JP2015190040A - Low alloy steel for steel forging and crank shaft - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low alloy steel for steel forging, having yield strength and toughness required for large steel forging and excellent in machinability.SOLUTION: The low alloy steel for steel forging of the present invention has a composition containing as a basic component, C:0.30 mass% to 0.40 mass%, Si:over 0 mass% to 0.45%, Mn:0.50 mass% to 1.90 mass%, Ni:0.70 mass% to 2.00 mass%, Cr:1.00 mass% to 2.30 mass%, Mo:over 0 mass to 0.35 mass%, V:over 0 mass% to 0.20 mass% and the balance Fe with inevitable impurities, 17% or more of a bainite structure, a martensite structure or a combination structure thereof and the balance a pearlite structure, a ferrite structure or a combination structure thereof and satisfies following formulation. (1.5×Mn+2×Ni)/Cr<2.35... (1).

Description

  The present invention relates to a low alloy steel for forged steel products and a crankshaft.

  Steel materials used for large crankshafts, which are transmission members of diesel engines used in ships and generators, are required to have high strength in order to improve the output of diesel engines and make them more compact.

  On the other hand, forging steel that can be used for a crank throw of an assembled crankshaft has been developed (see, for example, Japanese Patent No. 4150054).

  By the way, an assembly-type crankshaft is manufactured by joining a crank throw obtained by forming steel for forged steel and a crank journal by shrink fitting. Therefore, high yield strength (YS) is required for steel for forgings used for crank throw in order to prevent this shrink-fitted part from slipping during operation of the diesel engine. With increasing output of diesel engines, recently, high yield strength of 450 MPa or more is required.

  However, in steel for forgings using carbon steel, only yield strength of about 350 MPa or more and 380 MPa or less can be obtained. In addition, in the case of steel for forgings using a low alloy steel having a conventional composition, since cracking occurs when the steel becomes large, a steel for forgings that can be used for a large assembly type crankshaft with a shaft diameter exceeding 650 mm is used. It cannot be manufactured. Thus, it is difficult to manufacture steel for forgings that has a yield strength of 450 MPa or more and can be used for a large assembled crankshaft.

  In addition, crank throws and crank journals used for assembled crankshafts are molded into these shapes and then finished by cutting, so the steel for forgings used for assembled crankshafts also has excellent machinability. Required.

Japanese Patent No. 4150054

  The present invention has been made on the basis of the circumstances as described above, and has an object of providing a low alloy steel for forged steel products having yield strength and toughness required for large forged steel products and having excellent machinability. To do.

The invention made in order to solve the above problems is as follows: C (carbon): 0.30 mass% to 0.40 mass%, Si (silicon): more than 0 mass% to 0.45 mass%, Mn (manganese) : 0.50 mass% or more and 1.90 mass% or less, Ni (nickel): 0.70 mass% or more and 2.00 mass% or less, Cr (chromium): 1.00 mass% or more and 2.30 mass% or less, Composition containing Mo (molybdenum): more than 0% by mass and not more than 0.35% by mass, V (vanadium): more than 0% by mass and not more than 0.20% by mass, the balance being Fe (iron) and inevitable impurities A low alloy steel for forged steels having a bainite structure, a martensite structure or a combination structure thereof in an amount of 17% or more, the balance being a pearlite structure, a ferrite structure or a combination structure thereof and satisfying the following formula (1) It is.
(1.5 × Mn + 2 × Ni) / Cr <2.35 (1)

  The low alloy steel for forged steel products is such that the content of each composition of the steel material is within the above range and satisfies the above formula (1), and contains at least 17% of a bainite structure, a martensite structure, or a combination structure thereof. By doing so, excellent toughness can be obtained, and high yield strength required for large forged steel products such as large crankshafts can be secured. In addition, the low alloy steel for forged steel products has a steel composition that satisfies the above formula (1), whereby the Cr concentration in the cementite of the metal structure is increased, thereby reducing the resistance during cutting. Therefore, it is excellent in machinability.

