KR20130137101A - High strength large steel forging - Google Patents

Abstract

The large high strength steel forgings of the present invention have a high level of balance between strength and toughness, and also have high fatigue strength. The large-strength forged steel product of the present invention has a specific composition of C, Si, Mn, Ni, Cr, Mo, V, Al, and S as unavoidable impurities, and has a martensite structure or a mixed structure of martensite and bainite. It has a spherical austenite grain size of 19 µm or more and 70 µm or less, the maximum block diameter of martensite is 15 µm or less, and the minimum block diameter is 0.5 µm or more.

Description

HIGH STRENGTH LARGE STEEL FORGING

The present invention relates to a large, high strength forged steel product.

Large crankshafts and intermediate shafts used in ships and generators are required to have high strength, and they are generally manufactured by forging steel for forging. These large steel forgings are now required to have excellent toughness, which is usually in conflict with the strength, in addition to the further improvement in strength.

Therefore, in order to increase the strength and toughness of the forged steel product, (1) high strength steel for large forged steel products having limited component composition (see Japanese Patent Laid-Open No. 2005-344149) and (2) bainite and martensite Forging steels limited to the structure of the subject (see Japanese Patent No. 3896365), (3) Crankshafts that limit the grain size of the old austenite particles with the limitation of the composition of the components (Japanese Patent Publication No. 2010-248540) Reference), (4) nickel-based tempered steel specifying the grain size of aluminum (see Japanese Patent Application Laid-Open No. 2000-212705), and (5) forging steel specifying the concentration of magnesium and aluminum (Japanese Patent Laid-Open No. 2008-25021). And Japanese Patent Application Laid-Open No. 2009-173961), and (6) Forging products (see Japanese Patent Application Laid-Open No. 2003-147436) that have specified contents of sulfur and the like and hot forging conditions have been developed.

However, even in the steels of the above (1) to (3), the block diameter and the particle size of the formed structure are not appropriate, and the strength cannot be sufficiently exhibited, and the toughness and the fatigue strength can be improved in a good balance. none. In addition, in steel of said (4), since aluminum content is high, a nonmetallic inclusion and an intermetallic compound generate | occur | produce, and toughness and fatigue strength may fall. Although the steel of (5) is said to be high strength, it does not aim at high toughness. Although the forging product of (6) is said to be high strength and high toughness, presence of a predetermined amount of sulfur produces nonmetallic inclusions, such as MnS, and as a result, fatigue strength falls. As such, none of the conventional forged steels has a good balance of strength, toughness and fatigue strength.

Japanese Patent Publication No. 2005-344149 Japanese Patent No. 3896365 Japanese Patent Publication No. 2010-248540 Japanese Patent Publication No. 2000-212705 Japanese Patent Publication No. 2008-25021 Japanese Patent Publication No. 2009-173961 Japanese Patent Publication No. 2003-147436

This invention is made | formed based on the above-mentioned situation, and an object of this invention is to provide the large-strength high strength steel forgings which are balanced in the dimension with high strength and toughness, and has high fatigue strength.

According to an aspect of the present invention,

As a large high strength forged steel,

C: 0.31 mass% or more and 0.5 mass% or less,

Si: 0.02 mass% or more and 0.2 mass% or less,

Mn: 0.1 mass% or more and 0.6 mass% or less,

Ni: 2.6 mass% or more and 3.4 mass% or less,

Cr: 0.8 mass% or more and 1.9 mass% or less,

Mo: 0.25 mass% or more and 0.8 mass% or less,

V: 0.05 mass% or more and 0.2 mass% or less, and

Al: 0.005 mass% or more and 0.1 mass% or less as a basic component, remainder as Fe and an unavoidable impurity, and have the composition whose content of S as this unavoidable impurity is 0.008 mass% or less,

Martensitic structure, or a mixed structure of martensite and bainite,

Old austenite crystal grain size is 19 µm or more and 70 µm or less,

The maximum block diameter of martensite is 15 micrometers or less, and the minimum block diameter is 0.5 micrometer or more.

The large-strength high strength steel forgings are limited to the composition and structure as described above, and by setting the old austenite grain size and block diameter in the above range, the strength and toughness are well balanced, and the fatigue strength is high. have.

Here, the "large" in a large high strength forging product means having a spherical or columnar portion having a diameter of 150 mm or more, having a plate portion having a thickness of 150 mm or more, and having a size equal to or larger than these.

As described above, the large-strength high strength steel forging of the present invention is excellent in balance with strength and toughness, and also has high fatigue strength. Therefore, the large-strength forged steel product can be suitably used as a large crankshaft, intermediate shaft, or the like used in ships, generators, and the like.

