JP5319374B2 - Integrated crankshaft and manufacturing method thereof - Google Patents

Description

  The present invention relates to an integral crankshaft used for a marine engine or a diesel engine used for a generator, and more particularly to an integral crankshaft for a multi-cylinder engine.

  The integrated crankshaft is an engine component used to convert reciprocating power from an engine into rotational power in a small / medium-sized marine engine, a marine or on-shore power generation diesel engine, and the like. Due to recent demands for improving the energy efficiency of diesel engines, the load applied to the integrated crankshaft has increased, and high strength and high toughness are desired.

  In steel materials such as thick plates, Ni is well known as an additive element for improving strength and toughness, and utilization of Ni has been studied from the viewpoint of securing strength and toughness even in large forged steel products (Patent Documents). 1). However, when the Ni concentration exceeds 1.5%, the crystal grains of the forged steel products begin to coarsen, and when the Ni concentration exceeds 2.5%, the degree of coarsening increases considerably, which causes the problem that the toughness of the steel material decreases on the contrary. (FIG. 1 of Patent Document 1). Therefore, when aiming for high strength and high toughness, the addition amount of Ni has to be 2.5% at most.

  In addition, it is necessary that the characteristic value indicating toughness such as the absorbed energy value in the Charpy impact test is high, but in order to make the toughness more reliable, it is related to toughness (absorbed energy value). It is also desired that the crystal grain size of the deep austenite (hereinafter sometimes referred to as “γ”) grains be small. However, even if the crystal grain size is reduced, the old γ grains become coarse as the Ni concentration is increased as described above.

  On the other hand, steel materials containing 2.5% or more of Ni (for example, Patent Document 2 and Patent Document 3) are also known. However, these patent documents do not describe any old γ particle size. Patent Document 2 describes the use of a crankshaft for a press forging machine, and Patent Document 3 describes the use of a reduction gear of a reduction gear. It does not relate to an integral crankshaft for a multi-cylinder engine, but differs in the use as a steel part and the required characteristic level.

JP 2005-344149 A Japanese Patent Laid-Open No. 5-9662 Japanese Patent Laid-Open No. 55-8486

  As described above, the large crankshaft of Patent Document 1 has a problem that old γ grains become coarse when the amount of Ni added is 2.5% or more. On the other hand, if it is a small crankshaft (for example, about 15 mm in diameter) used for, for example, an automobile engine, a steel material (hereinafter referred to as “high Ni material”) in which the addition amount of Ni is 2.5% or more is described. May be described as “low Ni material”), because the crankshaft is thin, unlike a large forged steel product, the structure has been refined from the viewpoint of thermal history and plastic deformation. Cheap. More specifically, large forged steel products are large compared to small crankshafts such as automobile engines, and thus require high temperature and long time heating during hot plastic working, resulting in austenite. The structure tends to grow and become coarse. In addition, the strain introduced at the time of processing cannot be increased in large forged steel products due to the ability of the press, and the refinement of the structure by recrystallization by applying a large strain cannot be expected.

  Moreover, the process of hot plastic processing differs between a small crankshaft for automobile and a large crankshaft. That is, in a small crankshaft for automobiles or the like, the entire crank is formed simultaneously from a steel material such as a round bar, whereas in the large crankshaft to which the present invention is applied, one throw portion is provided. In this process, excessive heat is applied to the adjacent throw part, and it is difficult to refine the structure in terms of process.

  Furthermore, in the case of a large crankshaft, there is a problem that cooling takes place for a long time due to the mass effect, and grain growth continues during the cooling process. This is a problem that does not exist in small crankshafts for automobiles and the like.

  That is, the integrated crankshaft according to the present invention is not a small crankshaft for automobiles or the like, but an integrated crankshaft used for marine propulsion diesel engines, marine or onshore power generation diesel engines, and the like. is there. In particular, when the shaft diameter of the rotating shaft portion (journal) of the integrated crankshaft is 100 mm or larger, there is a specific problem that it is difficult to refine the structure. It is something to be solved.

  An integral crankshaft for a multi-cylinder engine requires a complicated forging process (hot plastic working) in which two or more throw parts are formed from one forged steel product. When the number of throw parts increases, it takes time to forge from the forged steel product to the shape of the integrated crankshaft. Therefore, it is necessary to preheat the forged steel product sufficiently. For example, preheating at 1150 ° C. or higher for 5 hours or longer is required. In such a case, the phenomenon of γ grain coarsening is more serious, and there is no means for solving this at present.