  Moreover, another invention made | formed in order to solve the said subject is a crankshaft manufactured from the said low alloy steel for forged products. Since the crankshaft is made of the low alloy steel for forged steel products, it has high yield strength and toughness as described above and is excellent in machinability.

  As described above, the low alloy steel for forged steel of the present invention has the yield strength and toughness required for large forged steel products and is excellent in machinability. Preferably used.

The graph which shows the relationship between the structure fraction and yield strength of the bainite structure in a Example, a martensitic structure, or those combination structures. The graph which shows the relationship between Mn, Ni, and Cr content in Example, and the torque resistance at the time of drilling

  Hereinafter, embodiments of the low alloy steel for forged steel products according to the present invention will be described.

<Metallic structure>
The metal structure of the low alloy steel for forgings is a bainite structure, martensite structure or a combination structure thereof (structure fraction) of 17% or more, and the balance is a pearlite structure, a ferrite structure or a combination structure thereof. is there. Thus, by making the structure fraction of the bainite structure, the martensite structure, or a combination structure thereof in the metal structure to be equal to or higher than the lower limit, the low alloy steel for forged steel is an assembly type large crankshaft for ships, etc. As high yield strength (450 MPa or more). In addition, as a method for measuring the structure fraction of the bainite structure, martensite structure or a combination thereof, for example, a specimen for microstructural observation is cut out from a low alloy steel for forged steel, and the surface of this specimen is in the forging direction. Can be mirror-polished, corroded with nital, and observed with an optical microscope.

<Composition>
The low alloy steel for forged steel is C: 0.30 mass% or more and 0.40 mass% or less, Si: more than 0 mass% and 0.45 mass% or less, Mn: 0.50 mass% or more and 1.90 mass%. Hereinafter, Ni: 0.70% by mass or more and 2.00% by mass or less, Cr: 1.00% by mass or more and 2.30% by mass or less, Mo: more than 0% by mass and 0.35% by mass or less, V: 0% by mass The composition contains a basic component of more than 0.20% by mass and the balance is Fe and inevitable impurities, and satisfies the following formula (1).
(1.5 × Mn + 2 × Ni) / Cr <2.35 (1)

  The lower limit of the C content of the low alloy steel for forged steel is 0.30% by mass, preferably 0.32% by mass. On the other hand, the upper limit of the C content of the low alloy steel for forged products is 0.40% by mass, and preferably 0.39% by mass. When the C content of the low alloy steel for forged steel is less than the above lower limit, sufficient hardenability cannot be obtained, and a sufficient microstructure can be obtained by tempering by tempering after normalization. There is a possibility that the organization fraction cannot be secured. Conversely, if the C content of the low alloy steel for forgings exceeds the above upper limit, the toughness is extremely lowered, and since the formation of pearlite during normalization is promoted by increasing the stability of cementite, Yield strength may be reduced. By setting the C content of the low alloy steel for forged steel to be in the above range, the hardenability and the yield strength of the low alloy steel for forged steel can be appropriately ensured.

  The lower limit of the Si content of the low alloy steel for forged steel products is more than 0% by mass. On the other hand, the upper limit of the Si content of the low alloy steel for forged steel products is 0.45 mass%, preferably 0.32 mass%. If the Si content of the low alloy steel for forged products exceeds the above upper limit, the toughness may be impaired. By setting the Si content of the low alloy steel for forged steel to be in the above range, the hardenability of the low alloy steel for forged steel can be improved and sufficient toughness can be ensured.

  As a minimum of Mn content of the low alloy steel for forged steel products, it is 0.50 mass%, and 0.80 mass% is preferred. On the other hand, the upper limit of the Mn content of the low alloy steel for forged steel products is 1.90% by mass, preferably 1.50% by mass. When the Mn content of the low alloy steel for forged steel is less than the above lower limit, sufficient hardenability cannot be obtained, and sufficient yield strength can be obtained by refining by tempering after normalization. There is a possibility that the tissue fraction cannot be secured and sufficient toughness cannot be obtained. On the other hand, if the Mn content of the low alloy steel for forged products exceeds the upper limit, the Cr concentration in the cementite is lowered and the machinability may be impaired. By setting the Mn content of the low alloy steel for forged steel in the above range, the yield strength and toughness of the low alloy steel for forged steel can be appropriately ensured.