1 is a graph showing the relationship between the ratio of depth x to martensite tissue fraction f _m (x) (%) with respect to the distance from the surface to the center in a large high strength forged steel product.
It is a figure which shows the relationship between the largest diameter of a martensite block and Charpy absorbed energy measured with the test piece of an Example or a comparative example.
It is a figure which shows the relationship between the tensile strength and Charpy absorbed energy measured by the test piece of an Example or a comparative example.
It is a figure which shows the relationship between the tensile strength and the fatigue strength measured by the test piece of an Example or a comparative example.
5 is a crystal orientation diagram of a test piece of Example 4. FIG.
6 is a crystal orientation diagram of a test piece of Example 11. FIG.
7 is a crystal orientation diagram of a test piece of Comparative Example 3. FIG.
8 is a crystal orientation diagram of a test piece of Comparative Example 7. FIG.
9 is a crystal orientation diagram of a test piece of Comparative Example 9. FIG.
10 is a TTT diagram used for analysis conditions of an analysis example.
It is a figure which shows the temperature dependence of each physical property value used for the analysis conditions of an analysis example.
It is a figure which shows the plastic behavior of each phase used for the analysis conditions of an analysis example.
It is a figure (a) which shows analysis conditions A and B in an analysis example, and (b) which shows the analysis result thereof.
It is a figure (a) which shows the relationship of the depth and Brinell hardness of the large high strength steel forgings of a reference example, and (b) which shows the relationship between the depth and martensite structure fraction.

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of the large high strength steel forgings of this invention is described.

<Composition>

The said large-strength high strength steel forgings are C: 0.31 mass% or more and 0.5 mass% or less, Si: 0.02 mass% or more and 0.2 mass% or less, Mn: 0.1 mass% or more and 0.6 mass% or less, Ni: 2.6 mass% or more and 3.4 mass% or less, Cr: 0.8 mass% or more and 1.9 mass% or less, Mo: 0.25 mass% or more and 0.8 mass% or less, V: 0.05 mass% or more and 0.2 mass% or less, and Al: 0.005 mass% or more and 0.1 mass% or less, and And the remainder of unavoidable impurities. The reason for limitation of each component is as follows.

(C: 0.31 mass% or more and 0.5 mass% or less)

As a minimum of content of carbon (C), it becomes 0.31 mass% and 0.33 mass% is preferable. On the other hand, as an upper limit of content of carbon (C), it becomes 0.5 mass% and 0.4 mass% is preferable. Carbon (C) improves hardenability and contributes to strength improvement. When carbon content is less than the said minimum, sufficient hardenability and strength will become difficult to obtain. On the contrary, when carbon content exceeds the said upper limit, toughness will fall extremely, and large ingot will promote reverse V segregation.

(Si: 0.02 mass% or more and 0.2 mass% or less)

As a minimum of content of silicon (Si), it becomes 0.02 mass% and 0.06 mass% is preferable. On the other hand, as an upper limit of content of silicon (Si), it becomes 0.2 mass% and 0.16 mass% is preferable. Silicon (Si) contributes to deoxidation and strength improvement. When silicon content is less than the said minimum, this effect cannot fully be exhibited. Conversely, when silicon content exceeds the said upper limit, reverse V segregation becomes remarkable and it becomes difficult to obtain a clean ingot.

(Mn: 0.1 mass% or more and 0.6 mass% or less)

As a minimum of content of manganese (Mn), it becomes 0.1 mass% and 0.3 mass% is preferable. On the other hand, as an upper limit of content of manganese (Mn), it becomes 0.6 mass% and 0.45 mass% is preferable. Manganese (Mn) improves hardenability and strength. When manganese content is less than the said minimum, it is hard to exhibit the said effect. Conversely, if manganese content exceeds the said upper limit, temper embrittlement is encouraged.

(Ni: 2.6 mass% or more and 3.4 mass% or less)

As a minimum of content of nickel (Ni), it becomes 2.6 mass% and 2.8 mass% is preferable. On the other hand, it is 3.4 mass% as an upper limit of content of nickel (Ni). Nickel (Ni) improves hardenability, strength and toughness. When nickel content is less than the said minimum, the said effect cannot fully be exhibited. Conversely, when the nickel content exceeds the above range, it becomes difficult to obtain old austenite grains of appropriate size. Moreover, by using less than the said upper limit, the usage-amount of expensive Ni can be suppressed and production cost can be suppressed.

(Cr: 0.8 mass% or more and 1.9 mass% or less)

As a minimum of content of chromium (Cr), it becomes 0.8 mass% and 1.4 mass% is preferable. On the other hand, as an upper limit of content of chromium (Cr), it becomes 1.9 mass%, and 1.65 mass% is preferable. Chromium (Cr) improves hardenability and toughness. When chromium content is less than the said minimum, the said effect cannot fully be exhibited. Conversely, if chromium content exceeds the said upper limit, it will promote reverse V segregation.

(Mo: 0.25 mass% or more and 0.8 mass% or less)

As a minimum of content of molybdenum (Mo), it becomes 0.25 mass% and 0.4 mass% is preferable. On the other hand, as an upper limit of content of molybdenum (Mo), it becomes 0.8 mass% and 0.6 mass% is preferable. Molybdenum (Mo) improves hardenability, strength and toughness. When molybdenum content is less than the said minimum, the said effect cannot fully be exhibited and inverse V segregation is encouraged. On the contrary, when molybdenum content exceeds the said upper limit, while it promotes micro segregation in a steel ingot, weight segregation will arise easily.