  The integrated crankshaft of the present invention that has solved the above-mentioned problems is as follows: C: 0.30 to 0.50% (meaning mass%, the same applies hereinafter), Si: 0.05 to 0.4%, Mn: 0 20 to 1.2%, Ni: 2.5 to 4.0%, Cr: 1.0 to 3.0%, Mo: 0.20 to 0.70%, V: 0.05 to 0.25 %, Al: 0.2% or less (excluding 0%), the balance is made of iron and inevitable impurities, the crystal grain size of the prior austenite grains is 6 or more in terms of ASTM grain size number, tensile strength Is an integrated crankshaft for a multi-cylinder engine.

  In the present invention, a “multi-cylinder engine” refers to an engine having two or more cylinders, and an integrated crankshaft that can be used for such an application has two or more throw parts (eccentric from the rotating shaft of the crankshaft). Part).

  The integrated crankshaft can take a mode in which the journal diameter is 100 mm or more. The integrated crankshaft is preferably used for diesel engine parts.

In addition, a preferable manufacturing method of the integrated crankshaft according to the present invention is a method in which the steel material having the chemical component is heated at 1150 ° C. or more for 5 hours or more and subjected to hot plastic working to form a slow portion at two or more locations, and then subzero. By carrying out the treatment, the retained austenite is reduced to 1% by volume or less, and then quenching / tempering treatment is performed. Further, the sub-zero treatment may be replaced by aging treatment of heating and holding at a temperature below the A ₁ transformation point.

  According to the present invention, in the conventional integrated crankshaft for a multi-cylinder engine, the problem of coarsening of prior austenite grains is overcome while increasing the strength of steel by increasing the amount of Ni added. The strength and toughness of the integrated crankshaft can be realized at the same time.

FIGS. 1A to 1C show XRD analysis results of the integrated crankshaft after hot plastic working. FIGS. 2A and 2B show EBSP analysis results of the integrated crankshaft after hot plastic working.

  As described above, with high Ni material, the old γ grains become coarse after hot plastic working (forging into the shape of the crankshaft), and when the old γ grains become coarse, the absorbed energy value of the Charpy impact test decreases. For example, the Japan Iron and Steel Institute, published by the Japan Institute of Metals, “Toughness of Steel”, Kyoto International Conference Hall, October 25, 26, 1971, p, 60 has a relationship between the old austenite grain size and vTrs, which is one of toughness values. The relationship between vTrs and the absorbed energy of the Charpy impact test is described in P107 of the document. Also from these facts, when the old γ grains become coarse, the absorbed energy value of the Charpy impact test decreases. However, it has not been clear so far how the old γ grains coarsen. In order to solve the problem of coarsening of old γ grains, the present inventors thought that it was first necessary to clarify the mechanism of coarsening of old γ grains, and worked on its elucidation.

  As described above, there is a problem that large forged steel products require a long time for cooling during heat treatment due to the mass effect, and grain growth continues in the cooling process (austenite → ferrite). On the other hand, regarding the heating process, the present inventors have carefully analyzed the γ grain formation site of the reverse transformation (ferrite → austenite) in the heating process of the high Ni material and the normal low Ni material. As a result, the high Ni material was found to have larger γ grains during reverse transformation than the low Ni material. And it was found that this is a cause of coarsening of the structure with a high Ni material. That is, even in the heating process in the quenching / tempering process, the residual γ is thermally decomposed into carbide (cementite) in the normal low Ni material, but in the high Ni material exceeding 2.5% in the heating process as well. It was found that the residual γ did not decompose and remained until the reverse transformation process of austenite. One reason is considered to be that Ni is an austenite forming element, so that high Ni material tends to remain as retained austenite in the structure after hot forging. Conversely, such a problem does not exist in the low Ni material.

