KR20160097140A - Large-sized forging steel and large-sized forged part - Google Patents

Abstract

One aspect of the present invention relates to a large forging steel, which has a composition comprising: 0.18 mass% or more and 0.35 mass% or less of C; 0 mass% or more and 0.3 mass% or less of Si; 1 mass% or more and 2.7 mass% or less of Mn; 0 mass% or more and 1 mass% or less of Ni; 0 mass% or more and 1 mass% or less of Cu; 1.5 mass% or more and 2.5 mass% or less of Cr; 0.35 mass% or more 0.55 mass% or less of Mo; 0 mass% or more and 0.15 mass% or less of V; 0.015 mass% or more and 0.05 mass% or less of Al; 30 mass ppm or more and 100 mass ppm or less of N; more than 0 mass ppm and equal to or less than 30 mass ppm of O; and the remaining consisting of Fe and inevitable impurities. A metal structure has bainite as a main agent, and satisfies following equations (1) and (2). Equation (1): 1.15 >=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14 >= 0.95, equation (2): 0.53 >=C+Si/30+Mn/20+Ni/60+Cr/20+Mo/15+V/10 >= 0.45. The large forging steel has excellent strength, toughness, and durability.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a large-

The present invention relates to steel for large forgings and large forgings.

For parts such as a crankshaft, an intermediate shaft, a propeller shaft, a connecting rod, a rudder stock, and a rudder horn, which are used as a transmitting member of a ship drive source, large forging steels are used. In order to realize improvement in output and compactness of a marine diesel engine, large forging steels used for these parts are required to have high strength, high toughness and high durability.

As a large forging steel having high strength and high toughness, a large forging steel which has studied its element composition and the like has been proposed (see Patent Document 1, Patent Document 2 and Patent Document 3).

Japanese Patent No. 3663170 Japanese Patent No. 3896365 Japanese Patent No. 4332070

An object of the present invention is to provide a large forging steel excellent in strength and toughness and also excellent in durability and a large forging part using the forging steel.

An aspect of the present invention relates to a method of manufacturing a semiconductor device, comprising: 0.18 mass% to 0.35 mass% of C (carbon); 0 mass% to 0.3 mass% (Nickel): 0 mass% to 1 mass%, Cu (copper): 0 mass% to 1 mass%, Cr (chromium): 1.5 mass% to 2.5 mass%, Mo (molybdenum): 0.35 mass Al (aluminum): 0.015 mass% or more and 0.05 mass% or less, N (nitrogen): 30 mass ppm or more and 100 mass ppm or less, O (1) and (2), wherein the metal body has bainite as a main component and the balance of Fe (iron) and inevitable impurities. And a large forging steel.

1.15? C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14? (One)

0.53? C + Si / 30 + Mn / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10? (2)

The above and other objects, features and advantages of the present invention will become apparent from the following detailed description and accompanying drawings.

According to the present invention, it is possible to provide a large-sized forging steel excellent in durability and excellent in strength and toughness by suppressing a material gap, and a large-sized forged part using the large-forging steel.

Fig. 1 is a graph showing the relationship between the value of the function G and the strength of a large forging steel at a low cooling rate.
Fig. 2 is a graph showing the relationship between the value of the function G and the toughness of a steel for a large forging at a low cooling rate.
Fig. 3 is a graph showing the relationship between the value of the function F and the strength of a large forging steel at a high cooling rate.
4 is a schematic front view showing a state in which SSRT (low strain rate test) is performed in the evaluation of hydrogen cracking resistance in the embodiment

The large forging steel is generally manufactured by annealing or quenching and then tempering. In this large forging steel, in general, there is a difference in material due to the difference in cooling rate between the inside and the surface. The present inventors paid attention to such a material gap. In the steel for large forgings, for example, the following material gaps are considered.

For large forged parts, which are the transmitting members of the ship's drive source, a thick forging steel for large forging becomes necessary. Specifically, if a large crank throw is manufactured as a large sized forged part, a large forging steel for a fork, such as an overall length of 3500 mm and a web width of 2000 mm, is required. When such a large forging steel is produced, the cooling rate tends to be different between the thickness direction of the large forging steel and the steel for each large forging. If the cooling rates are different, the structure of the produced large forging steel is likely to be different. Therefore, the large forging steel is liable to cause a material gap between the thickness direction and a plurality of large forging steel.

