20% and N:0.005 to 0.030%. A predetermined relationship regarding mass% of each of the elements of C, Si, Mn, P, Cu, Ni, Cr, V, Ti, N and S is satisfied.

Description

  TECHNICAL FIELD The present invention relates to steel for machine parts and non-heat treated steel machine parts using the same, and relates to steel for machine parts subjected to strain aging treatment and used for machine parts, and non-heat treated steel machine parts using the same. .

  Ferrite / pearlite type non-heat treated steel is used for mechanical parts such as connecting rods and hubs for engines of automobiles. In such machine parts, it is desired to increase the yield strength of the steel and improve the durability and the like, while it is desirable to secure the workability by suppressing the hardness at a part to be machined. In order to satisfy these conflicting requirements in a single machine component, a manufacturing method has been proposed in which a part requiring strength is subjected to strain aging treatment to increase partial mechanical strength.

  For example, in Patent Document 1, strain aging treatment is performed on a ferrite / pearlite non-heat treated steel having a specific composition by a coining process at a processing rate of 3 to 40% in a warm region of 200 to 700 ° C. It is disclosed that the partial mechanical strength can be greatly increased. Such steel is in mass%, C: 0.2-0.6%, Si: 0.05-2%, Mn: 0.3-1.5%, P: 0.01-0.2%, It has a component composition including Cr: 0.05 to 1%, V: 0.02 to 0.5%, and N: 0.009 to 0.03%. It is said that the partial mechanical strength (proof strength) can be greatly increased by hardening by precipitation of V carbonitride by N and fixing dislocations introduced by coining with N and C of the intrusion-type solid solution elements.

  Patent Document 2 discloses that steel having the same composition as that of Patent Document 1 is subjected to aging treatment at 400 to 650 ° C. after cold coining. After all, it is said that the partial mechanical strength can be greatly increased by hardening by precipitation of V carbonitride by N and fixing by movable dislocations N and C. Here, high-frequency induction heating is used as the heating means, and only the cold coining portion is subjected to aging treatment.

  In order to suppress the amount of V, which is relatively expensive, a method for improving the partial mechanical strength without using the precipitation of V carbonitride by N has also been proposed.

  For example, Patent Document 3 includes, in mass%, C: 0.15 to 0.6%, Si: 2.5% or less, Mn: 0.3 to 2.0%, and a ferrite / pearlite structure. It is disclosed that the tempered steel material can be subjected to an aging treatment at 100 to 450 ° C. after 3 to 30% of cold working to increase the partial mechanical strength of the cold worked portion. By performing the aging treatment at a relatively low temperature, it is possible to prevent thermal influence other than the cold-worked portion.

  Furthermore, in patent document 4, C: 0.20-0.60%, Si: 0.05-1.50%, Mn: 0.30-2.0%, Cr: 1.5% by mass% Hereinafter, Al: 0.001 to 0.06%, N: 0.0030 to 0.0300% are included, and optionally, V: 0.010 to 0.50%, B: 0.0005 to 0. Forging a steel containing one or more elements selected from 0050%, S: 0.10% or less, Pb: 0.30% or less, Bi: 0.30% or less, Ca: 0.01% or less It has been disclosed that after obtaining an intermediate forged product having a ferrite-pearlite structure, further forging can be applied to increase the partial mechanical strength in the temperature range of Ar1 point or lower to 200 ° C in the cooling process. . Warm forging refines crystal grains, increases dislocation density by further processing, and synergizes aging strengthening effects to intensively increase the yield strength (yield ratio) and fatigue strength of warm forging sites. States that you get.

JP 2006-52432 A JP 2005-59013 A JP-A-11-131134 JP 2003-55714 A

  As described above, in the method of manufacturing a mechanical part that increases the partial mechanical strength by applying strain aging treatment to a portion requiring strength, in order to be able to further improve the mechanical strength of the necessary portion while being excellent in machinability. It is preferable that the difference in mechanical strength before and after the strain aging treatment is larger.

  The present invention has been made in view of the above situation, and the object of the present invention is to provide a mechanical method for increasing the partial mechanical strength by applying strain aging treatment to a portion requiring strength. An object of the present invention is to provide a steel for machine parts that is excellent in workability and can further increase the mechanical strength of a necessary portion, and a non-heat treated steel machine part using the same.