  The present inventors have confirmed the problem of the present invention from an experimental approach. That is, it was verified whether there is a difference in structure after hot plastic working between a small Ni material, a high Ni material, and a high Ni material between a small material and a relatively large material. FIG. 1 shows (a) a low Ni material (Ni: 0.4%, shaft diameter: about 150 mm), (b) a high Ni material (Ni: 3%, shaft diameter: about 80 mm), and (c) a high Ni material. The results of X-ray crystal structure analysis (XRD: X-Ray Diffraction) of (Ni: 3%, shaft diameter: about 150 mm) are shown. “Α” shown in FIG. 1 indicates a ferrite structure, γ indicates an austenite structure, and “θ” indicates cementite. As is clear from FIG. 1, compared with the low Ni material, the high Ni material shows a lot of retained austenite in the structure of the crankshaft after hot plastic working, and in particular, the large high Ni material shows a large number of retained austenite. Has been. When such retained austenite remains until the quenching and tempering treatment in the next step, the retained austenite acts as a growth nucleus of austenite during reverse transformation.

  This retained austenite acting as a growth nucleus is derived from austenite (due to heating in the forging process) generated in a large forged steel product whose structure tends to coarsen due to the mass effect. From this point of view, the inventors analyzed the crystal orientation of the retained austenite after forging using an electron backscattering pattern (EBSP) as shown in FIG. FIG. 2A shows the retained austenite structure of the forged steel product, and FIG. 2B shows the ferrite structure. In FIG. 2 (a), it appears black, and in FIG. 2 (b), the portion other than black corresponds to the retained austenite structure. In particular, FIG. 2A shows that there are three prior austenite grain boundaries in the observation area, and FIG. 2B shows that the crystal orientations of the remaining austenite grains are aligned in one direction in each area. 2 (a) and 2 (b), it became clear that the residual austenite remaining in coarse old austenite grains is aligned in almost the same direction.

  When retained austenite with the same orientation acts as a growth nucleus of austenite at the time of reverse transformation, since it is originally in the same orientation, austenite grains grown from the retained austenite grow greatly in the next heating process such as quenching and tempering treatment. In the growth process, adjacent austenite grains having the same crystal direction are combined with each other to form coarse austenite grains. This is a high Ni material and is the cause of coarse austenite formation. The present inventors have found that, as a result of coalescence of the retained austenite in the same orientation, the austenite grains grow to coarse austenite grains having the same size as the above-mentioned prior austenite grains. As a result, it is considered that if the retained austenite is decomposed before the reverse transformation (quenching / tempering treatment), coarse austenite grains can be prevented from being formed by the quenching / tempering treatment, and the present invention has been completed. It is a thing.

There are roughly two methods for decomposing residual austenite, and one is to decompose the residual austenite by martensitic transformation of unstable residual austenite by subzero treatment. One also other is decomposed aging treatment residual austenite carbide by sustained fixed time heating below the A ₁ transformation point. As described above, by decomposing the retained austenite before the quenching / tempering process, it is possible to prevent the growth and coalescence of the retained austenite in the reverse transformation process, thereby preventing the formation of coarse austenite grains.

  The present invention has been made based on the above-described studies. The Ni concentration is 2.5% or more, the other chemical components are limited to a predetermined range, and the tensile strength is 1000 MPa or more. In the body crankshaft, the prior austenite grains are made to have a grain size number of 6 or more according to ASTM.

(Chemical component of crankshaft)
As described above, the present invention is characterized in that the Ni concentration is 2.5% or more, and the prior austenite grains are 6 or more in terms of the grain size number according to ASTM, and the basic composition of steel is not particularly limited. However, in order to satisfy the strength and toughness required for an integral crankshaft, it is desirable to satisfy the following basic composition in light of the general technical level of steel.

C: 0.30 to 0.50%
C is an element that enhances hardenability and contributes to improvement in strength, and in order to ensure sufficient strength and hardenability, it is 0.30% or more (preferably 0.32% or more, more preferably 0.34%). Above) It is necessary to contain. However, if the C content is excessive, the toughness is drastically reduced and reverse V segregation is promoted in large ingots, so it is 0.50% or less (preferably 0.48% or less, more preferably 0.46% or less. ) Is good to keep.

Si: 0.05-0.40%
Si acts as a strength improving element and needs to be contained in an amount of 0.05% or more (preferably 0.07% or more, more preferably 0.09% or more) in order to ensure sufficient strength. However, if the amount is too large, the reverse V segregation becomes remarkable and a clean steel ingot cannot be obtained. Therefore, it is necessary to make it 0.40% or less (preferably 0.37% or less, more preferably 0.34% or less). .

