WO2017126407A1 - 鍛造用鋼及び大型鍛鋼品 - Google Patents
鍛造用鋼及び大型鍛鋼品 Download PDFInfo
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- WO2017126407A1 WO2017126407A1 PCT/JP2017/000806 JP2017000806W WO2017126407A1 WO 2017126407 A1 WO2017126407 A1 WO 2017126407A1 JP 2017000806 W JP2017000806 W JP 2017000806W WO 2017126407 A1 WO2017126407 A1 WO 2017126407A1
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/30—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for crankshafts; for camshafts
Definitions
- the present invention relates to a forging steel and a large forged steel product using the forging steel.
- Forging steel is used for large forged steel products such as crankshafts, intermediate shafts, propulsion shafts, connecting rods, ladder stocks, ladder horns and the like used as transmission members for marine drive sources.
- large forged steel products such as crankshafts, intermediate shafts, propulsion shafts, connecting rods, ladder stocks, ladder horns and the like used as transmission members for marine drive sources.
- forging steels used in these large forged steel products are required to have high strength, high toughness, durability and improved productivity.
- Such large forged steel products are generally annealed or quenched and then tempered to increase strength and toughness.
- variations in material are generally caused by the difference in cooling rate between the inside and the surface.
- a large forged steel product of a transmission member of a marine drive source for example, if it is a large crank throw, a thick forging steel having a total length of 3500 mm and a web width of 2000 mm is required.
- the cooling rate is likely to differ between the forging steel thickness direction and each forging steel.
- the conventional forging steel has the disadvantage that it is difficult to achieve both strength and toughness and a reduction in material variation. Further, the conventional forging steel has a disadvantage that productivity is lowered because it takes too much time for gas cutting for journal hole processing.
- the present invention has been made based on the above-described circumstances, and an object of the present invention is to provide a forging steel that is excellent in strength, toughness, durability, and gas cutting ability.
- C 0.20 mass% or more and 0.35 mass% or less
- Si 0% by mass or more and 0.5% by mass or less
- Mn 0.50% by mass or more and 2.70% by mass or less
- Cu 0% by mass or more and 1% by mass or less
- Ni 0% by mass or more and 2.00% by mass or less
- Cr 1.00% by mass to 2.50% by mass
- Mo 0.10% by mass to 0.55% by mass
- V 0% by mass to 0.20% by mass
- N 30 mass ppm or more and 100 mass ppm or less
- Al 0% by mass or more and 0.050% by mass or less
- S 0% by mass or more and 0.020% by mass or less
- O more than 0 mass ppm and 50 mass ppm or less
- the balance Fe and a composition that is an inevitable impurity
- the metal structure is a bainite structure, and the average lath width is 3.0 ⁇ m or less
- the forging steel and high strength forged steel of the present invention are excellent in durability as well as strength and toughness.
- the present inventor has found that the structure of the steel for forging to be produced has an elemental composition that hardly varies depending on the cooling rate.
- the inventor analyzed a large number of forging steels having different elemental compositions, and the gas cutting performance was improved by setting the average lath width to a desired form after making the elemental composition less dependent on the cooling rate. This was found and the present invention was completed.
- the forging steel of the present invention made to solve the above problems is C: 0.20 mass% or more and 0.35 mass% or less, Si: 0% by mass or more and 0.5% by mass or less, Mn: 0.50% by mass or more and 2.70% by mass or less, Cu: 0% by mass or more and 1% by mass or less, Ni: 0% by mass or more and 2.00% by mass or less, Cr: 1.00% by mass to 2.50% by mass, Mo: 0.10% by mass to 0.55% by mass, V: 0% by mass to 0.20% by mass, N: 30 mass ppm or more and 100 mass ppm or less, Al: 0% by mass or more and 0.050% by mass or less, S: 0% by mass or more and 0.020% by mass or less, O: more than 0 mass ppm and 50 mass ppm or less, and the balance: Fe and a composition that is an inevitable impurity,
- the metal structure is a bainite structure, and the average lath width is 3.0 ⁇ m or less,
- the forging steel of the present embodiment has an average lath width of 3.0 ⁇ m or less, so that the gas cutting performance is improved. And the steel for forging of this embodiment is excellent in intensity
- the metal structure of the forging steel is mainly transformed into a bainite structure at the time of production, and satisfying the above formulas (1) and (2) at the time of transformation can reduce the material variation of the forging steel. it can.
- the mechanism is not clear, but by satisfying the above equation (1), the transformation start temperature can be reduced at a high cooling rate, and by satisfying the above equation (2), at a low cooling rate.
- the transformation start temperature can be prevented from increasing. Thereby, the material variation of the forging steel is suppressed due to the difference in the cooling rate, and the strength difference in the forging steel due to the material variation hardly occurs. Therefore, the forging steel according to the present embodiment is excellent in gas cutting property and also in strength, toughness and durability.
- the forging steel of this embodiment has a bainite structure as its metal structure.
- the forging steel is excellent in strength.
- a metal structure is a bainite structure
- the area fraction of a bainite structure occupies 90 area% or more with respect to all the structures.
- the lower limit is preferably 99% by area.
- a method for measuring the area fraction of the bainite structure a cross-section of the high strength steel for forged steel products subjected to nital etching is photographed with an optical microscope, and the micrograph is visually observed. It is possible to carry out by dividing the metal structure and obtaining the area ratio.
- the average lath width in the forging steel of this embodiment is 3 ⁇ m or less.
- the upper limit of the average lath width is more preferably 2.5 ⁇ m.
- the lower limit of the average lath width is not particularly limited, but may be 1 ⁇ m, for example.
- the average lath width was obtained by observing a scanning microscope (SEM) at a magnification of 1,000 times using a sample taken from a 1/4 thickness portion from the surface layer of the forging steel, and averaging the average values of the three fields of view.
- SEM scanning microscope
- the lath width For example, when the high-strength forging steel of this embodiment is a plate having a square cross section, the depth of 1/4 of the thickness of the forging steel from the surface is a portion having a thickness of 1/4 from the surface layer.
- the average block width in the forging steel of this embodiment is 50 ⁇ m or less.
- the upper limit of the average block width is preferably 40 ⁇ m, more preferably 30 ⁇ m.
- the lower limit of the average block width is not particularly limited, but can be 1 ⁇ m, for example, and is preferably 10 ⁇ m.
- the average block width was determined by the following procedure using a sample taken from a 1/4 thickness portion from the surface layer of the forging steel. First, as pretreatment, the specimen was wet-polished with emery paper and buffed with diamond paste (particle size: 3 ⁇ m), and then electropolished with chromic acid / glacial acetic acid to finish a mirror surface.
- the crystal orientation of bainitic ferrite was determined by analyzing the electron beam backscatter diffraction pattern (EBSP) obtained by using the pre-treated sample and SEM.
- the crystal orientation of the sample was measured at an acceleration voltage of 15 kV or 25 kV using a Hitachi S3100 scanning electron microscope equipped with a TSL OIM measuring device or a JSM-6500F field emission electron microscope.
- measurement software measurement software “OIM Data Collection 3.0” and “OIM Data Collection 3.5” manufactured by TSL were used.
- an analysis was performed using analysis software “OIM Analysis 3” manufactured by TSL, and a crystal orientation map was created.