  The lower limit of the Ni content of the low alloy steel for forged products is 0.70% by mass, preferably 0.74% by mass. On the other hand, the upper limit of the Ni content of the low alloy steel for forged products is 2.00% by mass, and preferably 1.50% by mass. When the Ni content of the low alloy steel for forged steel is less than the above lower limit, sufficient hardenability cannot be obtained, and a sufficient microstructure can be obtained by tempering by tempering after normalization. There is a risk that the organization fraction cannot be secured. Moreover, when the Ni content of the low alloy steel for forged steel products exceeds the above upper limit, the Cr concentration in the cementite is lowered, and the machinability may be impaired. By making the Ni content of the low alloy steel for forged steel within the above range, the hardenability and strength of the low alloy steel for forged steel can be appropriately ensured.

  The lower limit of the Cr content of the low alloy steel for forged steel products is 1.00% by mass, preferably 1.40% by mass. On the other hand, the upper limit of the Cr content of the low alloy steel for forged products is 2.30% by mass, and preferably 2.10% by mass. If the Cr content of the low alloy steel for forged steel is less than the above lower limit, sufficient hardenability cannot be obtained, and sufficient yield strength can be obtained by refining by tempering after normalization. The tissue fraction cannot be secured and sufficient strength may not be obtained. Conversely, if the Cr content of the low alloy steel for forged steel exceeds the upper limit, the cementite ratio in the metal structure increases and the toughness may be impaired. By setting the Cr content of the low alloy steel for forged steel to be in the above range, the hardenability, strength, and toughness of the low alloy steel for forged steel can be appropriately ensured.

  The lower limit of the Mo content of the low alloy steel for forged steel products is more than 0% by mass. On the other hand, the upper limit of the Mo content of the low alloy steel for forged products is 0.35% by mass, and preferably 0.32% by mass. If the Mo content of the low alloy steel for forged steel exceeds the above upper limit, fine carbides may be generated and the toughness may be impaired. When the low alloy steel for forged steel contains Mo in the above range, the hardenability is improved, and the microstructure fraction of the metal structure that can obtain sufficient yield strength by tempering after tempering after normalization is obtained. Can be secured, and sufficient strength can be obtained.

  The lower limit of the V content of the low alloy steel for forged steel is more than 0% by mass. On the other hand, the upper limit of the V content of the low alloy steel for forged products is 0.20% by mass, and preferably 0.10% by mass. When the V content of the low alloy steel for forged products exceeds the above upper limit, fine carbides may be generated and the toughness may be impaired. When the low alloy steel for forged steel contains V in the above range, the hardenability is improved, and the microstructure fraction of the metal structure at which sufficient yield strength can be obtained by tempering by tempering after normalization is obtained. Can be secured, and sufficient strength can be obtained.

  The low alloy steel for forged steel products contains Fe and inevitable impurities in the balance other than the basic components described above. Inevitable impurities include, for example, elements such as Cu (copper), Sn (tin), As (arsenic), Pb (lead), and Ti (titanium) brought in depending on the situation of raw materials, materials, manufacturing equipment, and the like. Is acceptable. In addition, it is effective to further contain other compositions, and the characteristics of the forged steel are further improved depending on the kinds of the contained compositions.

<Relational formula of each composition>
In the low alloy steel for forged steel products, the content of each element satisfies the following formula (1).
(1.5 × Mn + 2 × Ni) / Cr <2.35 (1)

  In the range where the content of each element satisfies the above formula (1), relatively brittle cementite having a Cr concentration of 24% or more is generated. It is considered that the formation of this relatively brittle cementite reduces the resistance during cutting and improves the machinability. In addition, if the Cr concentration in cementite is at least 22% or more, sufficient machinability required for cutting of a large crankshaft component can be obtained.