  The steel for machine parts according to the present invention is a steel for non-heat treated steel machine parts made of ferritic / pearlite non-heat treated steel and provided with a strengthened part by strain aging treatment, and in mass%, C: 0.20 -0.60%, Si: ≤1.5%, Mn: 0.30-1.50%, P: 0.01-0.20%, S: 0.01-0.20%, Cu: ≤ 1.0%, Ni: ≦ 1.0%, Cr: ≦ 1.0%, V: ≦ 0.10%, Ti: ≦ 0.20%, N: 0.005 to 0.030% The balance consists of Fe and impurities, and the mass% of the element M is represented by [M], and Ceq = [C] + 0.03 × [Si] + 0.55 × [Mn] + 0.26 × [P] +0.08 × [Cu] + 0.07 × [Ni] + 0.09 × [Cr] + 1.2 × [V] + 0.51 × ([Ti] −3.43 × [N]) − 0.7 × [S] , E 1 = −0.18 × [C] + 0.061 × [Mn] −0.178 × [S] + 0.502 × [V] + 0.354 × [Ti] +0.629, Eq2 = 260.6 + 85 × [ C] −49.3 × [Si] + 27 × [Mn] −288.8 × [V] −100.7 × [Ti], (806.6 × Ceq + 135.0) × Eq1 + Eq2 ≧ 700 It is characterized by.

  According to this invention, while being excellent in machinability, the mechanical component which further improved mechanical strength can be obtained by performing a strain aging process in a required location.

  The non-heat treated steel machine part according to the present invention is a non-heat treated steel machine part made of ferritic / pearlite non-heat treated steel and provided with a strengthened part by strain aging treatment, and in mass%, C: 0.20 -0.60%, Si: ≤1.5%, Mn: 0.30-1.50%, P: 0.01-0.20%, S: 0.01-0.20%, Cu: ≤ 1.0%, Ni: ≦ 1.0%, Cr: ≦ 1.0%, V: ≦ 0.10%, Ti: ≦ 0.20%, N: 0.005 to 0.030% The balance consists of Fe and impurities, and the mass% of the element M is represented by [M], and Ceq = [C] + 0.03 × [Si] + 0.55 × [Mn] + 0.26 × [P] +0.08 × [Cu] + 0.07 × [Ni] + 0.09 × [Cr] + 1.2 × [V] + 0.51 × ([Ti] −3.43 × [N]) − 0.7 × [S] , E 1 = −0.18 × [C] + 0.061 × [Mn] −0.178 × [S] + 0.502 × [V] + 0.354 × [Ti] +0.629, Eq2 = 260.6 + 85 × [ C] −49.3 × [Si] + 27 × [Mn] −288.8 × [V] −100.7 × [Ti], (806.6 × Ceq + 135.0) × Eq1 + Eq2 ≧ 700 The strengthened portion is characterized by giving a yield strength increase of 200 MPa or more with 0.2% proof stress.

  According to this invention, it is excellent in machinability and can further increase the mechanical strength.

  In the above-described invention, the hardness may be 25 HRC or less except for the reinforced portion. According to this invention, machinability can be further improved.

  In the above-described invention, the strengthened portion may have a yield strength of 700 MPa or more with a 0.2% proof stress. According to this invention, the mechanical strength can be further increased.

It is a flowchart which shows the manufacturing process of the non-heat-treated steel machine part by this invention. It is a chart of a component composition of steel of an example and a comparative example. It is a perspective view which shows the external appearance in a part of manufacturing process of a test piece. It is a one part heat processing diagram of the manufacturing process of a test piece. It is a figure which shows 0.2% yield strength after a strain aging process when (a) processing temperature and (b) processing rate are changed. It is a graph of FIG. It is a list figure of the test result of an Example and a comparative example.

[Method of manufacturing non-heat treated steel machine parts]
First, a typical method for manufacturing a non-heat treated steel machine part targeted in the present application will be described with reference to FIG.

  As shown in FIG. 1, a steel material for a machine part steel having a predetermined composition is heated (S1), and a rough shape of a non-tempered steel machine part is obtained by hot forging (S2). Such a rough shape is determined in consideration of a strain amount to be applied in a strain aging process described later.