- a line was drawn perpendicularly to the longitudinal direction of the lath cross section on the crystal orientation map, the number of intersections with boundaries having an orientation difference of 15 ° or more was counted, and the average intercept length was defined as the average block width.
- the forging steel of this embodiment is C (carbon): 0.20 mass% or more and 0.35 mass% or less, Si (silicon): 0% by mass or more and 0.5% by mass or less, Mn (manganese): 0.50% by mass or more and 2.70% by mass or less, Cu (copper): 0 mass% or more and 1 mass% or less, Ni (nickel): 0% by mass or more and 2.00% by mass or less, Cr (chromium): 1.00% by mass to 2.50% by mass, Mo (molybdenum): 0.10% by mass or more and 0.55% by mass or less, V (vanadium): 0% by mass to 0.20% by mass, N (nitrogen): 30 mass ppm or more and 100 mass ppm or less, Al (aluminum): 0% by mass or more and 0.050% by mass or less, S (sulfur): 0% by mass or more and 0.020% by mass or less, O (oxygen): more than 0
- C element is an element which improves hardenability and contributes to strength improvement.
- the lower limit of the content of C element is 0.20% by mass.
- the lower limit of the C element content is preferably 0.25% by mass.
- the upper limit of the content of C element is 0.35% by mass, and the upper limit is preferably 0.32% by mass.
- the content of C element is less than the above lower limit, sufficient strength and hardenability of the forging steel may not be ensured.
- the content of the C element exceeds the above upper limit, the toughness of the forging steel may be reduced, and the reverse V segregation of C may be promoted, so that the machinability of the forging steel may be reduced.
- Si element is an element that contributes to reducing the amount of oxygen as a deoxidizing element, and is added if necessary. That is, the lower limit of the content of Si element in the forging steel of this embodiment is 0% by mass, and Si may not be contained. Moreover, the upper limit of content of Si element is 0.5 mass%, As this upper limit, 0.3 mass% is preferable and 0.2 mass% is more preferable. When the Si element content exceeds the above upper limit, reverse V segregation of the Si element is promoted, so that the toughness and hydrogen cracking resistance of the forging steel may be reduced.
- the Mn element is an element that enhances hardenability and contributes to strength improvement.
- the lower limit of the content of Mn element in the forging steel of this embodiment is 0.50% by mass.
- the upper limit of the content rate of Mn element is 2.70 mass%, and as this upper limit, 2.50 mass% is preferable and 1.50 mass% is more preferable.
- the content of the Mn element is less than the above lower limit, there is a possibility that sufficient strength and hardenability of the forging steel cannot be ensured and variation in crystal grain size cannot be sufficiently suppressed.
- the content of the Mn element exceeds the upper limit reverse V segregation of the Mn element is promoted, so that the toughness and hydrogen cracking resistance of the forging steel may be lowered.
- Cu element content is an element contributing to toughness improvement, and is added as necessary. That is, the lower limit of the Cu element content in the forging steel of the present embodiment is 0% by mass, and the Cu element may not be contained.
- the upper limit of the content of Cu element in the forging steel of this embodiment is 1% by mass, and the upper limit is preferably 0.5% by mass. On the other hand, when the content of the Cu element exceeds the above upper limit, the production cost may increase or hot cracking may occur.
- Ni element is an element contributing to improvement of strength and toughness, and is added if necessary. That is, the lower limit of the Ni element content in the forging steel of this embodiment is 0% by mass, and the Ni element may not be contained.
- the upper limit of the Ni element content in the forging steel of this embodiment is 2.00% by mass, and the upper limit is preferably 1.00% by mass, and more preferably 0.80% by mass. When the content of Ni element exceeds the above upper limit, reverse V segregation of Ni element is promoted, so that the toughness of forging steel may be reduced.
- the Cr element is an element that enhances hardenability and contributes to improvement of toughness.
- the lower limit of the content of Cr element in the forging steel of this embodiment is 1.00% by mass.
- the upper limit of the Cr content in the forging steel of this embodiment is 2.50% by mass, and the upper limit is preferably 2.00% by mass and more preferably 1.60% by mass.
- the content of Cr element in the forging steel of this embodiment is less than the above lower limit, sufficient toughness and hardenability of the forging steel may not be ensured.
- the content of the Cr element exceeds the above upper limit, reverse V segregation of the Cr element is promoted, so that the machinability of the forging steel may be reduced.
- Mo element content Mo element is an element which contributes to improvement of hardenability, strength, and toughness.
- the lower limit of the Mo element content in the forging steel of this embodiment is 0.10% by mass, and the lower limit is preferably 0.30% by mass.
- the upper limit of the content of Mo element in the forging steel of the present embodiment is 0.55% by mass, and the upper limit is preferably 0.5% by mass.
- the content of Mo element is less than the above lower limit, sufficient hardenability, strength and toughness of the forging steel may not be ensured.
- the content of the Mo element exceeds the above upper limit, micro segregation or weight segregation of the Mo element is promoted, so that the toughness of the forging steel may be reduced.
- the V element is an element that improves the hardenability and contributes to improving the strength, and is added as necessary. That is, the lower limit of the V content in the forging steel of this embodiment is 0% by mass, and the lower limit is preferably 0.04% by mass, and more preferably 0.08% by mass. Moreover, the upper limit of the content of V element in the forging steel of the present embodiment is 0.20% by mass, and the upper limit is preferably 0.15% by mass. When the content of V element exceeds the above upper limit, microsegregation is promoted due to the low equilibrium partition coefficient of V, so that the toughness of the forging steel may be reduced.
- the N element is an element that contributes to securing toughness by forming nitrides to refine crystal grains.
- the lower limit of the content of N element in the forging steel of this embodiment is 30 ppm by mass.
- the upper limit of the content of N element in the forging steel of this embodiment is 100 ppm by mass, and as this upper limit, 80 ppm by mass is preferable, and 60 ppm by mass is more preferable.
- the content of N element is less than the above lower limit, the toughness of the forging steel may not be ensured.
- strain aging is caused as solid solution N, and the toughness of the forging steel may be reduced.
- the Al element is an element that contributes to the reduction of the amount of oxygen as a deoxidizing element, and is added if necessary. That is, the lower limit of the content of Al element in the forging steel of this embodiment is 0% by mass, and the Al element may not be contained. 0.010 mass% is preferable and, as for the minimum of content of Al element in the steel for forging of this embodiment, 0.015 mass% is more preferable. Moreover, the upper limit of the content of Al element in the forging steel of this embodiment is 0.050% by mass. When the content of Al element is 0.010% by mass or more or 0.015% by mass or more, it becomes possible to sufficiently reduce the amount of oxygen in the forging steel. On the contrary, when the content of Al element exceeds the above upper limit, the oxide becomes coarse and the toughness of the forging steel may be lowered.
- O atom content O atoms are present as oxides in the forging steel of the present embodiment, and the content of O atoms cannot be 0% by mass. Therefore, the lower limit of the content of O atoms in the forging steel of this embodiment is more than 0% by mass.
- the upper limit of the content of O atoms in the forging steel of this embodiment is 30 ppm by mass, and the upper limit is preferably 15 ppm by mass, and more preferably 10 ppm by mass. When the content of O atoms exceeds the above upper limit, the oxides are coarsened and the toughness of the forging steel may be reduced.
