JP3898179B2 - Manufacturing method of non-tempered steel parts and non-tempered steel parts using the same - Google Patents

Description

  The present invention relates to a method for producing a non-heat treated steel part and a non-heat treated steel part using the same.

JP 2003-55714 A

  Mechanical structural parts such as automobiles and construction machines are required to have strength and toughness, and have conventionally been manufactured by quenching and tempering after hot forging. For example, automotive engine parts, undercarriage parts, and other structural parts are formed by hot forging using carbon steel or alloy steel for mechanical structure, and then subjected to machining and tempering treatment. The desired shape and strength were obtained.

  However, the tempering treatment not only increases the number of heat treatments such as quenching and tempering, but also tends to cause distortion and tempering. It is uneconomical because it requires a complicated process. Therefore, in recent years, in order to save energy required for such tempering treatment, reduce costs, and simplify the manufacturing process, non-tempered steel that can be used as it is in hot forging has been developed. Many of such non-tempered hot forging steels are precipitation hardened non-tempered steels obtained by adding a small amount of so-called precipitation hardening alloying elements V, Nb, Ti, Zr, etc. to medium carbon steel, In the cooling step after hot forging, these are precipitated in the form of carbide, nitride, carbonitride, etc., and high strength is obtained by precipitation hardening.

  In particular, ferrite + pearlite type non-heat treated steel with a small amount of V added to medium carbon steel (C content 0.2 to 0.5 mass%) has a relatively high machinability and reduces machining costs. Since it is advantageous in planning, it is widely used for machine structural parts. This non-tempered steel is cooled after hot forging to a ferrite + pearlite structure, and V or Nb carbide or nitride (or carbonitride) is finely precipitated in the ferrite part to obtain a desired strength. is there. This eliminates the need for quenching and tempering after hot forging, simplifies the straightening process, and further reduces the number of defective products due to fire cracking, enabling a significant reduction in manufacturing costs. To do.

  When manufacturing automobile parts and the like by forging ferrite + pearlite type non-heat treated steel, hot forging is performed at a high temperature of about 1100 to 1200 ° C. For example, a connecting rod used for an automobile engine is formed by hot forging (about 1200 ° C.) of V-added non-tempered steel (S40VC). However, such high-temperature hot working tends to cause coarsening of crystal grains due to recrystallization, resulting in a decrease in yield strength and toughness. For example, in the case of yield strength, the limit is substantially 500 to 700 Pa. In addition, if the forging temperature is lowered, the ferrite + pearlite structure becomes finer, but the ferrite fraction increases, so the hardness tends to decrease, and the connecting rod with a small cross-sectional area that requires light weight is reliable. May lead to a decline.

  Therefore, in Patent Document 1, after hot forging steel to obtain an intermediate forged product having a ferrite + pearlite structure, forging is further performed in a temperature range of Ar 1 point or lower to 200 ° C. to obtain high yield strength and steel fatigue. A forged product of non-tempered steel having a degree is disclosed.

  However, the forged product of the non-heat treated steel of Patent Document 1 is difficult to increase the processing rate because the forging temperature is low, and in recent automobile applications where lighter and higher strength is required, There is a drawback that lack of toughness tends to be a problem. In addition, forcibly increasing the processing rate causes a problem that increases the number of defects such as processing cracks.

  An object of the present invention is to provide a method for producing a non-heat treated steel part capable of adopting a large processing rate, having a high production efficiency, and ensuring a high yield strength, and a non-heat treated steel part using the same. It is in.

Means for Solving the Problems and Effects of the Invention

In order to solve the above problems, the first of the methods for producing a non-tempered steel part of the present invention is:
Fe: 95% by mass or more, C: 0.18% by mass to 0.45% by mass, Si: 0.10% by mass to 2.00% by mass, Mn: 0.40% by mass to 1.80% by mass Hereinafter, steel containing Cr: 0.05 mass% or more and 0.50 mass% or less, Al: 0.003 mass% or more and 0.040 mass% or less, N: 0.005 mass% or more and 0.020 mass% or less Is austenitized by heating to 1000 ° C. or higher and 1300 ° C. or lower, after which the Fe-based phase becomes an austenite single phase and the pre-processing temperature set to 1300 ° C. or lower is Tp, and the average strain during the pre-processing is ε As
Xr≡1-exp (−ε × (750 / Tp) ⁻¹⁰ )> 0.8 (1)
Preliminary processing is performed by setting Tp and ε so as to satisfy the above condition, and further, forging a preliminary processing body obtained by the preliminary processing at 680 ° C. or higher and 850 ° C. or lower to obtain a part shape And the processing rate of the main processing is 30% or more in terms of compression processing rate.