<Mechanical properties>
The lower limit of the yield strength (YS) of the low alloy steel for forged steel products is preferably 450 MPa, and more preferably 500 MPa. If the yield strength of the low alloy steel for forged steel is equal to or higher than the above lower limit, a crankshaft having a shaft diameter of 650 mm or more can be manufactured with a demand for an assembly-type large crankshaft. Yield strength can be evaluated by, for example, a tensile test based on JIS-Z2241 (2011).

  The lower limit of the absorbed energy vE (absorbed energy at room temperature) of the low alloy steel for forged steel products is preferably 20J, and more preferably 30J. When the absorbed energy of the low alloy steel for forged steel is equal to or higher than the lower limit, the toughness required for the assembled large crankshaft can be satisfied. The absorption energy can be evaluated by, for example, a Charpy impact test based on JIS-Z2242 (2005).

<Manufacturing method of low alloy steel for forged steel and crankshaft>
The low alloy steel for forged steel products is manufactured by a manufacturing method including, for example, a melting process, a casting process, a heating process, a forging process, and a heat treatment process. Furthermore, the crankshaft is manufactured by processing the low alloy steel for forged steel products by a machining process.

(Melting process)
In the melting step, first, the steel adjusted to the above-described predetermined composition is melted using a high-frequency melting furnace, an electric furnace, a converter, or the like. Thereafter, the molten steel is subjected to vacuum treatment to remove gas components such as O (oxygen) and H (hydrogen) and impure elements.

(Casting process)
In the casting process, an ingot is cast using the steel whose components have been adjusted in the melting process. Ingot casting is mainly used for large steel forgings, and continuous casting can be used for relatively small forgings.

(Heating process)
In the heating step, the steel ingot is heated at a predetermined temperature for a predetermined time. Since the deformation resistance of the material increases at a low temperature, the heating temperature is set to, for example, 1150 ° C. or higher in order to perform processing within a good range of the material deformability. Moreover, in order to make the temperature of the surface and the inside of the steel ingot uniform, a predetermined heating time is required, and the heating time is, for example, 3 hours or more. The heating time is generally considered to be proportional to the square of the diameter of the workpiece, and the larger the material, the longer the heating and holding time.

(Forging process)
In the forging process, the steel ingot heated to a temperature of 1150 ° C. or higher in the heating process is forged. In order to crimp a casting defect such as a zest nest or microporosity, the forging ratio is preferably 3S or more.

(Heat treatment process)
In the heat treatment step, an annealing process and a normalizing process are performed, and then a tempering process is performed.

  First, the forged steel ingot is annealed. In the annealing treatment, the forged steel ingot is heated and held at a predetermined temperature (for example, 250 ° C. or higher and 350 ° C. or lower) for a predetermined time (for example, 5 hours or longer). Thereafter, the mixture is heated to an austenitizing temperature or higher (for example, 850 ° C. or higher and 920 ° C. or lower), held for a predetermined time (for example, 3 hours or longer), and then gradually cooled to a predetermined temperature (for example, 50 ° C. or higher and 200 ° C. or lower). Further, after the cooling, it is gradually heated to a predetermined temperature (for example, 500 ° C. or more and 800 ° C. or less) at a temperature rising rate of 30 ° C./hr or more and 70 ° C./hr or less, and held for a certain time (for example, 5 hours or more).

  In the normalizing process, after annealing, the steel material is cut out from the forged steel ingot cooled to room temperature, and first, the steel material is austenitized. Austenitization is performed by gradually heating at a temperature rising rate of 30 ° C./hr or more and 70 ° C./hr or less to the Ac3 transformation point (830 ° C.) or more and holding for a certain time (for example, 1 hour or more). In the case of a large steel material, a temperature difference occurs between the inside and outside of the material during heating. Therefore, it is necessary to gradually heat the material to the austenitizing temperature and hold the steel material for a certain time in order to make the temperature uniform between the surface and the inside. This holding time depends on the diameter of the steel material and needs to be longer for larger materials.