- the lower limit of the content of the S element in the forging steel of this embodiment is 0% by mass, and the S element may not be contained.
- the content of the Al element is 0.010 mass% or more
- the content of the O atom is 15 mass ppm or less
- the content of the S element is 0 mass. It is preferable that it is more than% and 0.0030 mass% or less. This further improves the fatigue characteristics of the forging steel.
- the forging steel of this embodiment contains Fe (iron) and inevitable impurities in the balance in addition to the components described above.
- Inevitable impurities include, for example, P (phosphorus), S, Sn (tin), As (arsenic), Pb (lead), Nb (niobium), Ti ( Mixing of elements such as titanium) is allowed.
- the upper limit of the content of the P element that is an inevitable impurity is preferably 0.1% by mass, and more preferably 0.01% by mass.
- the content of the P element in the forging steel of the present embodiment exceeds the above upper limit, there is a risk of promoting grain boundary fracture due to grain boundary segregation.
- the upper limit of the content of S element which is an inevitable impurity is preferably 0.020% by mass, and more preferably 0.010% by mass.
- the content of the S element in the forging steel of the present embodiment exceeds the above upper limit, the sulfide inclusions may increase and the strength may be deteriorated. Even if the content of S element is in the range below the above upper limit, the content of S element is further reduced by refining (desulfurization) of the forging steel of the present embodiment, and the above-described fatigue characteristics are improved. It is preferable that
- the forging steel of this embodiment satisfies the following formulas (1) and (2). 1.15 ⁇ C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ⁇ 0.89 (1) 0.53 ⁇ C + Si / 30 + Mn / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 ⁇ 0.40 (2)
- the metal structure of the forging steel of the present embodiment is mainly transformed into a bainite structure at the time of production, and at this time, satisfying the above formulas (1) and (2) reduces the material variation of the forging steel. be able to.
- the mechanism is not clear, but by satisfying the above equation (1), the transformation start temperature can be reduced at a high cooling rate, and by satisfying the above equation (2), at a low cooling rate. It is considered that the transformation start temperature can be prevented from increasing. Thereby, the material variation of the forging steel is suppressed due to the difference in the cooling rate, and the strength difference in the forging steel due to the material variation hardly occurs.
- the function F represented by the equation (3) has a positive correlation with the strength of the forging steel mainly at the low cooling rate where the average cooling rate after the quenching treatment is 1 ° C./min. Moreover, the function G of Formula (4) has a negative correlation with toughness at a low cooling rate.
- “Strength” means a value obtained by measuring tensile strength (TS) based on JIS-Z2241 (2011) using a JIS-Z2201 (2011) No. 14 test piece ( ⁇ 6 ⁇ G.30). “Toughness” means a value obtained by measuring absorbed energy (vE) at room temperature by a Charpy impact test based on JIS-Z2242 (2005) using a test piece (2 mmV notch) of JIS-Z2202 (2005). It means that it is excellent, so that a numerical value is large in both strength and toughness.
- the strength of the forging steel at the low cooling rate is improved, but when the value of the function G exceeds 0.53, the toughness of the forging steel is increased. It becomes less than 150 J, and there is a risk of insufficient toughness as forging steel.
- the value of the function G decreases, the toughness of the forging steel is improved.
- the value of the function G is less than 0.40, the strength of the forging steel at the low cooling rate is less than 650 MPa. There is a risk of insufficient strength. Therefore, in order to obtain the forging steel excellent in strength and toughness, it is necessary to satisfy the formula (2).
- the function F represented by the formula (3) has a positive correlation with the strength of the forging steel mainly at the high cooling rate where the average cooling rate after the quenching treatment is 10 ° C./min.
- the value of the function F is less than 0.90, the strength of the forging steel at a high cooling rate is less than 650 MPa, and the strength of the forging steel may be insufficient. Therefore, in order to obtain a forging steel excellent in strength, the value of the function F needs to be 0.90 or more.
- the increase in the strength of the forging steel at the high cooling rate is larger than the increase in the strength of the forging steel at the low cooling rate.
- the difference between the strength of the forging steel at the high cooling rate and the strength of the forging steel at the low cooling rate tends to increase. That is, the difference in the cooling rate tends to cause a difference in strength of the forging steel, which may reduce the durability.
- the function G is around 0.53
- the strength of the forging steel at the low cooling rate is about 700 MPa.
- TS tensile strength
- 700MPa 700MPa
- 800 MPa 800 MPa is more preferable.
- the forging steel has a tensile strength less than the lower limit, the forging steel may have insufficient strength.
- the tensile strength of the forging steel exceeds the above upper limit, the cooling temperature dependency of the strength of the forging steel tends to occur, and the durability of the forging steel may be insufficient.
- the upper limit of the difference in thickness (TS) is preferably 100 MPa, more preferably 75 MPa, and further preferably 50 MPa. When the difference exceeds the above upper limit, the cooling temperature dependency of the strength of the forging steel is likely to occur, and the durability of the forging steel may be insufficient.
- the lower limit of the absorbed energy of the forging steel of this embodiment measured at room temperature by the Charpy impact test 150J is preferable, and 180J is more preferable.
- an upper limit of the said absorbed energy 260J is preferable.
- the absorbed energy is less than the lower limit, the toughness of the forging steel may be insufficient.
- the said absorbed energy exceeds the said upper limit, there exists a possibility that the intensity
- the large forged steel product of this embodiment is manufactured using the forging steel of this embodiment. For this reason, the large forged steel product of this embodiment is excellent in durability as well as strength and toughness. Therefore, the large forged steel product of the present embodiment can be suitably used as a part for realizing improved output and compactness of marine and power generation diesel engines.
- the forging steel according to the present embodiment is manufactured, for example, through a melting process, a casting process, a heating process, and a material forging process, and a large forged steel product using the forging steel is subjected to a part forging process and a quenching pretreatment. It is manufactured by a manufacturing method including a process, a quenching process, and a machining process.
- 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 and 1350 ° C. or lower in order to perform processing within a favorable range of the deformability of the material. 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.
- Part forging process In the component forging process, the steel ingot (forging steel) forged in the material forging process is processed into a large forged steel product such as a crankshaft.
- the crankshaft can be machined by forging as a block in which the crank arm and crankpin are integrated, and by freezing and forging into a crankshaft shape by gas cutting and machining.
- Examples include the RR forging method and the TR forging method in which a forging process is performed so as to be a central part, and parts that are likely to deteriorate characteristics due to center segregation are integrally forged so as to be all the central parts of the crankshaft.
- the RR forging method and the TR forging method are preferable because the surface layer side of the crankshaft can be occupied by a portion with high cleanliness, and a crankshaft excellent in strength and durability can be easily obtained.
- the quenching process is a process in which the forged product cooled in the pre-quenching process is heated to a predetermined temperature and held for a predetermined time, and then cooled to a predetermined temperature.
- the quenching temperature is preferably 800 ° C. or more and 950 ° C. or less, and the holding time is preferably 1 hour or more.
- cooling temperature 450 to 530 degreeC is preferable.
- the rate of temperature rise is preferably 30 ° C./hr or more and 70 ° C./hr or less, and the cooling rate is preferably 15 ° C./min or less.
- Sub-zero treatment is also called deep cooling treatment, and is a treatment in which a forged product is immediately cooled from a quenching temperature to a cooling temperature of 0 ° C. or lower.