Mn: 0.20 to 1.2%
Mn is also an element that improves the hardenability and contributes to the improvement of strength, and 0.20% or more (preferably 0.25% or more, more preferably 0.30%) to ensure sufficient strength and hardenability. Above) It is necessary to contain. However, if the Mn content is excessive, reverse V segregation is promoted, so it is necessary to make it 1.2% or less (preferably 1.1% or less, more preferably 1.0% or less).

Ni: 2.5-4.0%
Ni is an element effective for improving the hardenability as well as enhancing the strength and toughness of Cr-Mo steels that are widely used as steels for forgings. High-grade Cr-Mo forging steels having a tensile strength of 1000 MPa or more. It is an essential element for the element and needs to be contained in an amount of 2.5% or more (preferably 2.6% or more, more preferably 2.7% or more). However, since Ni is an expensive element, it is made 4.0% or less (preferably 3.8% or less, more preferably 3.6% or less) from an industrial viewpoint.

Cr: 1.0-3.0%
Cr is an effective element that enhances hardenability and improves toughness. In order to exert these effects, 1.0% or more (preferably 1.2% or more, more preferably 1.4% or more) is contained. Let However, if excessively contained, it promotes reverse V segregation and makes it difficult to produce highly clean steel, so it is 3.0% or less (preferably 2.8% or less, more preferably 2.6% or less). .

Mo: 0.20 to 0.70%
Mo is an element that effectively works to improve hardenability, strength, and toughness, and 0.20% or more (preferably 0.25% or more, more preferably 0.30) in order to effectively exhibit these actions. % Or more), and if it is less than this, reverse V segregation is promoted, which is not preferable. However, when the Mo content is excessive, micro segregation in the steel ingot is promoted, and Mo is a heavy element and weight segregation is likely to occur. 65% or less, more preferably 0.60% or less).

V: 0.05-0.25%
V is an element that effectively acts to improve the hardenability and strength in a small amount, and 0.05% or more (preferably 0.06% or more, more preferably 0.07%) in order to exert such effects. % Or more). However, since V has a low equilibrium partition coefficient, microsegregation (normal segregation) is liable to occur if it is excessively contained, so 0.25% or less (preferably 0.22% or less, more preferably 0.19%). The following.

Al: 0.2% or less (excluding 0%)
Al is an element used as a deoxidizing element in the steel making process. However, Al fixes N in the form of AlN and the like, and inhibits the strengthening action of steel by blending N and V, etc., and also binds with various elements to generate non-metallic inclusions and intermetallic compounds, Since the toughness of the steel may be lowered, it is preferably 0.1% or less, more preferably 0.05% or less, for example. Often 0.01-0.03%.

  The preferred basic components of the integral crankshaft used in the present invention are as described above, and the remaining component is substantially iron. However, the integral crankshaft has a small amount of inevitable impurities (for example, P, S). , O, N, etc.) are allowed.

(Tensile strength 1000MPa or more)
In order to satisfy the strength required for the integrated crankshaft, the tensile strength is set to 1000 MPa or more. Preferably it is 1020 MPa or more, more preferably 1040 MPa or more. Such high strength can be basically achieved by setting the Ni concentration to 2.5% or more. However, as described above, in the integrated crankshaft for a multi-cylinder engine, the presence of high concentration Ni is present. This is a strength region that tends to cause coarsening of old γ grains (decrease in toughness).

(The old γ grain size number is 6 or more)
In the integrated crankshaft, it is of course necessary that the absorbed energy value in the Charpy impact test is high, but in order to increase the reliability of toughness, the crystal grain size of the prior austenite grains, which are closely related to toughness, is reduced. It is desirable to keep it. In the conventional integrated crankshaft for a multi-cylinder engine, as described above, austenite grains are coarsened in the presence of high concentration Ni. In the present invention, for example, due to the decomposition of residual austenite grains before quenching and tempering treatment as described above, the grain size number of prior austenite grains in the integrated crankshaft is 6 or more, preferably 6.5 or more in terms of ASTM grain size number. Preferably it can be 7 or more.