(V: 0.05 mass% or more and 0.2 mass% or less)

As a minimum of content of vanadium (V), it becomes 0.05 mass% and 0.07 mass% is preferable. On the other hand, as an upper limit of content of vanadium (V), it becomes 0.2 mass% and 0.13 mass% is preferable. Vanadium (V) significantly improves hardenability and strength by addition of a small amount, but micro segregation is likely to occur because of the small equilibrium distribution coefficient. When vanadium content is less than the said minimum, sufficient strength cannot be ensured. Conversely, when vanadium content exceeds the said upper limit, the generation of micro segregation is encouraged.

(Al: 0.005 mass% or more and 0.1 mass% or less)

As a minimum of content of aluminum (Al), it becomes 0.005 mass% and 0.008 mass% is preferable. On the other hand, as an upper limit of content of aluminum (Al), it becomes 0.1 mass% and 0.03 mass% is preferable. Aluminum (Al) is used as the deoxidation element. In addition, aluminum is capable of producing fine compounds such as AlN, which stops the growth of grains and makes grains finer. When aluminum content is less than the said minimum, this effect cannot fully be exhibited. Conversely, when aluminum content exceeds the said upper limit, aluminum will also combine with other elements, such as oxygen, and may produce an oxide and an intermetallic compound, and may reduce toughness and fatigue strength.

The basic component of the large-strength forged steel product is as described above, the balance component is substantially iron (Fe), but may contain a small amount of unavoidable impurities (for example, S, O, P, Cu, Sn, N, etc.). . Moreover, you may actively contain another element in the range which does not adversely affect the effect of the said large-strength high strength steel forgings. Examples of such other elements include Ti, Ca, and Mg. However, from the viewpoint of suppressing the formation of coarse inclusions, it is preferable to suppress the total of unavoidable impurities to 0.5% by mass or less.

(S: 0.008 mass% or less)

As content of sulfur (S), it becomes 0.008 mass% or less, and 0.003 mass% or less is preferable. Since sulfur (S) forms MnS in the forging product, when the content exceeds the upper limit, the fatigue strength is lowered. However, this content does not become 0 mass% industrially.

In addition, it is preferable that content of other unavoidable impurities is as follows.

(O: 0.0025 mass% or less)

As content of oxygen (O), 0.0025 mass% or less is preferable, and 0.002 mass% or less is more preferable. Oxygen (0) combines with various elements to form a non-metallic inclusion to lower the fatigue strength. Therefore, it is preferable to make oxygen content below the said upper limit. However, this content does not become 0 mass% industrially.

(P: 0.02 mass% or less)

As an upper limit of content of phosphorus (P), 0.02 mass% or less is preferable, and 0.01 mass% or less is more preferable. When content of phosphorus (P) exceeds the said upper limit, hot ductility falls and it becomes easy to produce the crack at the time of forging.

(Cu: 0.1 mass% or less)

As an upper limit of content of copper (Cu), 0.1 mass% or less is preferable, and 0.05 mass% or less is more preferable. When content of copper (Cu) exceeds the said upper limit, a crack etc. will arise easily at the time of hot working.

(Sn: 0.03 mass% or less)

As an upper limit of content of tin (Sn), 0.03 mass% or less is preferable, and 0.01 mass% or less is more preferable. If content of tin (Sn) exceeds the said upper limit, toughness may fall.

(N: 0.02 mass% or less)

As an upper limit of content of nitrogen (N), 0.02 mass% or less is preferable, and 0.01 mass% or less is more preferable. When content of nitrogen (N) exceeds the said upper limit, hot ductility will fall and it will become easy to produce the crack at the time of forging.

<Organization>

Next, the structure of the said large size high strength steel forgings is demonstrated.

The large high strength steel forgings consist of martensite structure or a mixed structure of martensite and bainite. The said large-strength high strength steel forgings consist only of these two types of structures, and can balance balance of strength and toughness. If other structures, such as ferrite and pearlite, exist in the said large-strength high strength steel forgings, strength and toughness may not be compatible.

The old austenite crystal grain size (average grain diameter) of the large-strength forged steel product is 19 µm or more and 70 µm or less. The former austenite grain size affects the block diameter. When the old austenite grain size becomes coarse, the block diameter also becomes large and sufficient toughness cannot be obtained. Therefore, this upper limit is set to 70 µm. Conversely, when the crystal grain diameter becomes too fine to less than 19 µm, the hardenability is lowered and the cornerstone ferrite is mixed, and as a result, the balance between strength and toughness is lowered. In addition, this old austenite grain size can be measured by the method as described in an Example.

In the martensite block diameter which is a lower structure of the martensite structure which forms the said large-strength high strength steel forgings, the largest block diameter is 15 micrometers or less, and the minimum block diameter is 0.5 micrometer or more. By making a martensite block diameter into the said range, a strength, toughness, and fatigue strength can be balanced well. In particular, by making this maximum block diameter atomize to 15 micrometers or less, stable toughness can be exhibited. On the other hand, if the block diameter is made too fine, the grain boundary density increases and the crack propagation rate increases, so the minimum block diameter is 0.5 µm or more.