- a treatment for cooling the forged product to about ⁇ 80 ° C. using methanol or ethanol and dry ice as a cryogen may be employed.
- a process of cooling the forged product to about ⁇ 130 ° C. using carbon dioxide as a cryogen may be employed, or a process of cooling the forged product using liquid nitrogen may be employed.
- the cooling temperature in the sub-zero treatment is preferably ⁇ 190 ° C. or higher and ⁇ 80 ° C. or lower.
- the block width can be reduced to 50 ⁇ m or less by performing the sub-zero treatment.
- the tempering process is a process in which the forged product subjected to the quenching process is gradually heated to a predetermined temperature, held for a certain period of time, and then cooled to room temperature.
- the tempering temperature is preferably from 550 ° C. to 650 ° C.
- the holding time is preferably from 5 hours to 20 hours.
- the rate of temperature rise is preferably 30 ° C./hr or more and 70 ° C./hr or less
- the cooling rate is preferably 15 ° C./min or less.
- the steel for forging of this embodiment is excellent in strength because the metal structure is a bainite structure.
- This metal structure is mainly transformed into a bainite structure at the time of producing the forging steel of the present embodiment.
- the transformation start temperature is lowered at a high cooling rate.
- the above formula (2) it is possible to suppress an increase in the transformation start temperature at a low cooling rate.
- strength and toughness can be secured by setting each elemental composition of the forging steel of this embodiment within the above range. Therefore, the forging steel has excellent durability as well as strength and toughness.
- the large forged steel product using the forging steel according to the present embodiment can be suitably used as a part for realizing output improvement and downsizing of a marine diesel engine, a power generation diesel engine, and the like.
- the forging steel described above may further contain at least one of 0 mass% to 0.07 mass% Nb and 0 mass% to 0.03 mass% B. That is, the forging steel of this embodiment is C: 0.20 mass% or more and 0.35 mass% or less, Si: 0% by mass or more and 0.5% by mass or less, Mn: 0.50% by mass or more and 2.70% by mass or less, Cu: 0% by mass or more and 1% by mass or less, Ni: 0% by mass or more and 2.00% by mass or less, Cr: 1.00% by mass to 2.50% by mass, Mo: 0.10% by mass to 0.55% by mass, V: 0% by mass to 0.20% by mass, N: 30 mass ppm or more and 100 mass ppm or less, Al: 0% by mass or more and 0.050% by mass or less, S: 0% by mass or more and 0.020% by mass or less, O: more than 0 mass ppm and 50 mass ppm or less, Nb (niobiobiadium tungsten
- the Nb element is an element that improves the hardenability and contributes to improving the strength, and is added as necessary. That is, the lower limit of the Nb content in the forging steel of this embodiment is 0% by mass, and the lower limit is preferably 0.01% by mass and more preferably 0.02% by mass.
- the upper limit of the Nb element content in the forging steel of this embodiment is 0.07% by mass, and the upper limit is preferably 0.05% by mass. When the content of the Nb element exceeds the upper limit, microsegregation is promoted due to the low equilibrium distribution coefficient of Nb, so that the toughness of the forging steel may be reduced.
- the B element is an element that improves the hardenability and contributes to improving the strength, and is added as necessary. That is, the lower limit of the B content in the forging steel of this embodiment is 0% by mass, and the lower limit is preferably 0.01% by mass. Moreover, the upper limit of B element content in the forging steel of this embodiment is 0.03% by mass, and the upper limit is preferably 0.02% by mass. When the content of B element exceeds the above upper limit, microsegregation is promoted due to the low equilibrium distribution coefficient of B, so that the toughness of the forging steel may be reduced.
- C 0.20 mass% or more and 0.35 mass% or less
- Si 0% by mass or more and 0.5% by mass or less
- Mn 0.50% by mass or more and 2.70% by mass or less
- Cu 0% by mass or more and 1% by mass or less
- Ni 0% by mass or more and 2.00% by mass or less
- Cr 1.00% by mass to 2.50% by mass
- Mo 0.10% by mass to 0.55% by mass
- V 0% by mass to 0.20% by mass
- N 30 mass ppm or more and 100 mass ppm or less
- Al 0% by mass or more and 0.050% by mass or less
- S 0% by mass or more and 0.020% by mass or less
- O more than 0 mass ppm and 50 mass ppm or less
- the balance Fe and a composition that is an inevitable impurity
- the metal structure is a bainite structure, and the average lath width is 3.0 ⁇ m or less
- the forging steel of the present embodiment has an average lath width of 3.0 ⁇ m or less, so that gas cutting performance is improved.
- the forging steel is excellent in strength because its metal structure is a bainite structure, and further has excellent durability as well as toughness.
- the forging steel preferably has an average block width of 50 ⁇ m or less, which further improves toughness.
- the forging steel is Nb: 0% by mass or more and 0.07% by mass or less, or B: 0% by mass or more and 0.03% by mass or less, It is preferable to further contain at least one of the following. This further improves the hardenability and strength.
- the Al content is 0.010 mass% or more
- the S content is more than 0 mass% to 0.0030 mass% or less
- the O content is 15 massppm or less. Is preferred. This further improves the fatigue characteristics.
- the large forged steel product according to another aspect of the present invention is manufactured using the high-strength forged steel described above. For this reason, the large forged steel product of this embodiment is excellent in durability with strength and toughness.
- the average block width was determined by performing EBSD measurement at a magnification of 400 times using a sample taken from the t / 4 position (t: plate thickness) of forged steel, and the longitudinal direction of the lath cross section on the obtained crystal orientation map. A line was drawn vertically, the number of intersections with a boundary having an orientation difference of 15 ° or more was counted, and the average intercept length was defined as the average block width.
- x described in the average block width of Table 3 indicates that cracking occurred in the forged steel and the average block width could not be determined.
- the tensile test was implemented about the test piece from the said t / 4 position of the forging steel of each component composition shown in Table 1 and Table 3.
- FIG. The tensile test was performed by measuring the tensile strength (TS) based on JIS-Z2241 (2011) using a JIS-Z2201 (2011) No. 14 test piece ( ⁇ 6 ⁇ G.30). It means that the steel for forging is excellent in strength, so that the numerical value of this tensile strength (TS) is large.
- the tensile test measures the tensile strength of the forging steel with an average cooling rate of 1 ° C./min and the tensile strength of the forging steel with an average cooling rate of 10 ° C./min.
- the strength 1 and strength 2 in Tables 2 and 4 are shown.
- both the tensile strength of the forging steel with an average cooling rate of 1 ° C./min and the tensile strength of the forging steel with an average cooling rate of 10 ° C./min are 650 MPa or more, “good” Judgment was made, and less than 650 MPa was judged as “not good”.
- the difference between the tensile strength of the forging steel with an average cooling rate of 1 ° C./min and the tensile strength of the forging steel with an average cooling rate of 10 ° C./min (strength difference in Tables 2 and 4). ) Is 100 MPa or less, the forging steel is excellent in durability and judged as “good”, and when it exceeds 100 MPa, it is judged as “not good”. This difference was noted as the intensity difference in Tables 2 and 4.
- the forging steel was judged to be excellent in toughness and “good”, and when it was less than 150 J, it was judged “not good”.