  In the above-described method of the present invention, the austenitization of the steel material is performed at a high temperature of 1000 ° C. or higher and 1300 ° C. or lower in order to sufficiently dissolve carbide, particularly V carbide (contributing to precipitation strengthening of the final member) into the matrix. If the heat treatment is too low, austenite cannot be formed, and precipitates such as carbides cannot be sufficiently dissolved. If the heat treatment is too high, the austenite grains are likely to be coarsened. Therefore, the heat treatment temperature is set to 1000 ° C. or higher and 1300 ° C. or lower (desirably 1000 ° C. or higher and 1250 ° C. or lower).

Next, by restricting the alloy composition of the steel as described above, and further optimizing the forging conditions in the preliminary processing so as to obtain an Xr value satisfying the expression (1), the number of nucleation numbers of recrystallization is obtained. And the growth rate are appropriately balanced, and the austenite structure after preliminary processing can be refined. Here, the average strain ε of the pre-processing is defined as ε≡ where Q (%) is the area reduction rate of the portion having the smallest diameter when the cross section of the round bar-shaped material subjected to the pre-processing is reduced by the pre-processing. (Q / 100) Defined with ^1/2 . In addition, the cross-sectional area before forging of the round bar-shaped material is A, and the area excluding burrs of the minimum cross-section after forging is B, and the area reduction rate Q (%) is
Q = (A−B) / A × 100
Defined by

  The Xr value is a value determined by the average strain ε and the preliminary processing temperature Tp as shown in the equation (1), and has a meaning as an index reflecting the degree of recrystallization of the material. Specifically, in the range of Xr> 0.8, austenite grain refinement by recrystallization can be made remarkable. The more appropriate temperature for the preliminary processing depends on the value of the average strain ε, but for example, when ε = 0.4, the preliminary processing temperature Tp is about 900 to 1050 ° C. FIG. 2 is a graph showing how the Xr value varies depending on the average strain ε at various preliminary processing temperatures Tp. In order to achieve an Xr value of 0.8 or higher, preliminary processing at approximately 900 ° C. or higher is required. When carbides such as V are precipitated during the pre-processing, the pre-processing itself is performed at a certain high temperature, so that the carbide grows rapidly and becomes coarse (these do not contribute to precipitation strengthening), and during cooling after the main processing, Sufficient carbides cannot be precipitated, leading to a decrease in yield strength. Therefore, it is necessary to set the preliminary processing temperature Tp to be equal to or higher than the precipitation temperature of the carbide (for example, 930 ° C. or higher). Further, in order to suppress coarse carbide precipitation, it is desirable to change directly from the austenitizing temperature to the pre-processing temperature, and at least not lower than the carbide precipitation temperature.

  Next, the main processing applied to the preliminary processed body is performed by compression processing with a processing rate of 30% or more. The reason why the compression processing is performed is that since the shear strain enters in the 45 ° direction, the crystal grains are divided and further refined. Then, by setting the compression processing rate to 30% or more, the yield strength is reduced to, for example, 30 compared with the conventional one by remarkably miniaturizing the crystal grains (depending on the temperature, moderate work hardening may contribute). % Or more can be increased. Here, the compression rate is defined as the rate of change of the diameter that has changed most in the compression direction in the cross section that has become the smallest diameter in the preliminary processing. In other words, the diameter of the thinnest cross section is C, and the length after compression is D, (C−D) / C × 100 (%).

  In the case of steel materials with the indicated composition, it is very difficult to perform compression processing at a processing rate as high as 30% or more due to cracking etc. in cold processing, and processing to reduce processing uniformity and deformation resistance. It is essential to set the temperature to 680 ° C. or higher. In particular, in the temperature range exceeding the pearlite transformation point (for example, 730 ° C. or higher), the austenite phase is the main component, so that the deformation resistance can be reduced, and the crystal grain breaking due to shear strain and thus the grain refinement effect can be made more prominent. . However, if the forging temperature exceeds 850 ° C., the work strain is likely to be lost due to recovery, and sufficient yield strength cannot be obtained.