  Next, deformation processing is applied to the strengthened portion that is intended to increase the mechanical strength within a predetermined temperature range while cooling it (strain aging treatment, S3). After cooling, it is further machined as necessary (S4) to obtain a non-tempered steel machine part having a ferrite / pearlite structure. In addition, in the non-strengthened part where processing (working strain) is not given by the strain aging treatment, the hardness is preferably 25 HRC or less from the viewpoint of machinability. The strain aging treatment S3 may be performed on the material once cooled after hot forging. In this case, after reheating, a deformation process is applied and cooled in a predetermined temperature range, or an aging process is performed by applying a deformation process at room temperature.

[Confirmation test of strain aging treatment]
Next, a tensile test piece was prepared by imitating the above-described manufacturing method, and the influence of the processing conditions in the strain aging treatment was confirmed. Such a test will be described. Here, two types of steel having the component compositions of Examples 1 and 2 shown in FIG. 2 were used.

  First, a method for creating a test piece will be briefly described with reference to FIG. 3 and a test method.

  Steel having the component composition shown in Examples 1 and 2 in FIG. 2 was melted and ingot-formed, and some steel materials having a round bar shape of 32 to 36D were obtained by rolling and forging. Each steel material was machined into a round bar shape of 32D × 90L (see FIG. 3A).

  In addition, referring to FIG. 4, the machined steel was heated and held at 1240 ° C. for 30 minutes, and hot forged into a plate-like body of 18T × 40W × 96L at 1150 ° C. (see FIG. 3 (b)). . The processing rate in such hot forging is 40%. Here, about one steel material, it air-cooled to room temperature as it was, the plate-shaped hot forging body was obtained, and the tensile test piece was cut out (reference test piece).

  The rest of the steel material is forged at a processing rate of 3 to 30% in a temperature range of 200 to 600 ° C. in the middle of air-cooling to obtain a plate-like body of 12.6 to 17.5 T × 40 W × 96 L, and then room temperature. (See FIG. 3C in particular). Tensile test pieces were cut out from the plate-like bodies after the strain aging treatment (test pieces after the strain aging treatment).

  About the tensile test piece, 0.2% yield strength was measured by the uniaxial tension test. The measurement results are shown in FIG. Here, FIG. 5A shows the measurement result of 0.2% proof stress when the processing rate in the strain aging treatment is constant at 15% and the processing temperature is changed in the range of 200 to 600 ° C. . FIG. 5B shows a measurement result of 0.2% proof stress when the processing temperature is constant at 500 ° C. and the processing rate is changed in the range of 0 to 30%. “Processing rate 0%” refers to the reference test piece. No. Reference numerals 1 and 2 are signs indicating that steel materials having the component compositions of Examples 1 and 2 in FIG. 2 were used, respectively.

  The test results will be described. Here, FIG. 6 is a graph of the test results shown in FIG. As shown in FIG. 1, no. In both cases, the processing temperature peaked around 300 ° C., and 0.2% proof stress was lowered on the higher temperature side. Compared to 461 MPa and 508 MPa of the reference test piece (see FIG. 5B, processing rate 0%), the 0.2% yield strength is the lowest 0.2%, which is about 150 MPa for the test piece after strain aging treatment at a processing temperature of 600 ° C. % Yield strength has been increased. On the other hand, the 0.2% yield strength increase of 250 MPa or more was obtained in the test piece after strain aging treatment at a processing temperature of 500 ° C. having the next lowest 0.2% yield strength. Based on such results, the processing temperature is preferably 200 to 500 ° C. In addition, while increasing the 0.2% proof stress in the strain aging treatment, on the other hand, considering the decrease in workability accompanying an increase in deformation resistance, a processing temperature of 400 ° C. is more preferable as the standard processing condition.