In addition, the cooling tends to become uneven in the portions where the shapes are different, for example, the web of the crankshaft and the fillet portion. If there is a difference in cooling rate depending on such a region, a material gap tends to occur in the obtained large forging steel.

For large forgings, even if the strength is high but the material gap is large, there is a difference in strength within the large forging steel due to the material gaps. Therefore, the steel for large forgings tends to be easily vibrated or deformed, so that the durability tends to deteriorate. Therefore, in the conventional large forging steel, it is difficult to achieve high strength, high toughness and reduction in material gaps.

DISCLOSURE OF THE INVENTION The present invention has been made based on the above-described circumstances, and it is an object of the present invention to provide a large forging steel having excellent strength and toughness and durability by suppressing material gaps, and a large forging part using the forging steel The purpose.

As a result of intensive studies, the inventors of the present invention have found that there is an element composition which is difficult to be formed into a different metal structure depending on the cooling rate in the produced large forging steel. In other words, the present inventor has analyzed a large number of large forgings having different element compositions, and found that there is an element composition having a low dependency of the cooling rate on the metal structure. It has been found that by controlling the composition of a large forging steel so as to have an element composition with a low dependence of the cooling rate, it is possible to suppress the material gap of the steel for large forging, and the present invention has been accomplished.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of large forging steel and large forged parts according to the present invention will be described.

[Steel for large forging]

<Metal structure>

The steel for the large forgings has bainite as its metal structure. Since the metal structure is mainly composed of bainite, the steel for large forging has excellent strength.

<Composition>

The steel for large forgings has a C (carbon) content of not less than 0.18 mass% and not more than 0.35 mass%, Si (silicon): not less than 0 mass% and not more than 0.3 mass%, Mn (manganese): not less than 1 mass% and not more than 2.7 mass% (Ni): 0 mass% to 1 mass%, Cu (copper): 0 mass% to 1 mass%, Cr (chromium): 1.5 mass% to 2.5 mass%, Mo (molybdenum) (Al): 0.015 mass% or more and 0.05 mass% or less, N (nitrogen): 30 mass ppm or more and 100 mass ppm or less , O (oxygen): 0 mass ppm to 30 mass ppm or less, and the balance of Fe (iron) and inevitable impurities. The respective content ratios are content ratios with respect to the steel for large forging.

[C (carbon)]

The lower limit of the C content of the steel for large forging is 0.18% by mass and more preferably 0.23% by mass. The upper limit of the C content of the steel for large forging is 0.35% by mass and more preferably 0.3% by mass. C is an element contributing to the strength improvement as well as the hardenability. If the C content of the large forging steel is less than the lower limit, there is a possibility that sufficient strength and hardenability of the large forging steel can not be secured. On the other hand, when the C content of the large forging steel exceeds the upper limit, there is a possibility that the toughness of the steel for large forging is lowered. Further, since the inverse V segregation of C is promoted, there is a possibility that the machinability of the steel for large forging is lowered.

[Si (silicon)]

The lower limit of the Si content of the steel for large forging is 0% by mass and Si may not be included. The upper limit of the Si content of the steel for large forging is 0.3% by mass, more preferably 0.2% by mass, and most preferably 0.1% by mass. Si is an element contributing to reduction of oxygen amount as a deoxidizing element, and is added as needed. On the other hand, when the Si content of the steel for large forging exceeds the upper limit, the inverse V segregation of Si is promoted, which may lower the toughness and the hydrogen cracking resistance of the steel for large forging.

[Mn (manganese)]

The lower limit of the Mn content of the steel for large forging is 1% by mass. The upper limit of the Mn content of the steel for large forging is 2.7% by mass, more preferably 2.5% by mass, and most preferably 1.5% by mass. Mn is an element contributing to strength improvement as well as enhancing hardenability. If the Mn content of the steel for large forging is lower than the lower limit, sufficient strength and hardenability of the large forging steel may not be secured. There is also a possibility that the difference in crystal grain size can not be sufficiently suppressed. On the other hand, when the Mn content ratio of the steel for large forging exceeds the upper limit, there is a fear that the torsion and the hydrogen cracking resistance of the large forging steel are lowered because the reverse V segregation of Mn is promoted.