  Next, in the present invention, V: 0.05% by mass or more and 0.40% by mass or less can be contained in the above-described steel material. By adding V, the proof stress of the member can be improved.

  Moreover, P: 0.03 mass% or more and 0.15 mass% or less can be contained in the above-mentioned steel materials. By adding P, the strength of the ferrite phase is increased, so that the yield strength of the member can be improved.

  Furthermore, Nb: 0.02 mass% or more and 0.10 mass% or less can be contained in the above-mentioned steel materials. By adding Nb, crystal grains can be refined, and the proof stress is further improved.

  Furthermore, in the present invention, S: 0.03 mass% to 0.20 mass%, Ca: 0.001 mass% to 0.010 mass%, Bi: 0.01 mass%, in addition to the steel material composition described above. One or two or more selected from 0.10% by mass or less and Pb: 0.02% by mass or more and 0.20% by mass or less can also be contained. By adding S, Ca, Bi, and Pb, the machinability of the steel material can be improved. Thereby, the workability of the manufactured non-heat treated steel part can be improved.

Hereinafter, the reasons for limiting the composition of the steel material in the present invention will be described.
C: 0.18% by mass or more and 0.45% by mass or less C is an element that determines the strength depending on the addition amount. In the present invention, C is contained by 0.18% by mass or more in order to obtain high strength. However, if the content exceeds 0.45% by mass, the pearlite grains become coarse and the toughness is significantly reduced, so the upper limit is made 0.45% by mass.

Si: 0.10% by mass or more and 2.00% by mass or less In addition to acting as an oxygen scavenger, it effectively dissolves in the ferrite structure and effectively improves yield strength and fatigue strength. For this reason, it is necessary to contain 0.10 mass% or more. However, if the amount is too large, a bainite structure is likely to occur during cooling, leading to a decrease in machinability, and the amount of decarburization after hot forging tends to increase. Preferably, it is 1.80 mass%.

Mn: 0.40% by mass or more and 1.80% by mass or less Mn reduces the transformation temperature, reduces the pearlite lamellar spacing, improves the pearlite hardness, and also increases the strength and toughness after hot forging. Therefore, 0.40% by mass or more is contained for securing the strength. However, if it is contained excessively, a bainite structure is likely to occur during cooling, and the machinability deteriorates. Therefore, it is 1.80 mass% or less. Preferably, it is 1.50 mass%.

Cr: 0.05 mass% or more and 0.50 mass% or less Cr has the effect | action which increases the carbon concentration of the steel surface, and acts on the wear resistance improvement of a member, etc. However, if it is contained excessively, a bainite structure is likely to be formed during cooling, and the machinability is lowered, so the content is made 0.50% by mass or less. Preferably, it is 0.30 mass%.

Al: 0.003 mass% or more and 0.040 mass% or less It acts as a deoxygenated atom, forms AlN by combining with N, suppresses coarsening of austenite grains, and contributes to refinement of the structure. However excessive effect contained is saturated, that Al ₂ O ₃ is increased, and 0.040 mass% or less so leading to machinability decrease. It may be contained as an inevitable impurity due to a deoxidizer or the like.

N: 0.005 mass% or more and 0.020 mass% or less Combines with Al to form AlN, suppresses coarsening of austenite grains, and contributes to refinement of the structure. Further, it forms nitrides or carbonitrides with V, Ti, Nb, etc. and contributes to strengthening of ferrite. Addition of 0.020% by mass or more is difficult. On the other hand, it may be contained as an inevitable impurity.

V: 0.05 mass% or more and 0.40 mass% or less V precipitates as carbide, nitride, or carbonitride in the ferrite transformed from austenite, and strengthens the ferrite. The crystal grains are refined. However, even if it contains excessively, an effect will be saturated and toughness will be reduced, so it shall be 0.40 mass% or less.

P: 0.03 mass% or more and 0.15 mass% or less P is an element effective for ensuring the strength of the ferrite phase and improving the machinability, but if added excessively, the toughness deteriorates and the impact In addition to reducing the strength, it also causes grain boundary segregation, so the upper limit is made 0.15% by mass. On the other hand, it may be contained as an inevitable impurity.