(Production example)
As a manufacturing method of the integrated crankshaft according to the present invention, for example, the following method will be described. First, the process of melting steel with the above-mentioned predetermined composition in an electric furnace, etc. → The process of removing impure elements such as S (sulfur) and gas components such as O (oxygen) by vacuum refining → The process of ingoting → Steel The process of forging the material after heating the lump → The process of forging into the shape of an integral crankshaft for a multi-cylinder engine (forming the throw part at two or more locations) → The process of decomposing residual austenite grains → Quenching / The step of hardening by tempering and then the finishing machining step may be performed sequentially.

  In the case of large-sized forging steel, ingot casting is mainly employed for casting, but depending on the size, a continuous casting method can also be employed.

  As a forging method to an integrated crankshaft for a multi-cylinder engine, a free forging method (a method of forging as a block in which a crank arm and a crankpin are integrated and finishing into an integrated crankshaft shape by gas cutting and machining) and , R. R. And T. R. Forging method (Forging process so that the axis of the steel ingot becomes the center of the integrated crankshaft, and the parts that are susceptible to deterioration of characteristics due to center segregation become all the cores of the integrated crankshaft. For example, when the latter forging method is adopted, the shaft surface layer side can be occupied by a portion with high cleanliness, and the integrated crankshaft has excellent strength and fatigue characteristics. Is easy to obtain. The integrated crankshaft thus obtained is useful as an integrated crankshaft for a multi-cylinder engine used for ships or generators.

  As described above, it is important to carry out the process of decomposing the residual γ contained in the integrated crankshaft after forging to the integrated crankshaft and before quenching / tempering treatment. Examples of the decomposition method include the following two methods.

1. Sub-zero treatment By performing sub-zero treatment after forging into the shape of the integrated crankshaft and before quenching and tempering treatment, unstable residual austenite remaining in the integrated crankshaft can be transformed into martensite. The purpose is to decompose residual austenite, and the temperature and time of the subzero treatment are not particularly limited, but the temperature is, for example, minus 50 ° C. or less, preferably minus 100 ° C. or less, more preferably minus 150 ° C. or less. The treatment time is, for example, 20 minutes or longer, preferably 40 minutes or longer, more preferably 1 hour or longer.

2. After forging to shape the aging solid type crankshaft, before quenching and tempering step, by performing the A ₁ transformation point or less of the heat treatment (aging treatment), it is possible to decompose the residual austenite to cementite. Similar to the sub-zero treatment, the purpose is to decompose residual austenite, and the temperature and time of the aging treatment are not particularly limited, but for example, 600 ° C. or higher, preferably 625 ° C. or higher, more preferably 650 ° C. or higher, for example 20 Hold for at least minutes, preferably at least 40 minutes, more preferably at least 1 hour.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Example Steel type A (high Ni material) having the components shown in Table 1 below was melted, heat treatment was performed at 1280 ° C. for 9 hours, simulating the forging process of the crankshaft, and many of these were cooled in the furnace. A sample piece was prepared. Then, before quenching and tempering treatment, as shown in Table 2 below, some samples are subjected to aging treatment (experiment numbers 1 to 9), other partial samples are subjected to subzero treatment (experiment numbers 10 to 15), and the rest The sample was subjected to quenching and tempering treatment without any aging treatment or subzero treatment (Experiment Nos. 16 to 18). In Experiment Nos. 10 to 12, sub-zero treatment is performed using liquid nitrogen, and the temperature of the decomposition process of residual γ in Table 2 is indicated as “−196 ° C.”.

  After aging treatment or sub-zero treatment and before quenching / tempering treatment (before quenching / tempering treatment for specimens that have not been subjected to aging treatment or sub-zero treatment), exist in the sample by X-ray crystal structure analysis Phase analysis was performed. Among the analysis results, the presence or absence of retained austenite is shown in Table 2. The X-ray crystal structure analysis is carried out according to the method described in “X-ray Diffraction Handbook” (published by Rigaku Corporation, February 21, 1998, first edition) if the detected amount of retained austenite exceeds 1% by volume. It was determined that there was residual austenite, and if the detected amount was 1% by volume or less, it was determined that there was no residual austenite.

  After that, tempering treatment (quenching / tempering treatment) was performed to ensure the strength of the crankshaft. The quenching conditions are also shown in Table 2.

  After quenching and tempering treatment, a part of each sample of Experiment Nos. 1 to 18 was further subjected to structure observation, and the prior austenite particle size was evaluated by the particle size number according to ASTM (E 112).