  In addition, as shown in FIG. 1, no. In both cases, the 0.2% yield strength can be increased as the processing rate is increased. In addition, the increase rate of 0.2% yield strength was 137 MPa and 110 MPa at a processing rate of 3%, whereas the increase rate of 0.2% yield strength was 239 MPa and 192 MPa at a processing rate of 7%. . Based on such results, the processing rate is preferably 7 to 30%. However, if the processing rate exceeds 15%, the increase in 0.2% proof stress becomes small with respect to the increase in the processing rate. In addition, while increasing the 0.2% proof stress in the strain aging treatment, on the other hand, considering the workability reduction accompanying the increase in the processing rate, a processing rate of 15% is more preferable as the standard processing condition.

[Mechanical testing of non-tempered steel machine parts]
Subsequently, a hardness test and a tensile test were performed on the steels having the composition of Examples 1 to 13 and Comparative Examples 1 to 5 shown in FIG. A method for preparing a test piece and a test method will be described.

  Similar to the confirmation test described above, a reference test piece having a component composition shown in FIG. 2 and a test piece after strain aging treatment were prepared. However, the strain aging treatment was performed under the processing conditions of a processing temperature of 500 ° C. and a processing rate of 15%.

  In the tensile test, 0.2% proof stress was measured by a uniaxial tensile test. FIG. 7 summarizes the 0.2% yield strength increase of the test piece after strain aging treatment with respect to the reference test piece. In the hardness test, Rockwell hardness was measured at the shoulder of the reference specimen. FIG. 7 shows the result.

  The test results will be described. As shown in FIG. 7, the hardness of the reference test pieces of Examples 1 to 13 is 16.4 to 25.0 HRC, and all have good machinability as a non-strengthened part of the non-heat treated steel machine part. Have. In addition, the 0.2% proof stress of the test piece after strain aging treatment is 725 to 900 MPa, and the increase range of the 0.2% proof stress is 235 to 317 MPa. Was obtained. That is, if it is steel for machine parts which has a component composition of Example 1 thru | or 13, it will be machinability in a non-strengthened part by giving the strain aging treatment by processing conditions with a processing temperature of at least 500 ° C and a processing rate of 15%. It is possible to obtain a non-heat treated steel machine part which is excellent and has a higher mechanical strength at the strengthened portion.

By the way, this inventor is the test piece after a strain aging treatment with respect to content of C, Si, Mn, P, Cu, Ni, Cr, V, Ti, N, and S about a series of steel containing Examples 1-13. Regression calculation was performed for 0.2% proof stress of Ycal to estimate 0.2% proof stress in the strengthened part of the specimen after strain aging treatment or the non-heat treated steel machine part. That is,
Ycal = (806.6 × Ceq + 135.0) × Eq1 + Eq2
It is. At this time, when the mass% of the element M is [M], Ceq, Eq1 and Eq2 are respectively given by the following equations.
Ceq = [C] + 0.03 × [Si] + 0.55 × [Mn] + 0.26 × [P] + 0.08 × [Cu] + 0.07 × [Ni] + 0.09 × [Cr] +1.2 * [V] +0.51 * ([Ti] -3.43 * [N])-0.7 * [S]
Eq1 = −0.18 × [C] + 0.061 × [Mn] −0.178 × [S] + 0.502 × [V] + 0.354 × [Ti] +0.629
Eq2 = 260.6 + 85 × [C] −49.3 × [Si] + 27 × [Mn] −288.8 × [V] −100.7 × [Ti]

  Ceq is the carbon equivalent. In the Ycal equation, (806.6 × Ceq + 135.0) × Eq1 is an estimate of the 0.2% proof stress of the reference specimen, that is, the 0.2% proof stress of the non-strengthened part of the non-heat treated steel machine part. It is a term to do. Furthermore, Eq2 was subjected to the same strain aging treatment, that is, the 0.2% proof stress increase range of the specimen after strain aging treatment given the strain aging treatment under the working condition of the processing temperature of 500 ° C. and the processing rate of 15%. This is a term for estimating an increase in 0.2% proof stress for a strengthened part of a non-heat treated steel machine part.