Nb: 0.02% by mass or more and 0.10% by mass or less Nb precipitates as carbonitride in ferrite transformed from austenite and strengthens the ferrite. The crystal grains are refined. However, even if Nb is contained excessively, the effect is saturated and the solid solubility is low, which may lead to a decrease in strength. Therefore, the upper limit is 0.10% by mass.

S: 0.03 mass% or more and 0.20 mass% or less S is an element that has an effect of improving machinability, but if contained excessively, fatigue strength decreases and forgeability is inhibited, so 0.20 mass. % Is the upper limit. On the other hand, it may be contained as an inevitable impurity.

Ca: 0.001% by mass or more and 0.010% by mass or less Ca is an element that improves machinability, but even if contained excessively, the effect is saturated, so 0.010% by mass is made the upper limit.

Bi: 0.01% by mass or more and 0.10% by mass or less Bi is an element that improves machinability, but excessive addition degrades toughness and saturates the effect, so the upper limit is 0.10% by mass. And

Pb: 0.02% by mass or more and 0.20% by mass or less Pb is an element that improves machinability, but even if contained excessively, the effect is saturated, so 0.20% by mass is the upper limit.

The second of the production methods of the present invention is
Fe: 95% by mass or more, C: 0.18% by mass to 0.45% by mass, Si: 0.10% by mass to 2.00% by mass, Mn: 0.40% by mass to 1.80% by mass Hereinafter, Cr: 0.05% by mass or more and 0.50% by mass or less, Al: 0.003% by mass or more and 0.040% by mass or less, Ti: 0.02% by mass or more and 0.10% by mass or less, N: 0 A steel material containing 0.005 mass% or more and 0.030 mass% or less is heated to 1000 ° C. or more and 1300 ° C. or less to be austenitized, and then the Fe-based phase becomes an austenite single phase and is set to 1300 ° C. or less. The pre-processing temperature is Tp, and the average strain during pre-processing is ε,
1-exp (−ε × (750 ÷ Tp) ⁻¹⁰ )> 0.8
Preliminary processing is performed by setting Tp and ε so as to satisfy the above condition, and further, forging a preliminary processing body obtained by the preliminary processing at 680 ° C. or higher and 850 ° C. or lower to obtain a part shape And the processing rate of the main processing is 30% or more in terms of compression processing rate.

  In order to manufacture a high yield strength non-heat treated steel part, it is effective to refine the crystal grains. In the second manufacturing method, Ti is added. Since Ti and N form TiN, when adding Ti, it is desirable to increase N to be added. Therefore, N is added in an amount of 0.005 mass% to 0.030 mass%. By adding Ti or N, crystal grains can be refined or crystal grain refinement by AlN can be ensured. About another point, since it is the same as that of the 1st manufacturing method, it abbreviate | omits.

Ti: 0.02% by mass or more and 0.10% by mass or less Ti combines with N to form TiN, and has the effect of refining crystal grains using this as a nucleus, which contributes to further improvement in yield strength. However, if added in a large amount, the cleanliness of the steel is impaired and the effect is saturated, so the upper limit is made 0.10% by mass.

N: 0.005 mass% or less 0.030 mass% or less Since nitrogen is fixed by addition of Ti, it can be added up to 0.030 mass%, which is higher than that of the first invention.

  The non-tempered steel part of the present invention can be realized by any of the above-described manufacturing methods, and consists of a ferrite + pearlite structure having an average crystal grain size of 5 μm or less. As a result, parts having higher proof stress than the conventional non-heat treated steel parts disclosed in Patent Document 1 and the like are realized (the lower limit of the average grain size that can be achieved under common-sense hot forging process conditions is about 50 μm). is there). The average crystal grain size means the average of the maximum length of each crystal grain in a cross section perpendicular to the forging direction. In addition, since the pearlite phase is hard, it contributes to improved yield strength, and the ferrite phase is soft, which contributes to improvement of machinability.From the viewpoint of optimizing the balance between the two, the non-tempered steel of the present invention is made of pearlite. It is desirable that the area ratio is 30% or more and 65% or less (particularly, the area ratio of ferrite and pearlite is about 50:50).