[Ni (nickel)]

The lower limit of the Ni content of the steel for large forging is 0% by mass, and Ni may not be included. The upper limit of the Ni content of the steel for large forging is 1% by mass, more preferably 0.5% by mass, and most preferably 0.2% by mass. Ni is an element contributing to improvement in strength and toughness, and is added as needed. On the other hand, when the Ni content of the steel for large forging exceeds the upper limit, since the reverse V segregation of Ni is promoted, there is a possibility that the toughness of the steel for large forging is lowered.

[Cu (copper)]

The lower limit of the Cu content of the steel for large forging is 0% by mass, and Cu may not be included. The upper limit of the Cu content of the steel for large forging is 1% by mass and more preferably 0.5% by mass. Cu is an element contributing to toughness enhancement and is added as needed. On the other hand, when the content of Cu in the large forging steel exceeds the upper limit, there is a fear of occurrence of hot cracking. In addition, there is a possibility that the manufacturing cost is increased.

[Cr (chromium)]

The lower limit of the Cr content of the large forging steel is 1.5% by mass. The upper limit of the Cr content of the steel for large forging is 2.5% by mass, more preferably 2% by mass, and further preferably 1.6% by mass. Cr improves hardenability and contributes to improvement of toughness. If the Cr content of the large forging steel is less than the lower limit, sufficient toughness and quenchability of the steel for large forging may not be secured. On the other hand, when the Cr content of the large forging steel exceeds the upper limit, since the reverse V segregation of Cr is promoted, there is a possibility that the machinability of the steel for large forging is lowered.

[Mo (molybdenum)]

The lower limit of the Mo content of the large forging steel is preferably 0.35 mass% and more preferably 0.45 mass%. The upper limit of the Mo content of the steel for large forging is 0.55% by mass and more preferably 0.5% by mass. Mo is an element contributing to improvement of hardenability, strength and toughness. If the content of Mo in the large forging steel is less than the lower limit described above, sufficient quenching property, strength and toughness of the large forging steel may not be secured. On the other hand, when the content of Mo in the large-forging steel exceeds the upper limit, micro-segregation of Mo and segregation of weight are promoted, and there is a possibility that the toughness of the steel for large forging is lowered.

[V (vanadium)]

The lower limit of the V content of the steel for large forging is 0 mass%, and V may not be included. The lower limit of the V content of the steel for large forging is more preferably 0.035% by mass. The upper limit of the V content of the steel for large forging is 0.15% by mass and more preferably 0.1% by mass. V is an element that contributes to the strength improvement, in addition to enhancing the hardenability, and is added as needed. On the other hand, when the V content of the steel for large forging exceeds the upper limit, the micro-segregation is promoted due to the low equilibrium partition coefficient of V, which may lower the toughness of the steel for large forging.

[Al (aluminum)]

The lower limit of the Al content in the steel for large forging is 0.015 mass%. The upper limit of the Al content in the steel for large forging is 0.05% by mass. Al is an element contributing to reduction of oxygen amount as a deoxidizing element. When the Al content of the steel for large forging is lower than the lower limit described above, there is a possibility that the sufficient amount of oxygen in the large forging steel can not be reduced. On the other hand, when the content of Al in the steel for large forging exceeds the upper limit, it may cause coarsening of the oxide, which may lower the toughness of the steel for large forging.

[N (Nitrogen)]

The lower limit of the N content of the steel for large forging is 30 mass ppm. The upper limit of the N content of the steel for large forging is 100 mass ppm, more preferably 80 mass ppm, and most preferably 60 mass ppm. N is an element contributing to securing toughness by forming nitride and grain refinement. If the N content of the large forging steel is less than the lower limit, there is a possibility that the toughness of the steel for a large forging can not be secured. On the other hand, when the N content of the large forging steel exceeds the upper limit, strain age may be caused as solid solution N, which may lower the toughness of the steel for large forging.