Here, when Ycal is applied to the above-described Examples 1 to 13, it is possible to accurately estimate the 0.2% proof stress increase range and the 0.2% proof stress of the test piece after strain aging treatment. Both Eq2 and Ycal of Examples 1 to 13 satisfy one target value, for example, 200 MPa or more and 700 MPa or more. That is, by adjusting the content of C, Si, Mn, P, Cu, Ni, Cr, V, Ti, N, and S, the 0.2% proof stress of the reference test piece, 0 of the test piece after strain aging treatment .2% proof stress increase range, 0.2% proof stress of strain-aged specimens can be adjusted and meet one target value of 700Mpa or more for 0.2% proof stress of specimens after strain aging treatment Can do. That is, it is a case where the following formula 1 is satisfied.
Ycal = (806.6 × Ceq + 135.0) × Eq1 + Eq2 ≧ 700 (Formula 1)

  Subsequently, test results in Comparative Examples 1 to 5 will be described.

  Comparative Example 1 has the same component composition as Examples 1 to 13, but Ycal was 695, which did not satisfy Formula 1, and the 0.2% proof stress of the test piece after strain aging treatment was 695 MPa, which was described above. Was below one target value.

  In Comparative Example 2, the S content was higher than in Examples 1 to 13, Ycal was 694, which did not satisfy Formula 1, and the 0.2% proof stress of the test piece after strain aging treatment was 698 MPa. That is, it was below the above one target value.

  In Comparative Example 3, the Si content was higher than in Examples 1 to 13, Ycal was 696, which did not satisfy Formula 1, and the 0.2% proof stress of the test piece after strain aging treatment was 696 MPa. That is, it was below the above one target value.

  In Comparative Example 4, the content of C was larger than in Examples 1 to 13, Ycal was 697, which did not satisfy Formula 1, and the 0.2% proof stress of the test piece after strain aging treatment was 697 MPa. That is, it was below the above one target value.

  In Comparative Example 5, the Cr content was higher than in Examples 1 to 13, but Ycal was 793, which satisfied Equation 1, but the 0.2% proof stress of the strain-aged test piece was 693 MPa. That is, it was below the above one target value. This is because Cr has changed the structure from ferrite and pearlite to a structure containing a large amount of bainite.

  As described above, according to the present embodiment, it is possible to obtain steel for machine parts that is excellent in machinability and can further increase the mechanical strength by performing strain aging treatment at a necessary location. Moreover, the non-tempered steel machine part which has the reinforcement | strengthening part which performed the strain aging process in this steel for machine parts, and raised the mechanical strength more can be obtained. In addition, even when it passed through the above-mentioned process of cooling once after hot forging and performing a strain aging treatment, it was confirmed that the 0.2% yield strength improvement equivalent to the above-mentioned Example can be obtained.

  Note that, as in Comparative Example 1 and Comparative Example 3, when the value of Eq2 is small, the 0.2% yield strength increase in the strain aging treatment is also small. Moreover, when Eq2 becomes too large, the yield strength ratio becomes low and the hardness becomes high, resulting in inferior machinability. Therefore, at least under the above processing conditions, the value of Eq2 is preferably in the range of 250 to 350.

  By the way, the range of the component composition considered as the steel for machine parts and the non-heat treated steel machine parts described so far is determined by the following guidelines.

  C is necessary to ensure the mechanical strength as a non-heat treated steel machine part, particularly the proof strength of the strengthened part. On the other hand, if contained excessively, the hardness is excessively increased, and the machinability as the steel for machine parts or the machinability at the non-strengthened part of the non-tempered steel machine part is lowered. Therefore, the content of C is mass% and is in the range of 0.20 to 0.60%.

  Si reduces the 0.2% proof stress increase range due to strain aging treatment, and excessively increases the deformation resistance in hot and warm conditions when it is excessively contained, so that the die in hot forging and strain aging treatment Will reduce the lifespan. Therefore, the Si content is in mass% and is in the range of 1.5% or less.

  Mn is necessary to ensure the proof stress as a non-tempered steel machine part. On the other hand, if contained excessively, the production of bainite is promoted to reduce the yield strength, and the machinability as the steel for machine parts or the machinability as the non-strengthened part of the non-tempered steel machine parts is lowered. Therefore, the content of Mn is mass% and is in the range of 0.30 to 1.50%.

  P is added in order to ensure the proof stress required as a non-heat treated steel machine part. On the other hand, if it is excessively contained, castability is lowered. Therefore, the content of P is in mass% and is in the range of 0.01 to 0.20%.