  Examples of the present invention will be described in detail. First, the production method and test method of the non-heat treated steel material of the present invention will be described. After melting inventive steels 1 to 11 and comparative steels A to C having chemical compositions shown in Table 1, round bars 10 having a diameter of 42 mm were manufactured by hot rolling as shown in FIG. 1 (a). From this round bar 10, the connecting rod 30 which is a non-tempered steel forged part shown in (c) is manufactured. The connecting rod 30 has a large end portion 33 (having a shaft insertion hole 33h), a small end portion 31 (having a shaft insertion hole 31h), and a flange portion 32 connecting the both.

  Prior to obtaining the shape of the connecting rod 30, the round bar 10 is preliminarily processed to obtain a preliminarily processed body 20 shown in FIG. The preliminary processed body 20 has a first shaft portion 21, a second shaft portion 22, and a third shaft portion 23 having a circular cross-sectional shape corresponding to the small end portion 31, the flange portion 32, and the large end portion 33 of the connecting rod 30, respectively. (The cross-sectional outer diameter decreases in the order of the third shaft portion 23, the first shaft portion 21, and the second shaft portion 22). The preliminary processing is performed by compression using the forging die 1 of FIG. The forging die 1 includes paired forging rolls 2 and 2, and as shown in an outer peripheral surface development view and a depth direction profile diagram in the figure, the outer peripheral surface of each roll 2 has a first shaft portion 21, Forging cavities 3a, 3b, 3c having widths and depths corresponding to the second shaft portion 22 and the third shaft portion 23 are formed in this order in the circumferential direction. These two rolls 2 and 2 are arranged to face each other on the outer peripheral surface in a state where the phases of the forging cavities 3a, 3b, and 3c are matched.

  After the round bar 10 is austenitized, the temperature is lowered to a predetermined preliminary processing temperature as it is, and both rolls 2, 2 are rotated at the start phase of the forging cavity 3a (first shaft portion 21) while rotating both rolls 2, 2. Inserted between. Then, the forging cavities 3a → 3b → 3c are sequentially compressed and forged to form a pre-processed body 20. Then, the preliminary processing body 20 is forged by main processing at a predetermined forging temperature, and the shape of the connecting rod 30 is obtained as shown in (c) (the shaft insertion holes 31h and 33h are formed by post-processing. Set). Each heating can be performed using high frequency heating. The cooling condition after this processing is blast cooling or air cooling.

  Table 1 shows the compositions of the inventive steels 1 to 11 of the present invention. Moreover, it shows similarly about the comparative steel A thru | or C used for the comparison. As shown in Table 1, the comparative steel A has a Mn of 0.22% by mass, which is less than that of the invention steel. In comparative steel B, Si is 0.05% by mass, which is less than that of the invention steel. Further, the comparative steel C has a C content of 0.55% by mass, which is larger than that of the inventive steel.

Table 2 shows the conditions for the preliminary processing and the main processing. As described above, the average strain ε in FIG. 1 is the portion (second shaft portion 22) having the smallest diameter due to the preliminary processing when the preliminary processing (b) is performed on the round bar 10 (a). ) The area reduction ratio of the part excluding the burr part with respect to the round bar 10 is defined as (Q / 100) ^1/2 as Q (%). The Xr value is a value determined by the equation (1) based on the average strain ε and the preliminary processing temperature Tp. The compression rate is defined as the rate of change of the diameter most changed in the compression direction with respect to the diameter of the round bar 10 in the cross section of the second shaft portion 22 that has the smallest diameter in the preliminary processing. That is, the diameter of the narrowest cross section in (b) is C, and the length after compression is D in (c), which is (C−D) / C × 100%.

  Comparative process A is a case where Xr value is as small as 0.534 using comparative steel A. Comparative process B is a case where Xr value is as small as 0.534 using comparative steel B. The comparative process C is a case where the austenitizing temperature is as high as 1280 ° C. and the forging temperature is as high as 1200 ° C. using the comparative steel C. The comparative process D is a case where the austenitizing temperature is as high as 1280 ° C. using the comparative steel B. The comparative process E is the case where the austenitizing temperature is as low as 600 ° C., the Xr value is as low as 0.018, and the forging temperature is as low as 500 ° C. using the inventive steel 1. The comparative process F is the same as the other inventive processes except that the comparative steel A is used.