[O (oxygen)]

The lower limit of the O content of the steel for large forging is 0 mass ppm or more. The upper limit of the O content of the steel for large forging is preferably 30 mass ppm, more preferably 15 mass ppm, further preferably 10 mass ppm. O is preferably small, but it exists as an oxide in the steel for large forging, and the content of O can not be 0 mass ppm. Therefore, the lower limit of the O content of the steel for large forging is more than 0% by mass. On the other hand, when the content of O in the steel for large forging exceeds the upper limit, there is a fear that the toughness of the steel for large forging is lowered due to coarsening of the oxide.

[Other components]

The above-mentioned large forging steel contains Fe and inevitable impurities in the remainder in addition to the above-mentioned components. Examples of the inevitable impurities include P (phosphorus), S (sulfur), Sn (tin), As (arsenic), Pb (lead), Nb Niobium), and Ti (titanium) are allowed to be incorporated.

The upper limit of the content of P, which is an inevitable impurity in the steel for large forging, is preferably 0.1% by mass, more preferably 0.01% by mass. If the P content of the steel for large forging exceeds the upper limit, there is a risk of promoting grain boundary fracture by grain boundary segregation. The lower limit of the P content in the steel for large forging is 0% by mass, and P is not necessarily included.

The upper limit of the content ratio of S, which is an inevitable impurity in the steel for large forging, is preferably 0.02% by mass, and more preferably 0.01% by mass. When the S content of the steel for large forging exceeds the upper limit, there is a fear that the sulfide inclusions are increased and the strength is deteriorated. The lower limit of the S content of the steel for large forging is 0 mass%, and S may not be included.

&Lt; Relationship between each component content &gt;

The steel for large forging satisfies the following formulas (1) and (2).

1.15? C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14? (One)

0.53? C + Si / 30 + Mn / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10? (2)

The metal structure of the steel for the large forging is transformed into a bainite structure mainly during the manufacture of the steel for large forging. At this time, by satisfying the above formulas (1) and (2), it is possible to reduce the material gap of the steel for large forging. Presently, although the mechanism is not clear, it is considered that by satisfying the above formula (1), it is possible to suppress the lowering of the transformation starting temperature at a high cooling rate, for example, an average cooling rate of 10 캜 / minute. By satisfying the above formula (2), it is considered that the increase in the transformation starting temperature at a low cooling rate, for example, an average cooling rate of 1 캜 / minute, can be suppressed. From these, it is possible to suppress the material gap of the steel for large forging due to the difference in the cooling speed, and it becomes difficult to cause a difference in strength in the steel for large forging due to the material gaps.

More specifically, the function G represented by the equation (5) between the inequality of the equation (4) and the inequality of the equation (2) between the inequality of the equation (1) It is derived by regression analysis of many large forging steels.

F = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (4)

G = C + Si / 30 + Mn / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / (5)

Fig. 1 shows the results of examining the relation between the various functions G of a plurality of kinds of forgings for large forgings and the strength of each large forgings (at a low cooling rate) obtained by cooling at a low cooling rate after quenching treatment. That is, FIG. 1 is a graph showing the relationship between the value of the function G and the strength of a large forging steel at a low cooling rate. From this result, as shown in Fig. 1, the function G represented by the formula (5) is a function of the strength and the strength of the large forging steel at the low cooling rate, in which the average cooling rate during cooling after the quenching treatment is 1 deg. I have found that there is a correlation.

Fig. 2 shows the results of examining the toughness of a large forging steel having various values of the function G and the toughness of each large forging steel (obtained at a low cooling rate) obtained by cooling at a low cooling rate after the quenching treatment. 2 is a graph showing the relationship between the value of the function G and the toughness of a steel for a large forgings at a low cooling rate. From this result, as shown in Fig. 2, the function G represented by the formula (5) has a negative correlation with the toughness at a low cooling rate, which is an average cooling rate during cooling after mainly quenching, of 1 deg. found.

On the other hand, "strength" means a value obtained by measuring the tensile strength (TS) based on JIS Z 2241 using a No. 14 test piece (φ6 × G.30) of JIS Z 2201. "Toughness" means a value obtained by measuring the absorption energy (vE) at room temperature by the Charpy impact test based on JIS Z 2242 using a test piece (2 mmV notch) of JIS Z 2202. The larger the value of both strength and toughness, the better the strength of the large forgings.