In the large high strength steel forgings, the martensite structure fraction f _m (x) (%) when the ratio of the depth to the distance from the surface to the center is x (0 ≦ x ≦ 1),

When O≤x≤O1, f _m (x) = 1OO,

104-40x≤f _m (x) ≤100, when 0.1 <x≤0.15,

When 0.15 <x≤0.2, 122-160x≤f _m (x) ≤100,

230-700x≤f _m (x) ≤100, when 0.2 <x≤0.3

110-300x≤f _m (x) ≤112-40x, when 0.3 <x≤0.35,

When 0.35 <x≤0.5, (22-20x) / 3≤f _m (x) ≤105-20x,

When 0.5 <x≤0.8, (32-40x) / 3≤f _m (x) ≤95, and

When 0.8 <x ≦ 1, it is preferable that 0 ≦ f _m (x) ≦ 95 (range (a) in FIG. 1).

On the other hand, the remaining tissues are bainite tissues. This martensite structure fraction can be measured using a mixing rule from the method described in the example, that is, the measurement result of hardness.

The "central part" of a large high strength steel forging means the deepest position from each position on the surface. For example, when a large high strength steel forging has a spherical portion, it means its center point. In the case of having a part, the center plane is equidistant from both sides. The term "distance from the surface to the center part" means a vertical distance from each part of the surface to the center part, for example, when having a spherical or circumferential part, the radius thereof, and when having a plate-shaped part, are half of the plate thickness.

In the case of controlling the martensite structure fraction f _m (x) in this way, the large-strength high strength steel forgings can reduce the occurrence of internal stress from the surface to the center as a result, and as a result, balance of strength, toughness and fatigue strength Can be made higher.

When the martensite structure fraction includes a depth region that is less than the above range, the tensile stress may remain in the vicinity of the surface, especially in the region near x = 0.2. This residual tensile stress causes a decrease in fatigue strength. A large high strength forged steel product is suitably used as a crankshaft or the like, but since the bending stress is repeatedly applied to the fillet portion on the crankshaft, particularly high fatigue strength of the surface of the fillet portion is required. Since the crankshaft and the like are finished by machining after heat treatment, the surface layer is ground. Therefore, if the residual tensile stress to a range of a constant depth (about 0 ≦ x ≦ 0.3) from the surface is reduced, higher fatigue strength can be provided even when used in a crankshaft or the like. On the other hand, when a martensite structure fraction contains the depth area exceeding the said range, the internal transformation stress by quenching becomes large and hardening cracks may arise easily.

In the large high strength steel forgings, in order to further reduce the occurrence of internal stress from the surface to the center as a whole, the martensite structure fraction f _m (x) is

When O≤x≤O1, f _m (x) = 1OO,

104-40x≤f _m (x) ≤100, when 0.1 <x≤0.15,

When 0.15 <x≤0.2, 122-160x≤f _m (x) ≤100,

150-300x≤f _m (x) ≤100, when 0.2 <x≤0.3

105-150x≤f _m (x) ≤112-40x, when 0.3 <x≤0.35,

105-150x≤f _m (x) ≤105-20x, when 0.35 <x≤0.5

80-100x ≦ f _m (x) ≦ 170-150x, when 0.5 <x ≦ 0.8, and

When 0.8 <x ≦ 1, it is preferable that 0 ≦ f _m (x) ≦ 130-100x (range (b) in FIG. 1).

By making martensitic structure fraction more than the said lower limit, generation | occurrence | production of the tensile stress in the vicinity of a surface can be reduced more. On the other hand, when the martensite structure fraction is less than or equal to the above upper limit, the internal tensile stress is reduced, and the risk of quenching cracking can be further reduced.

Moreover, in order to heighten the said effect (reduction of the generation of internal stress from the surface to the center as a whole) further, it is preferable that it is d (f _m (x)) / dx <= O when O <= <= 1.

<Performance, use>

Since the large high strength steel forgings have the above composition and structure, they have excellent strength and toughness, and have high fatigue strength. As this strength (tensile strength), it is preferable that it is 1,050 Mpa or more, and 1,080 Mpa or more is more preferable. In addition, tensile strength says the value measured based on JIS-Z2241 (1998).

The large-strength high-strength steel forgings have excellent strength, toughness and fatigue strength as described above, and thus can be suitably used as large crankshafts, intermediate shafts, and the like used in ships and generators. In particular, for example, a large crankshaft is required to further increase fatigue strength and tensile strength (eg, tensile strength of 950 MPa or more) in order to realize output improvement, compactness, and the like of a marine diesel engine and a six-foot diesel engine. The large, high strength steel forgings can sufficiently cope with this.

<Manufacturing Method>

It does not specifically limit as a manufacturing method of the said large size high strength steel forgings, It can obtain by forging and heat-processing the steel prepared by the said composition. Below, an example of the manufacturing method in which the said large-strength high strength steel forgings are integral crankshafts with a diameter of 150 mm or more is demonstrated.