  A test piece for measuring the yield strength is cut out from the non-heat treated steel connecting rod manufactured as described above, and the center smooth part has a diameter of 2.5 mm and both ends are attached to the testing machine with M4 screws. Were cut into tensile test pieces. Using this, a tensile test was performed at a speed of 10 mm / min with a testing machine in which a displacement meter and a load cell were attached to a hydraulic servo, and the yield strength (0.2%) was obtained from the displacement-load diagram. The results are shown in Table 2. In invention steps 1 to 13 using the inventive steels 1 to 11, the yield strength was 700 MPa or more. In the comparison steps A to H, the proof stress was less than 700 MPa. In the comparative process F, using the comparative steel A, the austenitizing temperature, the preliminary processing temperature, the average strain in the preliminary processing, the Xr value, the forging temperature, and the forging rate were all performed under the same conditions as in the invention process. That is, in the comparison process F, the amount of Mn added is different from that of the invention process. As a result, the yield strength was 528 MPa. Furthermore, in the comparative process A using the comparative steel A, the Xr value was reduced, but the proof stress showed a smaller value.

  In the comparison process B, since the comparison steel B is used and the Xr value is small, the proof stress is small. In comparative process D, although comparative steel B was used and the austenitizing temperature was increased, the yield strength was slightly increased to 636 MPa, but no significant improvement was observed. In the comparative process C, the comparative steel C is used to raise the austenitizing temperature and the forging temperature, but a large yield strength cannot be obtained. In the comparative process E, the inventive steel 1 is used to lower the austenitizing temperature and the forging temperature and reduce the Xr value, but the proof stress does not show a large value. Although the comparison process G has a small Xr value and the comparison process H has a small forging rate, a large yield strength cannot be obtained. However, it can be seen that when the inventive process is combined with the inventive steel, the yield strength is 700 MPa or more, indicating a large yield strength.

  FIG. 6 is an optical microscope tissue image of a connecting rod obtained in an example of the invention process, observed in a vertical section of the collar portion 32 (FIG. 1) (magnification 1000 times). The specific conditions are the case where the composition of Invention Steel 7 is adopted, austenitized at 1000 ° C., pre-processed at 970 ° C., and forged for main processing at 750 ° C. The Xr value is 1.0 and the compression rate is 70%. The pearlite area ratio is 40%. Very fine pearlite crystal grains are obtained, and the average crystal grain size is about 4 μm. As shown in FIG. 4, the average crystal grain size is obtained from an average value of E when the maximum length E is measured in the axial vertical section of each pearlite crystal grain 41 stretched by forging. On the other hand, FIG. 7 is a similar tissue image of the connecting rod obtained in the conventional process. The specific conditions are when the composition of Invention Steel 7 is adopted, austenitized at 1200 ° C., pre-processed at 1100 ° C., and forged for main processing at 1020 ° C. The Xr value is 1.0 and the compression rate is 70%. The average crystal grain size is as coarse as about 15 μm. The pearlite area ratio is 35%.

The mechanism by which the pearlite crystal grains can be refined in the process of the present invention is schematically shown in FIG. (A) shows the structure after the austenite heat treatment, and relatively large crystal grains are formed by the progress of recrystallization and Ostwald growth. By applying a preliminary process that satisfies the range of the Xr value, a large number of lattice defects such as dislocations are introduced as shown in FIG. Since this lattice defect tends to act as a nucleation site for recrystallization, the number of nuclei generated increases as the number of introduced lattice defects increases, and as a result, the recrystallized grains become finer, as shown in (c). Thus, it becomes the refined austenite crystal grain. Furthermore, when compression processing of 30% or more is applied at 680 ° C. or more and 850 ° C. or less, the austenite grains are crushed and dislocations are introduced as shown in (d). Here, since the temperature is not high enough to cause Ostwald growth, the fine grains remain. Thereafter, when below A ₃ transformation point (e), the transformed from dislocation defects in the ferrite, is formed in miniaturized form in order to divide the austenite grains. Then, when cooled below the A ₁ transformation point, residual austenite phase is pearlite transformation, is miniaturized pearlite structure as shown in (f) is obtained.