Here, from Fig. 1 and Fig. 2, the present inventor found the following. When the value of the function G is increased, the strength of the large forging steel at the low cooling rate is improved. On the other hand, if the value of the function G exceeds 0.53, the toughness of the steel for a large forging becomes less than 150 J, and there is a possibility that the strength for a large forging steel becomes insufficient. On the other hand, if the value of the function G decreases, the toughness of the steel for large forgings is improved. On the other hand, if the value of the function G is less than 0.45, the strength of the large forging steel at the low cooling rate becomes less than 650 MPa, and there is a possibility that the strength for the large forging steel becomes insufficient. Therefore, in order to obtain a large forging steel having excellent strength and toughness, it is necessary to satisfy the expression (2).

Similarly, FIG. 3 shows the results of investigating the relationship between strengths of large forging having various values of function G and strengths of large forging (obtained at high cooling rate) obtained by cooling at a high cooling rate after quenching treatment. That is, FIG. 3 is a graph showing the relationship between the value of the function G and the strength of a large forging steel at a high cooling rate. From this result, as shown in Fig. 3, the function F represented by the formula (4) is a ratio of the strength of the large forging steel at the high cooling rate, which is mainly the average cooling rate during cooling after quenching, I have found that there is a correlation.

3, the inventor of the present invention found the following. If the value of the function F is less than 0.95, the strength of the large-forging steel at the high cooling rate becomes less than 650 MPa, which may cause the strength as a large forging steel. Therefore, in order to obtain a large forging steel having excellent strength, it is necessary to set the value of the function F to 0.95 or more.

1 and Fig. 3, the present inventor has found the following. When the value of the function F becomes large, the increase in the strength of the large forging steel at the high cooling rate is greater than the increase in the strength of the large forging steel at the low cooling rate. Therefore, when the value of the function F increases, the difference between the strength of the large forging steel at the high cooling rate and the strength of the large forging steel at the low cooling rate tends to increase. That is, due to the difference in cooling rate, a difference in strength of the steel for a large forging tends to be generated, which may lower the durability. Here, when the function G is around 0.53, the strength of the large forging steel at the low cooling rate is about 700 MPa. On the other hand, when the value of the function F exceeds 1.15, the strength of the large forging steel at the high cooling rate exceeds 800 MPa, so that the difference in strength from the strength of the large forging steel at the low cooling rate exceeds 100 J, There is a danger that the forging steel may become durability shortage. Therefore, in order to obtain a large forging steel having excellent durability, it is necessary to set the value of the function F to 1.15 or less.

From the above, equation (1) is derived.

It is preferable that the large forging steel further has P (phosphorus) and S (sulfur), and satisfies the following formula (3). Thus, by satisfying the following formula (3), the large forging steel is excellent in hydrogen cracking resistance.

0.01 x C + 0.63 x Si + 0.1 x Mn + 3.64 x P + 4.24 x S -0.19 x Mo -0.01 x Ni ? (3)

At present, although the mechanism is not clear, satisfying the formula (3) reduces the amount of micro segregation which is the origin of hydrogen cracking, and also reduces the hardness of the segregated portion, It is presumed that gender is greatly improved.

The function H represented by the expression (7) on the left side of the expression (3) is preferably small, and the lower limit of the function H is preferably small, and may be zero.

H = 0.01 × C + 0.63 × Si + 0.1 × Mn + 3.64 × P + 4.24 × S -0.19 × Mo -0.01 × Ni (7)

On the other hand, in the formulas (1) to (5) and (7), an element symbol means a content rate [mass%] of the element. The term &quot; subject &quot; of the metal structure means that the area fraction accounts for 90% or more of the area of the entire structure, preferably the area fraction is 99% or more.

<Mechanical Properties>

The lower limit of the tensile strength (TS) of the steel for large forging is preferably 650 MPa, more preferably 700 MPa. The upper limit of the tensile strength of the steel for large forging is preferably 850 MPa, more preferably 800 MPa. If the tensile strength of the large forging steel is less than the lower limit, there is a possibility that the strength of the large forging steel is insufficient. On the other hand, when the tensile strength of the large forging steel exceeds the upper limit, the strength of the large forging steel is likely to depend on the cooling rate, which may result in insufficient durability of the large forging steel.