  Next, in the normalizing process, after the temperature of the steel material becomes uniform due to austenitization, the steel material is cooled. The lower limit of the cooling rate at this time is preferably 0.5 ° C./min, and more preferably 1 ° C./min. Moreover, as an upper limit of the said cooling rate, 8 degree-C / min is preferable and 5 degree-C / min is more preferable. If the cooling rate is less than the lower limit, the bainite structure, the martensite structure, or a combination structure thereof may decrease, and a sufficient yield strength may not be obtained. On the other hand, if the cooling rate exceeds the upper limit, cracking may occur due to thermal stress or transformation stress during cooling.

  The said low alloy steel for forged products is obtained by performing a tempering process after the said cooling. In the tempering of the steel material, the steel material is gradually heated to a predetermined temperature at a temperature rising rate of 30 ° C./hr to 70 ° C./hr and held for a certain time (for example, 5 hours or more). Tempering is performed at, for example, 550 ° C. or higher in order to adjust the balance of strength, ductility, and toughness and to remove internal stress (residual stress) generated by the phase transformation. However, when the temperature becomes high, the steel material softens due to the coarsening of carbides, recovery of the dislocation structure, and the like, and sufficient strength cannot be secured.

(Machining process)
The forged steel low alloy steel after the heat treatment step is forged into a crank throw or a crank journal, and then subjected to finish machining including cutting or grinding, whereby the crankshaft component can be obtained. Then, the crankshaft is obtained by shrink fitting the crank throw and the crank journal subjected to finish machining.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples.

[Evaluation of steel grade and tempering method]
First, the applicability to large crankshafts was evaluated for steel types and tempering methods.

(Example 1)
In Example 1, a rope having the composition shown in Table 1 was hot-forged between 900 ° C. and 1230 ° C., and then formed into a crank throw shape with a shaft diameter of 950 mm by gas cutting. Next, the molded crank throw was heated to 870 ° C., cooled by air cooling, and normalized. Further, the cooled crank throw was heated to 620 ° C. and tempered.

(Comparative Examples 1-3)
In Comparative Examples 1 to 3, steel having the composition shown in Table 1 was hot-forged between 900 ° C. and 1230 ° C., and then formed into crank throw shapes having shaft diameters of 620 mm, 720 mm, and 1062 mm by gas cutting. . Next, these molded crank throws were heated to 870 ° C., cooled by air cooling, and normalized. Further, the cooled crank throw was heated to 620 ° C. and tempered.

(Comparative Example 4)
In Comparative Example 4, a rope having the composition shown in Table 1 was hot-forged between 900 ° C. and 1230 ° C., and then molded into a crank throw shape with a shaft diameter of 720 mm by gas cutting. Next, the molded crank throw was heated to 870 ° C., and then cooled with a polyalkylene glycol-based polymer solution in which the concentration of the aqueous solution was adjusted to 25% by mass, followed by quenching. Further, the cooled crank throw was heated to 620 ° C. and tempered.

[Scoring evaluation]
The tempered crank throw was observed by visual inspection to check for cracks.

[Yield strength evaluation]
A test piece was collected from the vicinity of the shrink-fitted portion of the tempered crank throw, and a tensile test was performed in accordance with JIS-Z2241 (2011). The shape of the test piece was a JIS-Z2241 (2011) No. 14A test piece, φ6 × G. L. The yield strength was measured at 30 mm. In this test, those having a yield strength of 450 MPa or more were accepted.

  In this test, a crank throw with no cracks and a yield strength of 450 MPa or more was designated as a comprehensive evaluation “A”. Moreover, the thing whose yield strength was less than 450 MPa was made into comprehensive evaluation "B", and the thing in which the crack was recognized was made into comprehensive evaluation "C". These measurement results are shown in Table 1.

[Measurement result]
In the crank throw of Example 1, no cracking was observed, and a yield strength of 450 MPa or more was obtained. Thereby, it turned out that the steel which contained the basic component of this invention and tempered by the process of tempering after normalization can be used suitably for a large-sized crankshaft.

  On the other hand, although the crank throw using the steels of Comparative Examples 1 to 3 did not show any burning cracks, it can be said that the yield strength is low and it cannot be applied to an assembly-type large crankshaft.