First, the steel prepared by the above-mentioned predetermined | prescribed component composition is melt | dissolved using an electric furnace, a high frequency melting furnace, a converter, etc. Thereafter, impurities (sulfur, oxygen, etc.) are removed (reduced) by vacuum refining or the like. After the removal of impurities, the steel is formed by casting. Ingot casting is mainly used as this casting method, but in the case of a relatively small forged steel product, a continuous casting method may be used.

Next, the round bar raw material before forming a crankshaft is forged. As heating temperature at this time, in order to forge in the range where the deformation ability of steel is favorable, it is good to set it as 1,150 degreeC or more, More preferably, it is 1,200 degreeC or more. When this heating temperature is low, an increase in deformation resistance is caused and manufacturing efficiency is lowered. Moreover, what is necessary is just to set it as 3 hours or more as heating time. This heating time is necessary in order to equalize the temperature of the ingot surface and the inside. This heating time is generally proportional to the square of the diameter of the workpiece, and is, for example, 3 hours or longer when the large crankshaft is manufactured.

After forging with a round bar material, it is forged into the shape of an integrated crankshaft. This forging is preferably performed by the CGF (Continuous Grain Flow) forging method. The CGF forging method is a method of forging a steel core in such a manner that the shaft core of the integral crankshaft is subjected to forging, and the portion easily prone to deterioration of characteristics due to central segregation so as to integrally forge the entire core of the integrated crankshaft. As said CGF forging, RR forging, TR forging, etc. are mentioned. These are preferable because the crankshaft surface layer side can be occupied by a portion with high cleanliness, and an integrated crankshaft having excellent strength and fatigue characteristics is easily obtained.

Hereinafter, the forging method will be described in detail with reference to the RR forging method.

In RR forging, the obtained forged raw material is heated at 1,150 ° C or more for 3 hours or more, and each slope is hot formed. As a specific procedure, first, the round bar material obtained by the above-mentioned process is machined, and it is set as RR forging material. Thereafter, the pin shaft, the pair of block portions, and the journal shaft corresponding to one cylinder are partially heated, and the pressing force of the press is converted into a lateral force by a wedge mechanism, thereby simultaneously translating the lateral compression force and the eccentric force into the RR material. Forge one cylinder. This operation is repeated the required number of cylinders, and is finished with one crankshaft. Here, the heating temperature may be 1,150 ° C. or more, more preferably 1,200 ° C. or more, in order to forge in the range where the deformation ability of steel is favorable. When this heating temperature is low, the deformation resistance is increased, and the production efficiency is lowered. This heating time is necessary in order to equalize the temperature of the ingot's surface and the inside, and at the time of manufacturing a large crankshaft, for example, it becomes 3 hours or more.

After RR forging, before performing temper treatment (quenching and tempering treatment), you may perform the process of decomposing | removing the retained austenite (Y) contained in a forging. The phase transformation during the temper treatment is utilized for the structure refinement, but when the residual Y present after the forging is stable, the residual Y continues to exceed the Ac1 temperature during the temper treatment heating. This residual Y is the residue of Y during the forging heat treatment, and originally has the same direction in the old austenite particles after forging. Therefore, when Y transformation progresses and residual Y mutually contacts, the interface cannot become a grain boundary, and the Y particle diameter at the time of Y transformation completion becomes coarse like the initial Y particle diameter. For this reason, the process of decomposing residual Y is performed.

As a method of decomposing residual austenite, the aging treatment etc. which heat-hold at the temperature (600-680 degreeC) below Ac1 transformation point are mentioned, for example. The heating holding time at this time is 5 hours or more, and preferably 20 hours or more. By this aging treatment, residual austenite is decomposed and the retained austenite can be 1% or less in volume ratio. In addition, as a method of decomposing residual austenite, a sub-zero treatment can be used.

Next, a tempering treatment (quenching and tempering treatment) is performed. First, before quenching, it is gradually heated (heating rate 30-70 degreeC / hour) to the temperature (840-940 degreeC) more than Ac3 transformation point, and hold | maintains for a fixed time (3 to 9 hours). From the viewpoint of suppressing the former austenite grain grain coarsening, the quenching is preferably treated at a relatively low temperature (840 to 940 ° C) of Ac 3 or more. In addition, in the case of a large product, since a temperature difference arises in and out of a material at the time of a heating, it heats gradually to the heating temperature before quenching, and hold | maintains for a fixed time in order to make the temperature of the steel surface and inside uniform. On the other hand, the required holding time depends on the steel diameter and the like, and the larger the holding time, the longer the holding time. For this reason, after a sufficient holding time, the temperature is made uniform to the inside of the steel, and the following quenching is performed.

Quenching is performed using refrigerant | coolant, such as oil or a polymer, and the martensite structure or the structure which consists of martensite and bainite is obtained. In order to obtain such a structure, the average cooling rate in hardening is performed at 3 degree-C / min or more. As for this cooling rate, 5 degrees C / min or more and 100 degrees C / min or less are more preferable, and 10 degrees C / min or more and 60 degrees C / min or less are more preferable.