A difference in tensile strength (TS) between a large forging steel (at a high cooling rate) obtained by cooling at a high cooling rate after quenching treatment and a large forging steel (at a low cooling rate) obtained by cooling at a low cooling rate after quenching treatment Is preferably 100 MPa, and more preferably 50 MPa. Specifically, except that the large-forging steel produced at an average cooling rate of 10 ° C / min during cooling after quenching treatment and the average cooling rate during cooling after quenching treatment were changed to 1 ° C / min, The upper limit of the difference in tensile strength (TS) of the steel for large forgings produced by the present invention is preferably 100 MPa, more preferably 50 MPa. When the difference exceeds the upper limit, the cooling rate dependency of the strength of the large forging steel tends to occur, which may result in insufficient durability of the large forging steel.

The lower limit of the absorption energy of the large forging steel measured at room temperature by the Charpy impact test is preferably 100 J, more preferably 150 J, and even more preferably 180 J. The upper limit of the absorption energy is preferably 260J. When the absorbed energy is lower than the lower limit, there is a possibility that the toughness of the steel for a large forging is insufficient. On the other hand, when the absorbed energy exceeds the upper limit, there is a possibility that the strength of the large forging steel is lowered.

The upper limit of the hydrogen cracking susceptibility S value of the large forging steel is preferably 67%, more preferably 50%, even more preferably 40%, and particularly preferably 30%. If the hydrogen cracking susceptibility S value exceeds the upper limit, cracking may occur due to grain boundary fracture due to hydrogen penetration and accumulation at the grain boundary of the steel for large forging. On the other hand, the lower limit of the hydrogen cracking susceptibility S value is not particularly limited, and the lower the better. Here, the &quot; hydrogen cracking susceptibility S value &quot; means a value obtained by immersing an aqueous solution obtained by mixing 0.5 mol / L of H ₂ SO ₄ and 0.01 mol / L of potassium thiocyanate (KSCN) When the breaking stress (elongation) of the large forging steel subjected to cathodic electrolysis at 0.5 A / dm ² is represented by S1, and the breaking stress of the large forging steel measured in the atmosphere without immersion in the aqueous solution, 1-S1 / S0) x100.

<Large forged parts>

The large forged parts are manufactured by forging the large forging steel. Therefore, the steel for large forging is excellent in strength and toughness as well as in durability. Therefore, the large-sized forged part can be suitably used as a part for realizing improvement of output and compactness of marine diesel engine.

<Manufacturing Method>

The large forging steel is produced, for example, by a solvent process, a casting process, a heating process and a material forging process. The large forged parts using the forgings for large forgings are manufactured by a manufacturing method including parts forging process, quenching pretreatment process, quenching process process and machining process.

(Solvent process)

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

(Casting process)

In the casting step, a steel ingot is cast using the steel adjusted in the solvent step. In the case of a large forging steel, ingot casting is mainly employed, but a continuous casting method may be employed.

(Heating process)

In the heating step, the ingot is heated at a predetermined temperature for a predetermined time. When the temperature becomes low, the deformation resistance of the material increases. The heating temperature is set to, for example, 1150 DEG C or more and 1350 DEG C or less in order to perform processing in a range where the deformability of the material is satisfactory. In addition, a predetermined heating time is required in order to make the temperature of the surface and the inside of the ingot uniform. 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 longer the heating time is, the larger the material is.

(Material forging process)

In the material forging process, the heated ingot is forged in the heating process. In order to squeeze casting defects such as shrinkage voids and microporosity, 3S or more is preferable as the roughing forming ratio. Thus, the steel for the large forging can be obtained.

(Component forging process)

In the component forging process, the forged ingot (steel for large forging) is processed into a large forged part such as a crankshaft in the material forging process. For example, as a working method for a crankshaft, there is a free forging method in which a crank arm and a crank pin are integrally formed into a block and forged, and the crank shaft is finished by gas cutting and machining. As another method (a method of machining into a crankshaft), forging is performed so that the axial center of the steel ingot is included in the axial center of the crankshaft, and the portion that is liable to cause deterioration of characteristics by center segregation is integrally forged RR forging and TR forging are exemplified. Among them, the RR forging method and the TR forging method can occupy the surface layer side of the crankshaft as a portion with high cleanliness, and it is preferable to obtain a crankshaft excellent in strength and durability.