  In addition, the crank throw using the steel of Comparative Example 4 is hardened despite the relatively large shaft diameter, so that a temperature difference between the surface layer and the inside tends to occur, and thermal stress or transformation is caused by the temperature difference generated during quenching. It is thought that stress was generated and cracking occurred.

[Evaluation of steel composition and microstructure]
Next, the steel composition and the metal structure were evaluated.

(Examples 2-5, Comparative Examples 5-15)
In Examples 2 to 5 and Comparative Examples 5 to 15, steels having the compositions shown in Table 2 were melted to produce test steels for forged products. First, steel having the composition shown in Table 1 was melted using a high-frequency furnace, and a 40 kg steel ingot having a width of 170 mm, a thickness of 120 mm, and a height of 230 mm was cast. Next, the feeder part of the obtained steel ingot was excised and heated at 1230 ° C. for 5 hours to 10 hours. Next, with a free forging press machine, the heated steel ingot was compressed to ½ in height ratio, the steel ingot center line was rotated by 90 ° and forged, and stretched to width 90 mm × thickness 90 mm × length 450 mm. Later, the material was allowed to cool in the atmosphere. Thereafter, the material cooled to 250 ° C. or higher and 300 ° C. or lower was held in a heating furnace for 10 hours or more, then heated at 50 ° C./hr and held at 870 ° C. for 5 hours. Thereafter, the material was furnace cooled to 100 ° C. or higher and 150 ° C. or lower, heated again at 50 ° C./hr, held at 650 ° C. for 15 hours, and then allowed to cool in the atmosphere and annealed.

  Next, a square member having a width of 20 mm, a thickness of 20 mm, and a length of 200 mm was cut out from the above-mentioned raw material that had been allowed to cool to room temperature, and austenitized using a small simulated furnace. In the austenitizing treatment, the cut material (the above-mentioned square bar) is heated to 870 ° C. at 50 ° C./hr and held for 1 hour, and then the average cooling rate in the temperature range from 870 ° C. to 500 ° C. is 1.0 ° C. / Cooling was performed to reach min. Then, after cooling the said raw material to room temperature, after hold | maintaining at 620 degreeC for 10 hours as a tempering process, it stood to cool in air | atmosphere. In addition, the average cooling rate of 1.0 ° C./min in the temperature range from 870 ° C. to 500 ° C. after the above-described austenitizing treatment is a cooling rate that simulates cooling after normalization in a large forged steel product. .

(Comparative Examples 16-19)
Comparative Examples 16 to 19 are the same as Example 2 except that steel having the same composition as Examples 2 to 5 was used and the average cooling rate in cooling after normalization was set to 0.3 ° C./min. The test material was manufactured by the manufacturing method.

(Comparative Examples 20 and 21)
In Comparative Examples 20 and 21, a test material was produced by the same production method as in Example 2 above, using steel having the composition shown in Table 2. The steel used in Comparative Examples 20 and 21 has a composition that does not have the above formula (1).

(Examples 6 to 9)
Examples 6 to 9 are the same as Example 2 except that the steel has the same composition as Examples 2 to 5 and the average cooling rate in cooling after normalization is 5.0 ° C./min. The test material was manufactured by the manufacturing method.

(Comparative Examples 22 and 23)
Comparative Examples 22 and 23 are the same as Example 2 except that the steel has the same composition as Comparative Examples 20 and 21 and the average cooling rate in cooling after normalization is 5.0 ° C./min. The test material was manufactured by the manufacturing method.

[Measuring mechanical properties]
A test piece was collected from the manufactured test material and subjected to a tensile test in accordance with JIS-Z2241 (2011). The shape of the test piece was a JIS-Z2241 (2011) No. 14A test piece, φ6 × G. L. The yield strength (YS) was measured at 30 mm. In this test, those with a yield strength of 450 MPa or more were accepted.