- This absorbed energy is shown in the toughness in Tables 2 and 4.
- x described as toughness in Table 4 indicates that cracking occurred in the forged steel and the toughness could not be determined.
- the gas cutting property is “good” when the forged steel is cut at a cutting speed of 170 mm / min, an oxygen pressure of 5.5 kg / cm 2 and a propane gas pressure of 0.55 kg / cm 2 and the gas cut surface roughness is 5.5 ⁇ m or less. And when it exceeds 5.5 ⁇ m, it was determined as “not good”.
- the gas cut surface roughness was measured according to JIS-B0601 (2013). This gas cut surface roughness is shown in the gas cutting properties of Tables 2 and 4.
- the forging steel of 15 examples has the tensile strength of forging steel with an average cooling rate of 1 ° C./min, the tensile strength, durability and toughness of forging steel with an average cooling rate of 10 ° C./min. In addition, it was confirmed that both gas cutting property and gas cutting property were good.
- the forging steel of 42 comparative examples has the tensile strength of forging steel with an average cooling rate of 1 ° C./min, the tensile strength, durability, and toughness of the forging steel with an average cooling rate of 10 ° C./min. It was also confirmed that at least one of the gas cutting properties was not good.
- each comparative example will be examined.
- the forging steel No. 34 is a comparative example whose composition does not satisfy the range defined in the present invention. This No. 16-No.
- Each of the 34 forging steels has at least the tensile strength of the forging steel with an average cooling rate of 1 ° C./min, the tensile strength of the forging steel with an average cooling rate of 10 ° C./min, and toughness. One is not good.
- the forging steel No. 39 is a comparative example that does not satisfy at least one of the above formulas (1) and (2). This No. 35-No.
- the forging steel No. 39 has a large difference between the tensile strength of the forging steel with an average cooling rate of 1 ° C./min and the tensile strength of the forging steel with an average cooling rate of 10 ° C./min. Not good.
- the forging steel No. 42 is a comparative example that does not satisfy the average lath width defined by the present invention. This No. 40-No. Since the forging steel No. 42 does not have the average lath width defined in the present invention, the gas cutting property is not good.
- the forging steel of 53 examples has the tensile strength of forging steel with an average cooling rate of 1 ° C./min, the tensile strength, durability, and toughness of forging steel with an average cooling rate of 10 ° C./min. In addition, it was confirmed that both gas cutting property and gas cutting property were good. In particular, no. 43-No. The forging steel of 46 examples had an average block width of 20 ⁇ m to 27 ⁇ m and was confirmed to exhibit excellent tensile strength, durability, toughness, and gas cutting ability.
- the forging steel No. 57 is a comparative example in which the sub-zero treatment was performed at a cooling temperature outside the range of ⁇ 190 ° C. to ⁇ 80 ° C., which is a preferable cooling temperature.
- No. 54-No. Forging steel No. 55 is a comparative example in which sub-zero treatment was performed at a temperature below the lower limit of the preferred cooling temperature, cracking occurred in the forging steel, and the toughness was not good.
- No. 56-No. Steel for forging No. 57 is a comparative example in which sub-zero treatment was performed at a temperature exceeding the upper limit of the preferable cooling temperature, the average block width exceeds 50 ⁇ m which is the upper limit of the preferable range, and the toughness is not good.