  S forms sulfides and enhances machinability as steel for machine parts or as a non-strengthened part of non-tempered steel machine parts. On the other hand, if it is excessively contained, the productivity is lowered. Therefore, the S content is in the range of 0.01 to 0.20% by mass%.

  Cu, Ni and Cr can be contained in the steel for machine parts as impurities. When these elements are contained excessively, the formation of bainite is promoted and the yield strength is lowered, and the machinability as a steel for machine parts or the machinability as a non-strengthened part of non-tempered steel machine parts is lowered. End up. Therefore, the contents of Cu, Ni and Cr are each in the range of 1.0% or less by mass%.

  V can improve the yield strength of non-tempered steel machine parts by precipitation strengthening by precipitation of V carbonitride. On the other hand, when it adds excessively, cost can be increased. Therefore, the content of V is in mass% and is in the range of 0.10% or less.

  Ti can improve the yield strength of non-tempered steel machine parts by precipitation strengthening by precipitation of Ti carbonitride. On the other hand, when it adds excessively, cost can be increased. Therefore, the Ti content is in mass% and is in the range of 0.20% or less.

  N can be included in the steel to a certain extent. 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 0.005 to 0.030%.

  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)

A steel for non-tempered steel machine parts made of ferritic pearlite non-tempered steel and given a strengthened part by strain aging treatment,
In mass%, C: 0.20 to 0.60%, Si: ≦ 1.5%, Mn: 0.30 to 1.50%, P: 0.01 to 0.20%, S: 0.01 ˜0.20%, Cu: ≦ 1.0%, Ni: ≦ 1.0%, Cr: ≦ 1.0%, V: ≦ 0.10%, Ti: ≦ 0.20%, N: 0.0. 005 to 0.030%, with the balance being Fe and impurities,
The mass% of the element M is represented by [M].
Ceq = [C] + 0.03 × [Si] + 0.55 × [Mn] + 0.26 × [P] + 0.08 × [Cu] + 0.07 × [Ni] + 0.09 × [Cr] +1.2 * [V] +0.51 * ([Ti] -3.43 * [N])-0.7 * [S]
Eq1 = −0.18 × [C] + 0.061 × [Mn] −0.178 × [S] + 0.502 × [V] + 0.354 × [Ti] +0.629
Eq2 = 260.6 + 85 × [C] −49.3 × [Si] + 27 × [Mn] −288.8 × [V] −100.7 × [Ti]
Then,
(806.6 × Ceq + 135.0) × Eq1 + Eq2 ≧ 700
Steel for machine parts, characterized by Non-heat treated steel machine parts made of ferritic pearlite non-heat treated steel and given a strengthened part by strain aging treatment,
In mass%, C: 0.20 to 0.60%, Si: ≦ 1.5%, Mn: 0.30 to 1.50%, P: 0.01 to 0.20%, S: 0.01 ˜0.20%, Cu: ≦ 1.0%, Ni: ≦ 1.0%, Cr: ≦ 1.0%, V: ≦ 0.10%, Ti: ≦ 0.20%, N: 0.0. 005 to 0.030%, with the balance being Fe and impurities,
The mass% of the element M is represented by [M].
Ceq = [C] + 0.03 × [Si] + 0.55 × [Mn] + 0.26 × [P] + 0.08 × [Cu] + 0.07 × [Ni] + 0.09 × [Cr] +1.2 * [V] +0.51 * ([Ti] -3.43 * [N])-0.7 * [S]
Eq1 = −0.18 × [C] + 0.061 × [Mn] −0.178 × [S] + 0.502 × [V] + 0.354 × [Ti] +0.629
Eq2 = 260.6 + 85 × [C] −49.3 × [Si] + 27 × [Mn] −288.8 × [V] −100.7 × [Ti]
Then,
(806.6 × Ceq + 135.0) × Eq1 + Eq2 ≧ 700
Because
A non-tempered steel machine part characterized by giving a yield strength increase of 200 MPa or more at 0.2% proof stress in the strengthened portion.   The non-heat treated steel machine part according to claim 2, wherein the hardness is 25HRC or less except for the strengthened portion.   The non-heat treated steel machine part according to claim 3, wherein the reinforced portion has a yield strength of 700 MPa or more with a 0.2% proof stress.