  Another part of the sample was subjected to a tensile test after tempering at 600 ° C. for 10 hours, and the tensile strength was measured to evaluate whether the strength was secured to 1000 MPa or more. The test method is based on “JIS Z2241” described in the JIS handbook issued in 2000. The test piece shape is a No. 14 test piece of “JIS Z2201”.

  In some experiments, Charpy impact test was performed to measure the absorbed energy (vE) (unit: J). The test method is based on “JIS Z2242” described in the JIS handbook issued in 2000. The specimen shape is a 2 mmV notch of “JIS Z2202”. The absorption energy value was determined to be 30 J or more as a pass criterion.

Comparative Example Steel type B (low Ni material) having the components shown in Table 1 was melted, and thereafter, the same test pieces as those in the above examples were prepared and the characteristics were evaluated. However, various conditions such as heat treatment are as shown as test numbers 19 to 21 in Table 2.

[Discussion]
From Table 2, in the high Ni material of the present invention (experiment numbers 1 to 6, 10 to 12), a strength of 1050 MPa or more is ensured, and the crystal grain size of the prior austenite grains is 6 or more in terms of ASTM grain size number. In the integrated crankshaft for cylinder engines, the miniaturization of the structure that was not possible in the past has been realized.

  On the other hand, even in high Ni materials, when the crystal grain size of the prior austenite grains is less than 6 in ASTM grain size number (experiment numbers 7 to 9, 13 to 18), the absorbed energy values are all less than 30 J, and the required toughness Characteristics were not obtained.

  On the other hand, with the low Ni material (experiment numbers 19 to 21), the requirement for the crankshaft strength of 1000 MPa or more was not achieved in the first place. Therefore, the Charpy impact test was not performed.

  As shown in Table 2, aging treatment and sub-zero treatment were cited for the decomposition of residual austenite. However, it is only necessary that the residual austenite can be decomposed, and the decomposition of residual austenite by stress-induced transformation by stress introduction is also possible. It is thought that there is an effect in miniaturization.

  Further, in the aging treatment, even if the temperature is lower than that in the above embodiment, the retained austenite can be decomposed if the time is increased accordingly. Also, even if residual austenite does not decompose when aging treatment is performed under certain conditions, the presence or absence of decomposition of residual austenite is evaluated by XRD after aging treatment. If the heat treatment is repeated until the residual austenite is decomposed, the formation of coarse prior austenite grains can be prevented by the subsequent quenching and tempering treatment. Thus, the aging condition can be achieved not only once but various conditions. As described above, whether or not the residual austenite has decomposed is determined as “existing” if the detected amount of residual austenite by XRD exceeds 1%, and if the detected amount is 1% or less, the residual austenite “ “None”. Similarly to the aging treatment, the temperature or time of the sub-zero treatment can be adjusted as appropriate within a range in which the retained austenite can be judged as “none”.

Claims (5)

C: 0.30 to 0.50% (meaning mass%, hereinafter the same),
Si: 0.05-0.4%
Mn: 0.20 to 1.2%,
Ni: 2.5-4.0%,
Cr: 1.0-3.0%,
Mo: 0.20 to 0.70%,
V: 0.05-0.25%,
Al: 0.2% or less (excluding 0%),
, The balance is made of iron and inevitable impurities, the crystal grain size of the prior austenite grains is 6 or more in terms of ASTM grain size, and the tensile strength is 1000 MPa or more. axis.   The integral crankshaft according to claim 1, wherein the journal diameter is 100 mm or more.   The integral crankshaft according to claim 1 or 2 used for a diesel engine.   The steel material having the chemical component according to claim 1 is heated at 1150 ° C. or more for 5 hours or more, subjected to hot plastic forming to form a slow portion at two or more locations, and then subjected to sub-zero treatment to obtain 1 volume of retained austenite. % Or less, followed by quenching and tempering. The steel material having the chemical component according to claim 1 is heated at 1150 ° C. or more for 5 hours or more, subjected to hot plastic working to form a slow portion at two or more locations, and then subjected to aging treatment at a temperature not higher than A ₁ transformation point. A method for producing an integrated crankshaft for a multi-cylinder engine, wherein the residual austenite is reduced to 1% by volume or less by applying, followed by quenching and tempering.