- the forging steel No. 59 is a comparative example in which at least one of the Nb element content and the B element content deviates from the preferred range.
- No. No. 58 forging steel is a comparative example in which the Nb element content exceeds 0.07% by mass, which is the upper limit of the preferred range.
- the forging steel No. 59 is a comparative example in which the content of B element exceeds 0.03% by mass which is the upper limit of the preferable range, and none of the toughness is good.
- the forging steel and the large forged steel product of the present invention can be suitably used for, for example, marine diesel engines and power generation diesel engines.
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Abstract
Description
C:0.20質量%以上0.35質量%以下、
Si:0質量%以上0.5質量%以下、
Mn:0.50質量%以上2.70質量%以下、
Cu:0質量%以上1質量%以下、
Ni:0質量%以上2.00質量%以下、
Cr:1.00質量%以上2.50質量%以下、
Mo:0.10質量%以上0.55質量%以下、
V:0質量%以上0.20質量%以下、
N:30質量ppm以上100質量ppm以下、
Al:0質量%以上0.050質量%以下、
S:0質量%以上0.020質量%以下、
O:0質量ppm超50質量ppm以下、並びに
残部:Fe及び不可避的不純物である組成を有し、
金属組織がベイナイト組織であり、かつ、平均ラス幅が3.0μm以下であり、
下記式(1)及び(2)を満たすことを特徴とする。
1.15≧C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14≧0.89 ・・・(1)
0.53≧C+Si/30+Mn/20+Ni/60+Cr/20+Mo/15+V/10≧0.40 ・・・(2)
C:0.20質量%以上0.35質量%以下、
Si:0質量%以上0.5質量%以下、
Mn:0.50質量%以上2.70質量%以下、
Cu:0質量%以上1質量%以下、
Ni:0質量%以上2.00質量%以下、
Cr:1.00質量%以上2.50質量%以下、
Mo:0.10質量%以上0.55質量%以下、
V:0質量%以上0.20質量%以下、
N:30質量ppm以上100質量ppm以下、
Al:0質量%以上0.050質量%以下、
S:0質量%以上0.020質量%以下、
O:0質量ppm超50質量ppm以下、並びに
残部:Fe及び不可避的不純物である組成を有し、
金属組織がベイナイト組織であり、かつ、平均ラス幅が3.0μm以下であり、
下記式(1)及び(2)を満たすことを特徴とする。
1.15≧C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14≧0.89・・・(1)
0.53≧C+Si/30+Mn/20+Ni/60+Cr/20+Mo/15+V/10≧0.40・・・(2)
以下、本発明に係る鍛造用鋼の実施形態について説明する。
本実施形態の鍛造用鋼は、その金属組織がベイナイト組織である。このように金属組織がベイナイト組織であることにより、当該鍛造用鋼は強度に優れる。
本実施形態の鍛造用鋼は、
C(炭素):0.20質量%以上0.35質量%以下、
Si(ケイ素):0質量%以上0.5質量%以下、
Mn(マンガン):0.50質量%以上2.70質量%以下、
Cu(銅):0質量%以上1質量%以下、
Ni(ニッケル):0質量%以上2.00質量%以下、
Cr(クロム):1.00質量%以上2.50質量%以下、
Mo(モリブデン):0.10質量%以上0.55質量%以下、
V(バナジウム):0質量%以上0.20質量%以下、
N(窒素):30質量ppm以上100質量ppm以下、
Al(アルミニウム):0質量%以上0.050質量%以下、
S(硫黄):0質量%以上0.020質量%以下、
O(酸素):0質量ppm超50質量ppm以下、並びに
残部:Fe及び不可避的不純物である組成を有する。
C元素は、焼き入れ性を高めると共に強度向上に寄与する元素である。本実施形態の鍛造用鋼においてC元素の含有量の下限は0.20質量%である。このC元素の含有量の下限としては0.25質量%が好ましい。また、C元素の含有量の上限は0.35質量%であり、この上限としては0.32質量%が好ましい。C元素の含有量が上記下限未満である場合、鍛造用鋼の十分な強度と焼き入れ性とが確保できないおそれがある。一方、C元素の含有量が上記上限を超える場合、鍛造用鋼の靭性が低下するおそれや、Cの逆V偏析が助長されるため鍛造用鋼の被削性が低下するおそれがある。
Si元素は、脱酸元素として酸素量低減に寄与する元素であり、必要により添加される。すなわち、本実施形態の鍛造用鋼におけるSi元素の含有量の下限は0質量%であり、Siは含まれていなくてもよい。また、Si元素の含有量の上限は0.5質量%であり、この上限としては0.3質量%が好ましく、0.2質量%がより好ましい。Si元素の含有量が上記上限を超える場合、Si元素の逆V偏析が助長されるため、鍛造用鋼の靭性や耐水素割れ性が低下するおそれがある。
Mn元素は、焼き入れ性を高めると共に強度向上に寄与する元素である。本実施形態の鍛造用鋼におけるMn元素の含有量の下限は0.50質量%である。また、Mn元素の含有率の上限は2.70質量%であり、この上限としては2.50質量%が好ましく、1.50質量%がより好ましい。Mn元素の含有量が上記下限未満である場合、鍛造用鋼の十分な強度と焼き入れ性とが確保できないおそれや結晶粒度のばらつきを十分に抑制できないおそれがある。一方、Mn元素の含有量が上記上限を超える場合、Mn元素の逆V偏析が助長されるため、鍛造用鋼の靭性や耐水素割れ性が低下するおそれがある。
Cu元素は、靭性向上に寄与する元素であり、必要により添加される。すなわち、本実施形態の鍛造用鋼におけるCu元素の含有量の下限は0質量%であり、Cu元素は含まれていなくてもよい。本実施形態の鍛造用鋼におけるCu元素の含有量の上限は1質量%であり、この上限としては0.5質量%が好ましい。一方、Cu元素の含有量が上記上限を超える場合、製造コストが増大するおそれや熱間割れが生じるおそれがある。
Ni元素は、強度及び靭性の向上に寄与する元素であり、必要により添加される。つまり、本実施形態の鍛造用鋼におけるNi元素の含有量の下限は0質量%であり、Ni元素は含まれていなくてもよい。また、本実施形態の鍛造用鋼におけるNi元素の含有量の上限は2.00質量%であり、この上限としては1.00質量%が好ましく、0.80質量%がより好ましい。Ni元素の含有量が上記上限を超える場合、Ni元素の逆V偏析が助長されるため、鍛造用鋼の靭性が低下するおそれがある。
Cr元素は、焼き入れ性を高めると共に靭性向上に寄与する元素である。本実施形態の鍛造用鋼におけるCr元素の含有量の下限は1.00質量%である。また、本実施形態の鍛造用鋼におけるCr含有量の上限は2.50質量%であり、この上限としては、2.00質量%が好ましく、1.60質量%がより好ましい。本実施形態の鍛造用鋼におけるCr元素の含有量が上記下限未満である場合には、鍛造用鋼の十分な靭性と焼き入れ性とが確保できないおそれがある。一方、Cr元素の含有量が上記上限を超える場合、Cr元素の逆V偏析が助長されるため、鍛造用鋼の被削性が低下するおそれがある。
Mo元素は、焼き入れ性、強度及び靭性の向上に寄与する元素である。本実施形態の鍛造用鋼におけるMo元素の含有量の下限は0.10質量%であり、この下限としては0.30質量%が好ましい。また、本実施形態の鍛造用鋼におけるMo元素の含有量の上限は0.55質量%であり、この上限としては0.5質量%が好ましい。Mo元素の含有量が上記下限未満である場合、鍛造用鋼の十分な焼き入れ性、強度及び靭性が確保できないおそれがある。一方、Mo元素の含有量が上記上限を超える場合、Mo元素のミクロ偏析や重量偏析が助長されるため、鍛造用鋼の靭性が低下するおそれがある。