  Further, the absorbed energy (vE) (absorbed energy at room temperature) of the test material was measured by a Charpy impact test, and the toughness was evaluated. The Charpy impact test was performed according to JIS-Z2242 (2005), and the 2 mmV notch of JIS-Z2242 (2005) was adopted as the shape of the test piece at this time. In this test, the absorption energy of 20 J or more was determined to be acceptable.

[Observation of metal structure]
A specimen for observing the metal structure was cut out from the test material, and the specimen was corroded with 3% nital solution and observed with an optical microscope. Specifically, observation and photographing were performed with an optical microscope at a magnification of 100 times, and a lattice of 11 rows × 11 columns was applied to the photographed photo to count lattice points corresponding to a bainite structure or a martensite structure. Then, the number of lattice points was divided by the total number of lattices 121 to obtain the structure fraction (ratio in the metal structure) of the bainite structure, the martensite structure, or a combination structure thereof in each field of view. The same measurement was performed for each test material in three fields of view, and the average value was taken as the structure fraction of the bainite structure, martensite structure, or combination thereof in each material. In Table 2, this tissue fraction was described as “B / M fraction”.

[Concentration analysis of alloying elements in cementite]
The concentration analysis of the alloy elements in the cementite was performed by quantitative analysis using EDX attached to a scanning electron microscope (SEM). EDX is a technique for performing elemental analysis and composition analysis by detecting characteristic X-rays generated by electron beam irradiation and performing spectral analysis with energy. In Table 2, for Comparative Examples 5 to 8, 10, and 12 to 19 in which the column of “Cr concentration in cementite” is hatched, the concentration analysis of the alloy elements in cementite is not performed.

[Machinability evaluation]
As an evaluation of machinability, the test material was subjected to a measurement test of a torque resistance value during drilling. A square material cut at a length of 12 mm from a test material having a width of 20 mm × thickness of 20 mm × 200 mm was used as a test piece, and a drill hole was formed on the cut surface of the test piece. Using a carbide drill of φ5 mm (“860.1-0500-019A1-PM4234” from Sandvik Co., Ltd.), two 10 mm deep holes were drilled with one drill per test piece. Drilling is performed at a flow rate of 10 L / min at a rotational speed of 4,000 rpm, a feed rate of 0.25 mm / rev, and a lubricating oil (water-soluble lubricant (Yushiroken "EC50" from Yushiro Chemical Industry Co., Ltd.) diluted 50 times). This was done while supplying to the drilling section. In the machinability evaluation, an average value of torque resistance was measured from when the drill contacted the test piece until the drilling of a depth of 10 mm was completed, and 180 N · cm or less was accepted. In Table 2, the machinability is not evaluated for Comparative Examples 5 to 8, 10, and 12 to 19 in which the “torque resistance” column is hatched.

  In this test, the overall evaluation “A” was determined when all the yield strength, absorbed energy, and machinability were determined to be acceptable. In addition, those with a yield strength (YS) of less than 450 MPa are comprehensively evaluated “B”, those with an absorbed energy (vE) of less than 20 J are comprehensively evaluated “C”, and torque resistance during drilling exceeds 180 N · cm. Was evaluated as “D”. These measurement results are shown in Table 2.

[Measurement result]
Each of Examples 2 to 9 had a high yield strength and excellent toughness and machinability, and was a comprehensive evaluation A. It can be said that these low alloy steels for forged steel can be suitably used as large crankshafts.

  In contrast, in Comparative Examples 5 to 15, at least one of yield strength, toughness, and machinability did not satisfy the acceptable range. These test materials are prepared using steel having a composition that does not satisfy the basic component range of the present invention. Among the basic components of the present invention, C, Mn, Ni, and Cr are elements that contribute to strength improvement, and have a composition having an element that is less than the lower limit of the content of these elements defined in the present invention (Comparative Example 5). , 8, 10, 12) It can be said that the yield strength is lowered. Among the basic components of the present invention, C, Si, Cr, Mo, and V are elements that impair toughness when they are excessive, and have elements that exceed the upper limit of the content of these elements defined in the present invention. The toughness of the composition (Comparative Examples 6, 7, 13, 14, 15) is reduced. In addition, among the basic components of the present invention, if Mn and Ni are excessive, the Cr concentration in the cementite structure decreases, so that the composition having elements exceeding the upper limit of the content of these elements defined in the present invention. The thing (Comparative Examples 9 and 11) has reduced machinability.