In large forged steels, there is a risk of cracking when water quenching is performed. Therefore, quenching of the large crankshaft is generally oil quenching or polymer quenching. The cooling rate during quenching depends on the size of the forged steel, but for crankshafts with a diameter of 500 mm, the average cooling rate between 800 and 500 ° C is about 20 ° C / min for oil and about 50 ° C / min for polymer. If the diameter is larger than that (for example, 1, OOmm), the cooling rate becomes smaller.

In order to make both the strength and toughness of the large-size high strength steel forgings compatible, it is necessary to control the martensite structure or the mixed structure of martensite and bainite. Therefore, even when the quenching cooling rate is about 20 ° C./min (for oil quenching), for example, to be applied to a large crankshaft of 150 mm or more in diameter, the conditions for realizing such a structure are examined. Reached.

Moreover, in hardening, it is preferable to temper after cooling to 200 degrees C or less. By cooling to 200 degrees C or less in this way, transformation can be completed completely. If the cooling is insufficient, untransformed residual austenite remains, which causes irregularities in properties.

Tempering is gradually heated to a predetermined temperature (550 ° C. to 620 ° C.) (heating rate 30 to 70 ° C./hour) to maintain a constant time (5 to 20 hours). This tempering is performed at 550 ° C. or higher in order to adjust the balance between strength and toughness and to remove internal stress (residual stress) during quenching. However, if the temperature is too high, it is softened by coarsening of carbide, recovery of dislocation structure, and the like, and sufficient strength cannot be ensured.

Thus, the said large-strength high strength steel forgings can be obtained from the tempered forging manufactured by performing the finishing machining process which includes grinding of at least one part of a surface layer as needed. In addition, the large high strength steel forgings of this invention are not limited to the manufacturing method mentioned above, For example, it can also manufacture by free forging. In addition, large-strength high strength steel forgings other than the large crankshaft can also be obtained by the same production method.

Example

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

[How to measure]

Each measurement performed in the Example was performed by the following method, respectively.

1. Old Austenitic (Y) Grain Size (㎛)

Based on ASTM (E112-96), after determining the particle size number by the comparative method in the following procedure, the crystal grain size (nominal particle size) of the old austenite grains was determined.

(1) The photograph of a magnification of 100 times with an optical microscope is compared with a standard drawing, and the corresponding particle size number N is determined.

(2) The particle size number N can be determined according to the number n of crystal grains in 25 mm square (625 mm ² : microscope field of view) observed at 100 times the microscope, and Equation 1 below holds true.

[Equation 1]

(3) Since it is considered that there are n particles in 62,500 µm ² , the crystal grain size d (µm) can be calculated by the following expression (2).

&Quot; (2) &quot;

On the other hand, about the old austenite crystal grain size, ten places (10 field of view) were measured and each average particle diameter was calculated | required.

2. Martensite block diameter (㎛)

In the substructure of martensite, Ras populations in almost the same direction are called blocks (martensite blocks). The azimuth difference between blocks is 15 degrees or more (diagonal boundary). Then, the martensite block diameter (micrometer) was calculated | required from the crystal orientation map obtained by FESEM-EBSP method by the following method.

(1) An EBSP measurement is performed on a 120 µm x 120 µm field of vision in 0.3 µm steps to obtain a crystal orientation.

(2) From the crystal azimuth diagram, the area | region where the orientation difference with an adjacent crystal | crystallization is enclosed by 15 degrees or more is identified, and the area is calculated, respectively.

(3) The square root of each area is taken (√ (area)) to find the block diameter.

On the other hand, about martensite block diameter, it measured ten places (10 field of view), and calculated | required the maximum diameter and the minimum diameter of the block in each visual field, and calculated | required each average diameter (average of the largest diameter and average of the minimum diameter).

3. Tensile properties (0.2% yield strength: YS (MPa), tensile strength: TS (MPa), elongation: EL (%) and cross sectional shrinkage rate: RA (%))

It measured based on JIS-Z2241 (1998). The test piece shape is a 14 test piece described in JIS-Z2201 (1998), and has a φ6 × G.L. It was 30 mm.

4. Charpy absorbed energy: vE (J)

It measured based on JIS-Z2242 (2005). The test piece shape employ | adopted the 2 mmV notch described in JIS-Z2242 (2005). Three tests were carried out, respectively, and the absorbed energy was taken as the average value.

5. Rotary bending fatigue strength FS (MPa) and endurance limit ratio

The rotation bending fatigue test was done with the test method shown below, and fatigue strength was evaluated.

Test piece: 10 mm x G.L. 30 mm smooth test pieces (5 pieces)

Test method: rotational bending (stress ratio = -1, rotational speed = 3,000 to 3,600 rpm)

Evaluation method: The differential method (20 MPa of differential stress)

Fatigue Strength [FS] = Break Stress (MPa)-Dependent Stress (MPa)

Durability Limit = Fatigue Strength [FS] / Tensile Strength [TS]

[Examples 1 to 13 and Comparative Examples 1 to 14]

Steel grades a to q of the components shown in Table 1 were solvented. In addition, in Table 1, "-" shows that it is below a detection limit value. Steel grade a was melted and blown and refined in an electric furnace to cast 70 ton steel ingots. In addition, the steel grades b-q were each melted using the high frequency furnace, and 40 kg steel ingot was cast.