Process explanatory drawing which shows an example of this invention. The figure which shows the relationship between preliminary processing temperature, preliminary processing average distortion, and Xr value. The schematic diagram which shows an example of the forging roll used for preliminary processing. The figure explaining the concept of a crystal grain diameter. The figure explaining the mechanism of structure refinement | miniaturization by this invention. The optical microscope observation image which shows the cross-sectional structure | tissue of the member obtained in an invention process. The optical microscope observation image which shows the cross-sectional structure | tissue of the member obtained at a comparison process.

Explanation of symbols

10 Round bar 20 Pre-processed body 30 Connecting rod

Claims (9)

C : 0.18 mass% or more and 0.45 mass% or less, Si: 0.10 mass% or more and 2.00 mass% or less, Mn: 0.40 mass% or more and 1.80 mass% or less, Cr: 0.05 Contains from 0.5% by mass to 0.50% by mass, Al: 0.003% by mass to 0.040% by mass, N: 0.005% by mass to 0.020% by mass , and the balance from Fe and inevitable impurities The steel material is heated to 1000 ° C. or more and 1300 ° C. or less to be austenitized, and then the Fe-based phase becomes an austenite single phase and the pre-processing temperature set to 1300 ° C. or less is Tp, and the average strain during the pre-processing Is ε,
1-exp (−ε × (750 ÷ Tp) ⁻¹⁰ )> 0.8
Preliminary processing is performed by setting Tp and ε so as to satisfy the above conditions, and further, main processing for obtaining a part shape by forging a preliminary processing body obtained by the preliminary processing at 680 ° C. or higher and 850 ° C. or lower is performed. A method for producing a non-tempered steel part, wherein the processing rate of the main processing is 30% or more in terms of the compression processing rate. C : 0.18% by mass to 0.45% by mass, Si: 0.10% by mass to 2.00% by mass, Mn: 0.40% by mass to 1.80% by mass, Cr: 0.05 Mass% or more and 0.50 mass% or less, Al: 0.003 mass% or more and 0.040 mass% or less, Ti: 0.02 mass% or more and 0.10 mass% or less, N: 0.005 mass% or more. A steel material containing 030 mass% or less and the balance Fe and inevitable impurities is heated to 1000 ° C. or higher and 1300 ° C. or lower to be austenitized, and then the Fe-based phase becomes an austenite single phase and 1300 ° C. or lower. The pre-processing temperature set to Tp and the average strain during pre-processing as ε,
1-exp (−ε × (750 ÷ Tp) ⁻¹⁰ )> 0.8
Preliminary processing is performed by setting Tp and ε so as to satisfy the above condition, and further, forging a preliminary processing body obtained by the preliminary processing at 680 ° C. or higher and 850 ° C. or lower to obtain a part shape And a processing rate of the main processing is 30% or more in terms of compression processing rate.   The manufacturing method of the non-heat-treated steel part of Claim 1 or 2 which uses what contains V: 0.05 mass% or more and 0.40 mass% or less as said steel material.   The manufacturing method of the non-heat-treated steel part of any one of Claim 1 thru | or 3 which uses what contains P: 0.03 mass% or more and 0.15 mass% or less as said steel materials.   The method for producing a non-tempered steel part according to any one of claims 1 to 4, wherein the steel material contains Nb: 0.02 mass% or more and 0.10 mass% or less.   As the steel material, S: 0.03% by mass to 0.20% by mass, Ca: 0.001% by mass to 0.010% by mass, Bi: 0.01% by mass to 0.10% by mass, Pb The method for producing a non-tempered steel part according to any one of claims 1 to 5, wherein one containing at least one selected from 0.02 mass% to 0.20 mass% is used. .   The method for producing a non-tempered steel part according to any one of claims 3 to 6, wherein the preliminary processing is performed at a temperature higher than a precipitation temperature of V carbide.   A non-tempered steel part manufactured by the manufacturing method according to any one of claims 1 to 7, wherein the non-tempered steel part is composed of a ferrite and a pearlite structure having an average crystal grain size of 5 µm or less. Steel parts.   The non-heat treated steel part according to claim 8, wherein the area ratio of pearlite observed in the cross-sectional structure is 30% or more and 65% or less.