V元素は、焼き入れ性を高めると共に強度向上に寄与する元素であり、必要により添加される。すなわち、本実施形態の鍛造用鋼におけるV含有量の下限は0質量%であり、この下限としては0.04質量%が好ましく、0.08質量%がより好ましい。また、本実施形態の鍛造用鋼におけるV元素の含有量の上限は0.20質量%であり、この上限としては0.15質量%が好ましい。V元素の含有量が上記上限を超える場合、Vの平衡分配係数の低さに起因してミクロ偏析が助長されるため、鍛造用鋼の靭性が低下するおそれがある。
N元素は、窒化物を形成して結晶粒を細粒化し、靭性の確保に寄与する元素である。本実施形態の鍛造用鋼におけるN元素の含有量の下限は30質量ppmである。また、本実施形態の鍛造用鋼におけるN元素の含有量の上限は100質量ppmであり、この上限としては80質量ppmが好ましく、60質量ppmがより好ましい。N元素の含有量が上記下限未満である場合、鍛造用鋼の靭性が確保できないおそれがある。一方、N元素の含有量が上記上限を超える場合、固溶Nとしてひずみ時効をもたらし、鍛造用鋼の靭性を低下させるおそれがある。
Al元素は、脱酸元素として酸素量低減に寄与する元素であり、必要により添加される。すなわち、本実施形態の鍛造用鋼におけるAl元素の含有量の下限は0質量%であり、Al元素は含まれていなくてもよい。本実施形態の鍛造用鋼におけるAl元素の含有量の下限は、0.010質量%が好ましく、0.015質量%がより好ましい。また、本実施形態の鍛造用鋼におけるAl元素の含有量の上限は0.050質量%である。Al元素の含有量が0.010質量%以上又は0.015質量%以上であることで、鍛造用鋼の十分な酸素量低減が可能となる。逆に、Al元素の含有量が上記上限を超える場合、酸化物の粗大化を招き、鍛造用鋼の靭性が低下するおそれがある。
O原子は、本実施形態の鍛造用鋼中に酸化物として存在し、O原子の含有量は0質量%にできない。従って、本実施形態の鍛造用鋼のO原子の含有量の下限は0質量%超である。一方、本実施形態の鍛造用鋼におけるO原子の含有量の上限は30質量ppmであり、この上限としては15質量ppmが好ましく、10質量ppmがより好ましい。O原子の含有量が上記上限を超える場合、酸化物の粗大化を招き、鍛造用鋼の靭性が低下するおそれがある。
本実施形態の鍛造用鋼におけるS元素の含有量の下限は0質量%であり、S元素は含まれていなくてもよい。ただし、本実施形態の鍛造用鋼は、上記Al元素の含有量が0.010質量%以上であり、上記O原子の含有量が15質量ppm以下であると共に、S元素の含有量が0質量%超0.0030質量%以下であることが好ましい。これによって、鍛造用鋼の疲労特性がより向上する。
本実施形態の鍛造用鋼は、上述した成分以外に残部にFe(鉄)及び不可避的不純物を含む。また、不可避的不純物としては、例えば原料、資材、製造設備等の状況によって持ち込まれるP(リン)、S、Sn(スズ)、As(ヒ素)、Pb(鉛)、Nb(ニオブ)、Ti(チタン)等の元素の混入が許容される。
本実施形態の鍛造用鋼は、下記式(1)及び(2)を満たす。
1.15≧C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14≧0.89・・・(1)
0.53≧C+Si/30+Mn/20+Ni/60+Cr/20+Mo/15+V/10≧0.40・・・(2)
F=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14・・・(3)
G=C+Si/30+Mn/20+Ni/60+Cr/20+Mo/15+V/10・・・(4)
本実施形態の鍛造用鋼の引張強さ(TS)の下限としては、650MPaが好ましく、700MPaがより好ましい。また、本実施形態の鍛造用鋼の引張強さの上限としては、850MPaが好ましく、800MPaがより好ましい。鍛造用鋼の引張強さが上記下限未満である場合、鍛造用鋼の強度が不足するおそれがある。一方、鍛造用鋼の引張強さが上記上限を超える場合、鍛造用鋼の強度の冷却温度依存性が発生し易くなり、鍛造用鋼の耐久性が不足するおそれがある。
本実施形態の大型鍛鋼品は、本実施形態の鍛造用鋼を用いて製造される。このため、本実施形態の大型鍛鋼品は強度及び靭性と共に耐久性にも優れる。従って、本実施形態の大型鍛鋼品は、船舶用、発電用ディーゼルエンジンの出力向上やコンパクト化を実現するための部品として好適に用いることができる。
本実施形態の鍛造用鋼は、例えば溶製工程、鋳造工程、加熱工程、素材鍛造工程を経ることで製造され、当該鍛造用鋼を用いた大型鍛鋼品は、部品鍛造工程、焼き入れ前処理工程、焼き入れ処理工程及び機械加工工程を備える製造方法により製造される。
鋳造工程では、溶製工程で成分調整した鋼を用いて鋼塊(インゴット)を鋳造する。鍛造用鋼の場合は、主としてインゴット鋳造が採用されるが、連続鋳造法を採用することも可能である。
加熱工程では、所定の温度で所定時間、鋼塊を加熱する。低温になると材料の変形抵抗が増大するので、材料の変形能の良好な範囲で加工を行うために、加熱温度は例えば1150℃以上1350℃以下とする。また、鋼塊の表面と内部との温度を均一にするために所定の加熱時間が必要であり、加熱時間は例えば3時間以上とする。加熱時間は、一般的に被加工物の直径の2乗に比例すると考えられており、大型材ほど加熱保持時間は長くなる。
素材鍛造工程では、加熱工程で加熱された鋼塊を鍛造する。ザク巣やミクロポロシティ等の鋳造欠陥を圧着させるために、鍛錬成形ではガス切断性向上のため以下の条件が必要となる。このようにして本実施形態の鍛造用鋼が得られる。
この温度は再結晶により旧オーステナイトが微細になる最適温度である。この温度範囲での累積圧下率を20%以上、好ましくは25%以上とすることによって、適正サイズのラス組織が得られる。この温度範囲から外れると粗大な組織となりガス切断性が確保できない。また、この温度範囲において、S=A0/A≧3.0、好ましくはS≧3.5とするとよい。ここで、A0は鍛造前の断面積を示し、Aは鍛造後の断面積を示す。
部品鍛造工程では、素材鍛造工程で鍛造された鋼塊(鍛造用鋼)をクランク軸等の大型鍛鋼品に加工する。例えばクランク軸への加工方法としては、クランクアームとクランクピンを一体としたブロックとして鍛造し、ガス切断及び機械加工によってクランク軸形状に仕上げる自由鍛造法や、鋼塊の軸心がクランク軸の軸心部となる様に鍛造加工し、中心偏析により特性の劣化を起こし易い部分をクランク軸の全ての軸心部となるように一体に鍛造加工するRR鍛造法及びTR鍛造法が例示される。中でもRR鍛造法及びTR鍛造法が、クランク軸の表層側を清浄度の高い部分で占めさせることができ、強度及び耐久性に優れたクランク軸が得られ易いので好ましい。
焼き入れ処理工程では、焼き入れ処理を行った後、焼き戻し処理を行う。焼き入れ処理の前に、鍛造品を冷却する焼き入れ前処理工程が実施される。焼き入れ処理は、焼き入れ前処理工程で冷却された鍛造品を、所定温度まで昇温して所定時間保持した後、所定温度まで冷却する処理である。焼き入れ温度としては800℃以上950℃以下が好ましく、上記保持時間としては1時間以上が好ましい。また冷却温度としては、450℃以上530℃以下が好ましい。また、昇温速度としては、30℃/hr以上70℃/hr以下が好ましく、冷却速度としては、15℃/min以下が好ましい。
焼き入れ処理工程後の鍛造品から、ガス切断後に切削、研削を含む仕上げ機械加工を施すことで、本実施形態の大型鍛造用部品を得ることができる。
本実施形態の鍛造用鋼は、その金属組織がベイナイト組織であるため、強度に優れる。この金属組織は、本実施形態の鍛造用鋼の製造時に主としてベイナイト組織に変態したものであり、この変態時に、上記式(1)を満たすことで、高冷却速度での変態開始温度の低温化が抑制でき、上記式(2)を満たすことで、低冷却速度での変態開始温度の高温化を抑制できる。このように冷却速度による変態開始温度の相違を抑制することで、鍛造用鋼の材質ばらつきを抑制することができる。さらに本実施形態の鍛造用鋼の各元素組成を上記範囲内とすることにより、強度及び靭性が確保できる。従って、当該鍛造用鋼は、強度及び靭性と共に耐久性に優れる。また、平均ラス幅が所定範囲内であるので、ガス切断性に優れ、生産性の向上を図ることができる。このため、本実施形態の鍛造用鋼を用いた大型鍛鋼品は、船舶用ディーゼルエンジンや発電用ディーゼルエンジン等の出力向上やコンパクト化を実現するための部品として好適に用いることができる。