The steel ingot (70 tons) of the steel grade a was forged hot to obtain a round bar-shaped forging material having a diameter of 500 mm. In addition, about steel grades b-q were forged to 90 mm x 90 mm x 600 mm, and then allowed to cool in air. About each forging material of steel grades a-q, after cooling to room temperature, the small piece of 20 mm x 20 mm x 180 mm was cut out from each forging material. About each small piece, the sample piece was produced by heat-processing on the conditions shown in Table 2 which simulated the forging process of a crankshaft, and furnace-cooling this.

Thereafter, each sample piece was subjected to a temper treatment (quenching and tempering treatment) to secure the strength of the crankshaft. About hardening conditions, the hardening process which simulated the heating and cooling rate of the crankshaft of diameter 500mm was performed. Specifically, the average cooling rate of the temperature range of 870-500 degreeC is heated after heating up to 870-940 degreeC at the temperature increase rate of 40 degreeC / hour using a small simulation furnace, and hold | maintaining at that temperature for 3 to 8 hours. Quenching was performed by the cooling which becomes 20-50 degreeC / min. The tempering treatment was maintained at a temperature of 560 to 610 ° C for 13 hours, and the furnace was cooled (shown in Table 2 for the treatment conditions for each sample piece). In this manner, sample pieces (forgings) of Examples 1 to 13 and Comparative Examples 1 to 14 were obtained.

About the sample pieces of Examples 1-13 and Comparative Examples 1-14, the state of microstructure (martensite structure (M) or bainite structure (B)) was observed. In addition, the measurement method evaluated the former austenite grain size, martensite block diameter, tensile properties, Charpy absorbed energy and fatigue characteristics (fatigue strength FS and endurance limit ratio). The measurement results are shown in Table 3.

In addition, in Table 3, "-" shows that it was not measured.

The relationship between the largest diameter of a martensite block and the Charpy absorbed energy measured by the test piece of an Example or a comparative example is shown in FIG. The relationship between the tensile strength measured by the test piece of an Example or a comparative example and Charpy absorbed energy is shown in FIG. The relationship between the tensile strength and the fatigue strength measured by the test piece of an Example or a comparative example is shown in FIG.

Fig. 5 shows the crystal orientation of the test piece of Example 4, Fig. 6 shows the crystal orientation of the test piece of Example 11, Fig. 7 shows the crystal orientation of the test piece of Comparative Example 3. Fig. 8 shows the crystal orientation of the test piece of Comparative Example 7. 9 shows the crystal orientation of the test piece of Comparative Example 9.

[Review]

Since Examples 1-13 satisfy | fill the requirements of this invention also in the composition of steel and manufacturing conditions, the steel forgings which have a desired characteristic were obtained. In Comparative Examples 1 to 14, since the composition of the steel did not satisfy the requirements of the present invention, forged steel products having desired characteristics were not obtained. In Comparative Examples 15 to 17, the composition of the steel satisfies the requirements of the present invention. However, since the quenching temperature is not appropriate, the old austenite grain size does not satisfy the requirements of the present invention, and forged steel products having desired characteristics cannot be obtained. Did.

As shown in FIG. 2, it can be seen that when the maximum block diameter of martensite is 15 μm or less, good toughness (Charpy absorbed energy) is obtained. As shown in FIG. 3, it can be seen that the high strength steel for large forged steel products of the present invention has better toughness (impact characteristics) even though it is higher in strength (strength of 1,050 MPa or more) than conventional steel. In general, the material is tougher when the strength is higher, but by optimizing the chemical composition and the metal structure, it is possible to provide a large high strength forged product having a good balance of strength and toughness (for example, strength of 1,050 MPa or more).

4 shows the relationship between tensile strength and fatigue strength. The fatigue strength of the forged steel product of the present invention is improved by about 10% or more with respect to conventional steel. The endurance limit ratio (= fatigue strength / tensile strength) is equivalent to that of conventional steel, and a proportional relationship between tensile strength and fatigue strength is maintained. That is, the increase in the notch sensitivity accompanying high strength is not confirmed.

[Example of interpretation]

Heat transfer and thermal stress analysis was performed using the general-purpose program FORGE2009 for the transformation stress inside the steel material by quenching. Specific conditions are as follows. Assuming a round bar shape, modeling was performed using a two-dimensional axial symmetry model, and only the unit length was modeled in the axial direction, and the upper and lower surfaces were insulated. The heat transfer and the thermal stress analysis which performed cooling to near normal temperature by making initial stage temperature 870 degreeC uniformity were performed. In addition, each material physical property value used for analysis is shown in FIGS. Analysis was performed on the following analysis conditions A and B. FIG.

(Interpretation condition A)

Inside the round bar steel with a ratio of depth to distance from the surface to the center from 0 to 0.35, 100% martensite tissue, 95% martensite tissue and 5% bainite tissue, as indicated by the dotted lines in FIG. 13 (a). Tissue fraction.