  In Comparative Examples 16 to 19, since the average cooling rate in cooling after normalization was reduced, the bainite structure and the martensite structure in the test material were reduced, and the bainite structure and the martensite structure were present in any of the test materials. I was not able to admit. In Comparative Examples 16 to 19, since the bainite structure and the martensite structure are small, it is considered that the yield strength is lowered.

  Comparative Examples 20 and 21 use steel having a composition that satisfies the range of the basic components of the present invention, but the contents of Mn, Ni, and Cr do not satisfy the above formula (1). Therefore, it is considered that the Cr concentration in the cementite is lowered and the machinability is lowered.

  In Examples 6 to 9, since the average cooling rate in the cooling after normalization was increased, the ratio of the bainite structure or martensite structure in the test material was increased, and all of the metal structures of the test material were bainite structure and martensite. It became a site organization. Therefore, it can be said that the Cr concentration in cementite did not decrease and the machinability did not decrease. In Examples 6 to 9, the yield strength is higher than that in Examples 2 to 5, and it can be said that the yield strength can be improved by controlling the average cooling rate in cooling after normalization.

  In Comparative Examples 22 and 23, since the average cooling rate in the cooling after normalization was larger than those in Comparative Examples 20 and 21, the ratio of the bainite structure or the martensite structure in the test material was higher than that in Comparative Examples 20 and 21. It has become. However, since the contents of Mn, Ni, and Cr do not satisfy the above formula (1), the Cr concentration in cementite does not increase, and it is considered that machinability was not improved with respect to Comparative Examples 20 and 21. .

  Regarding the measurement results of Examples 2 to 9 and Comparative Examples 5 to 23, the relationship between the structure fraction (B / M fraction) and the yield strength (YS) of the bainite structure, martensite structure or a combination structure thereof is illustrated. It is shown in 1. The black circle plots in FIG. 1 show the measurement results of the example, and the square plots show the measurement results of the comparative example. From FIG. 1, the yield strength tends to improve as the structure fraction of the bainite structure, martensite structure, or combination thereof increases, and the structure fraction of the bainite structure, martensite structure, or combination thereof increases by 17% or more. It can be seen that a high yield strength of 450 MPa or more can be obtained.

  Moreover, the relationship between the value of the left side of said Formula (1) and torque resistance is shown in FIG. 2 about the measurement result of the said Examples 2-9 and the comparative examples 9, 11, 20-23. The black circle plots in FIG. 2 show the measurement results of the example, and the square plots show the measurement results of the comparative example. From FIG. 2, the torque resistance tends to increase as the value on the left side of the above formula (1) increases. When the value on the left side of the above formula (1) is less than 2.35, the torque resistance becomes 180 N · cm or less. It can be seen that excellent machinability can be obtained.

  As described above, the low alloy steel for forged steel products has high yield strength and toughness required for large forged steel products, and is excellent in machinability. Used.

Claims (2)

C: 0.30 mass% or more and 0.40 mass% or less,
Si: more than 0% by mass and 0.45% by mass or less,
Mn: 0.50% by mass or more and 1.90% by mass or less,
Ni: 0.70% by mass or more and 2.00% by mass or less,
Cr: 1.00% by mass to 2.30% by mass,
Mo: more than 0% by mass and 0.35% by mass or less,
V: containing a basic component of more than 0% by mass and 0.20% by mass or less, with the balance being Fe and inevitable impurities,
The bainite structure, martensite structure or a combination structure thereof is 17% or more, the balance is a pearlite structure, a ferrite structure or a combination structure thereof,
Low alloy steel for forgings that satisfies the following formula (1).
(1.5 × Mn + 2 × Ni) / Cr <2.35 (1)   A crankshaft manufactured from the low alloy steel for forged steel product according to claim 1.

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