上述した鍛造用鋼が、0質量%以上0.07質量%以下のNb、又は0質量%以上0.03質量%以下のBの少なくとも一方をさらに含有することも可能である。つまり、本実施形態の鍛造用鋼は、
C:0.20質量%以上0.35質量%以下、
Si:0質量%以上0.5質量%以下、
Mn:0.50質量%以上2.70質量%以下、
Cu:0質量%以上1質量%以下、
Ni:0質量%以上2.00質量%以下、
Cr:1.00質量%以上2.50質量%以下、
Mo:0.10質量%以上0.55質量%以下、
V:0質量%以上0.20質量%以下、
N:30質量ppm以上100質量ppm以下、
Al:0質量%以上0.050質量%以下、
S:0質量%以上0.020質量%以下、
O:0質量ppm超50質量ppm以下、
Nb(ニオブ):0質量%以上0.07質量%以下、
B(ホウ素):0質量%以上0.03質量%以下、並びに
残部:Fe及び不可避的不純物の組成を有することも可能である。これにより、本実施形態の鍛造用鋼の焼き入れ性及び強度を向上させることができる。
Nb元素は、焼き入れ性を高めると共に強度向上に寄与する元素であり、必要により添加される。すなわち、本実施形態の鍛造用鋼におけるNb含有量の下限は0質量%であり、この下限としては0.01質量%が好ましく、0.02質量%がより好ましい。また、本実施形態の鍛造用鋼におけるNb元素の含有量の上限は0.07質量%であり、この上限としては0.05質量%が好ましい。Nb元素の含有量が上記上限を超える場合、Nbの平衡分配係数の低さに起因してミクロ偏析が助長されるため、鍛造用鋼の靭性が低下するおそれがある。
B元素は、焼き入れ性を高めると共に強度向上に寄与する元素であり、必要により添加される。すなわち、本実施形態の鍛造用鋼におけるB含有量の下限は0質量%であり、この下限としては0.01質量%が好ましい。また、本実施形態の鍛造用鋼におけるB元素の含有量の上限は0.03質量%であり、この上限としては0.02質量%が好ましい。B元素の含有量が上記上限を超える場合、Bの平衡分配係数の低さに起因してミクロ偏析が助長されるため、鍛造用鋼の靭性が低下するおそれがある。
C:0.20質量%以上0.35質量%以下、
Si:0質量%以上0.5質量%以下、
Mn:0.50質量%以上2.70質量%以下、
Cu:0質量%以上1質量%以下、
Ni:0質量%以上2.00質量%以下、
Cr:1.00質量%以上2.50質量%以下、
Mo:0.10質量%以上0.55質量%以下、
V:0質量%以上0.20質量%以下、
N:30質量ppm以上100質量ppm以下、
Al:0質量%以上0.050質量%以下、
S:0質量%以上0.020質量%以下、
O:0質量ppm超50質量ppm以下、並びに
残部:Fe及び不可避的不純物である組成を有し、
金属組織がベイナイト組織であり、かつ、平均ラス幅が3.0μm以下であり、
下記式(1)及び(2)を満たすことを特徴とする。
1.15≧C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14≧0.89・・・(1)
0.53≧C+Si/30+Mn/20+Ni/60+Cr/20+Mo/15+V/10≧0.40・・・(2)
Nb:0質量%以上0.07質量%以下、又は
B:0質量%以上0.03質量%以下、
の少なくとも一方をさらに含有することが好ましい。これによって焼き入れ性及び強度がより向上する。
表1及び表3に示す組成を有し、所定の平均ラス幅(表1及び表3に示す平均ラス幅)を有するNo.1~No.42及びNo.51~No.67の450mm厚の板状の鍛鋼について後述する評価を行った。平均ラス幅は、鍛鋼のt/4位置(t:板厚)から採取したサンプルを用いて、倍率:1,000倍にて走査型顕微鏡(SEM)観察を行い、3視野の平均値をその鋼種の平均ラス幅とした。また、No.51~No.58の鍛鋼については、表4に示す冷却温度でサブゼロ処理を実施し、平均ブロック幅を決定した。平均ブロック幅は、鍛鋼のt/4位置(t:板厚)から採取したサンプルを用いて、倍率:400倍にてEBSD測定を行い、得られた結晶方位マップ上でラス断面の長手方向と垂直に線を引き、15°以上の方位差を持つ境界との交点の数を数え、その平均切片長さを平均ブロック幅とした。なお、表3の平均ブロック幅に記した×は、鍛鋼に割れが発生して平均ブロック幅を決定できなかったことを示す。
強度評価として、表1及び表3に示す各成分組成の鍛造用鋼の上記t/4位置からの試験片について引張試験を実施した。引張試験は、JIS-Z2201(2011)の14号試験片(φ6×G.30)を用いJIS-Z2241(2011)に基づいて引張強さ(TS)を測定することで行った。この引張強さ(TS)の数値が大きいほど、鍛造用鋼が強度に優れることを意味する。引張試験は、平均冷却速度を1℃/minとした鍛造用鋼の上記引張強さと、平均冷却速度を10℃/minとした鍛造用鋼の上記引張強さとについて測定し、それぞれの測定結果を表2及び表4の強度1及び強度2に記した。平均冷却速度を1℃/minとした鍛造用鋼の上記引張強さと、平均冷却速度を10℃/minとした鍛造用鋼の上記引張強さとの双方が650MPa以上である場合、「良好」と判断し、650MPa未満を「良好でない」と判断した。また、平均冷却速度を1℃/minとした鍛造用鋼の上記引張強さと、平均冷却速度を10℃/minとした鍛造用鋼の上記引張強さとの差(表2及び表4における強度差)が100MPa以下である場合、鍛造用鋼が耐久性に優れ、「良好」と判断し、100MPa超の場合「良好でない」と判断した。この差を表2及び表4の強度差として記した。
靭性評価として、表1及び表3に示す各成分組成の鍛造用鋼(t/4位置)についてシャルピー衝撃試験を実施した。シャルピー衝撃試験は、JIS-Z2202(2005)の試験片(2mmVノッチ)を用いJIS-Z2242(2005)に基づいて室温で吸収エネルギー(vE)を測定することで行った。吸収エネルギーの数値が大きいほど鍛造用鋼が靭性に優れることを意味する。なお、シャルピー衝撃試験は、平均冷却速度を1℃/minとした鍛造用鋼について実施した。上述のようにこの吸収エネルギーの数値が150J以上である場合、鍛造用鋼が靭性に優れ、「良好」と判断し、150J未満の場合「良好でない」と判断した。この吸収エネルギーを表2及び表4の靱性に記した。なお、表4の靱性に記した×は、鍛鋼に割れが発生して靱性を判断できなかったことを示す。
ガス切断性は、切断速度170mm/min、酸素圧5.5kg/cm2、プロパンガス圧0.55kg/cm2にて鍛鋼を切断し、ガス切断面粗さが5.5μm以下であるものを「良好」と判断し、5.5μm超の場合「良好でない」と判断した。なお、ガス切断面粗さは、JIS-B0601(2013)に準拠して測定した。このガス切断面粗さを表2及び表4のガス切断性に記した。
Claims (6)
- C:0.20質量%以上0.35質量%以下、
Si:0質量%以上0.5質量%以下、
Mn:0.50質量%以上2.70質量%以下、
Cu:0質量%以上1質量%以下、
Ni:0質量%以上2.00質量%以下、
Cr:1.00質量%以上2.50質量%以下、
Mo:0.10質量%以上0.55質量%以下、
V:0質量%以上0.20質量%以下、
N:30質量ppm以上100質量ppm以下、
Al:0質量%以上0.050質量%以下、
S:0質量%以上0.020質量%以下、
O:0質量ppm超50質量ppm以下、並びに
残部:Fe及び不可避的不純物
である組成を有し、
金属組織がベイナイト組織であり、かつ、平均ラス幅が3.0μm以下であり、
下記式(1)及び(2)を満たすことを特徴とする鍛造用鋼。
1.15≧C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14≧0.89・・・(1)
0.53≧C+Si/30+Mn/20+Ni/60+Cr/20+Mo/15+V/10≧0.40・・・(2) - 平均ブロック幅が50μm以下である請求項1に記載の鍛造用鋼。
- Nb:0質量%以上0.07質量%以下、又は
B:0質量%以上0.03質量%以下、
の少なくとも一方をさらに含有する請求項1又は請求項2に記載の鍛造用鋼。 - 上記Alの含有量が0.010質量%以上、
上記Sの含有量が0質量%超0.0030質量%以下、
上記Oの含有量が15質量ppm以下である請求項1又は請求項2に記載の鍛造用鋼。 - 上記Alの含有量が0.010質量%以上、
上記Sの含有量が0質量%超0.0030質量%以下、
上記Oの含有量が15質量ppm以下である請求項3に記載の鍛造用鋼。 - 請求項1に記載の鍛造用鋼を用いた大型鍛鋼品。
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