(Interpretation condition B)

The fraction of tissue inside the round bar steel when the ratio of the depth to the distance from the surface to the center portion from 0 to 0.1 is 100% martensite structure and the center portion is 100% bainite structure, as shown by the solid line in FIG. .

The analysis result in analysis conditions A and B is shown to FIG. 13 (b). As shown in FIG. 13 (b), in the case of the analysis condition A, the internal transformation stress increases. In addition, in the case of analysis condition B, it turns out that a stress remains in the vicinity of ratio 0.2 of depth. By setting it as the structure fraction between analysis conditions A and B, the whole internal stress from a surface to a center can be controlled low.

Reference Example 1

Steel grade a having the component composition shown in Table 1 above was dissolved, blown and refined in an electric furnace to cast a 70 ton steel ingot. The steel grade A (70 ton) ingot was hot forged by a free forging press to form a round bar forging material having a diameter of 500 mm (radius 250 mm), and then cooled in air. This round bar forging material simulated the heating before RR forging, and heat-processed for 3 hours at 1,280 degreeC. Before quenching and tempering treatment, the aging treatment (holding at a temperature of 650 ° C. for 20 hours) was cooled to room temperature. About quenching conditions, it heated at the temperature increase rate of 40 degreeC / hour, and polymer quenched after hold | maintaining at 870 degreeC for 8 hours. Then, as tempering process, after hold | maintaining at 580 degreeC for 15 hours, it cold-cooled to 350 degreeC, and then air-cooled to room temperature, and obtained the large high strength steel forgings of the reference example 1.

The obtained high strength steel forgings were ground so that the respective depths (25 mm, 40 mm, 70 mm, 100 mm, 130 mm, 160 mm, 190 mm, 220 mm and 250 mm) became the surface from the surface to the center direction. Brinell hardness HB, tensile strength (MPa), tissue fraction (%) and spherical austenite (Y) crystal grain size (μm) at each depth were measured. In addition, about the tensile strength in the large-strength high-strength steel forging of this reference example 1, it is a conversion value computed based on the hardness conversion table (SAE J417) from Brinell hardness HB measured. The measurement and calculation method of Brinell hardness and martensite structure fraction are as follows. The measurement results are shown in Table 4 together with the heat treatment conditions at the time of manufacture.

Brinell hardness

It measured based on JIS-Z2243 (2008).

Martensitic tissue fraction (%)

The structure fraction was computed from the result of hardness (Brinnell hardness: HB) measurement using the following formula by the mixing rule. On the other hand, the bainite structure fraction (%) was also calculated as the bainite structure other than the martensite structure.

HB = HB _M × f _m (x) / 100 + HB _B × (1-f _m (x) / 100)

f _m (x): martensite tissue fraction (%)

HB _M : Brinell hardness of martensite

(Measured hardness of part of total martensite: 368)

HB _B : Brinell hardness of bainite

(Measured hardness of part of total bainite: 352)

In addition, the relationship between each depth and Brinell hardness of the large-strength high strength steel forgings of Reference Example 1 is shown in FIG. 14 (a), and the relationship between each depth and martensite structure fraction is shown in FIG.

The large high strength steel forgings of the present invention are excellent in strength and toughness, and also have high fatigue strength. Therefore, the said large-strength high strength steel forgings can be used suitably as a large crankshaft, an intermediate shaft, etc. used for a ship or a generator.

Claims (2)

C: 0.31 mass% or more and 0.5 mass% or less,
Si: 0.02 mass% or more and 0.2 mass% or less,
Mn: 0.1 mass% or more and 0.6 mass% or less,
Ni: 2.6 mass% or more and 3.4 mass% or less,
Cr: 0.8 mass% or more and 1.9 mass% or less,
Mo: 0.25 mass% or more and 0.8 mass% or less,
V: 0.05 mass% or more and 0.2 mass% or less, and
Al: 0.005 mass% or more and 0.1 mass% or less as a basic component, remainder as Fe and an unavoidable impurity, and have the composition whose content of S as this unavoidable impurity is 0.008 mass% or less,
Martensitic structure, or a mixed structure of martensite and bainite,
Old austenite crystal grain size is 19 µm or more and 70 µm or less,
A large high strength steel forging with martensite having a maximum block diameter of 15 µm or less and a minimum block diameter of 0.5 µm or greater. The method of claim 1,
The martensite structure fraction f _m (x) (%) when the ratio of the depth to the distance from the surface to the center is x (0≤x≤1),
When O≤x≤O1, f _m (x) = 1OO,
104-40x≤f _m (x) ≤100, when 0.1 <x≤0.15,
When 0.15 <x≤0.2, 122-160x≤f _m (x) ≤100,
230-700x≤f _m (x) ≤100, when 0.2 <x≤0.3
110-300x≤f _m (x) ≤112-40x, when 0.3 <x≤0.35,
When 0.35 <x≤0.5, (22-20x) / 3≤f _m (x) ≤105-20x,
When 0.5 <x≤0.8, (32-40x) / 3≤f _m (x) ≤95, and
Large high strength steel forgings, where ₀ ≦ f _m (x) ≦ 95 when 0.8 <x ≦ 1.

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