(Pre-quenching process)

In the quenching pretreatment step, the forged product in the component forging step is heated to a predetermined temperature, held for a predetermined time, and then cooled to room temperature. The heating temperature is preferably 550 deg. C or higher and 650 deg. C or lower. The holding time is preferably 10 hours or more. In the temperature range of 500 占 폚 or more, it may be heated to a holding temperature at a heating rate of 50 占 폚 / hour or less. By performing the quenching pretreatment process before performing the quenching treatment, it is possible to reduce the number of the matched precipitates in the forgings.

(Quenching treatment process)

In the quenching treatment process, quenching treatment is performed, and then tempering treatment is performed. The quenching treatment is a process in which the forged product cooled in the quenching pre-treatment process is heated to a predetermined temperature, held for a predetermined time, and then cooled to a predetermined temperature. The quenching temperature is preferably 800 占 폚 or higher and 950 占 폚 or lower. The holding time is preferably 1 hour or more. The cooling temperature is preferably 450 ° C or higher and 530 ° C or lower. The temperature raising rate is preferably 30 deg. C / hr or more and 70 deg. C / hr or less. The cooling rate is preferably 15 ° C / minute or less.

The tempering treatment is a process in which the forged product subjected to quenching treatment is gradually heated to a predetermined temperature, held for a predetermined time, and then cooled to room temperature. The tempering temperature is preferably 550 deg. C or higher and 650 deg. C or lower. The holding time is preferably 5 hours or more and 20 hours or less. The temperature raising rate is preferably 30 deg. C / hr or more and 70 deg. C / hr or less. The cooling rate is preferably 15 ° C / minute or less. By performing the tempering, the balance of the strength, ductility and toughness is adjusted, and the internal stress (residual stress) caused by the phase transformation is removed.

(Machining process)

If necessary, a part of the surface layer is subjected to finishing machining including cutting or grinding from the forged product after the quenching treatment process to obtain the large forging component.

<Advantages>

The steel for large forgings has excellent strength because its metal structure is mainly composed of bainite. The metal structure is transformed into a bainite structure mainly at the time of manufacturing the steel for large forging. By satisfying the above formula (1) at the time of transformation, it is possible to suppress the lowering of the transformation start temperature at a high cooling rate can do. By satisfying the above formula (2), it is possible to suppress the increase of the transformation starting temperature at a low cooling rate. Thus, by suppressing the difference in the transformation start temperature depending on the cooling rate, it is possible to suppress the material gap of the steel for large forging. In addition, strength and toughness can be secured by setting the composition of each element of the large forging steel within the above-mentioned range. Therefore, the large forging steel has excellent durability along with strength and toughness. Therefore, the large-sized forged parts using the forgings for large forging can be suitably used as parts for realizing improvement in output and compactness of marine diesel engines and power generation diesel engines.

Although the present specification discloses various techniques of the present invention as described above, a main technique among them is summarized below.

An aspect of the present invention relates to a method of manufacturing a semiconductor device, comprising: 0.18 mass% to 0.35 mass% of C (carbon); 0 mass% to 0.3 mass% (Nickel): 0 mass% to 1 mass%, Cu (copper): 0 mass% to 1 mass%, Cr (chromium): 1.5 mass% to 2.5 mass%, Mo (molybdenum): 0.35 mass Al (aluminum): 0.015 mass% or more and 0.05 mass% or less, N (nitrogen): 30 mass ppm or more and 100 mass ppm or less, O (1) and (2), wherein the metal body has bainite as a main component and the balance of Fe (iron) and inevitable impurities. And a large forging steel.

1.15? C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14? (One)

0.53? C + Si / 30 + Mn / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10? (2)

The steel for large forgings has excellent strength because its metal structure is mainly composed of bainite. The metal structure is transformed into a bainite structure mainly at the time of manufacturing the steel for large forging. By satisfying the above formula (1) at the time of transformation, it is possible to suppress the lowering of the transformation start temperature at a high cooling rate can do. By satisfying the above formula (2), it is possible to suppress the increase of the transformation starting temperature at a low cooling rate. By suppressing the difference in transformation start temperature according to the cooling rate in this way, it is possible to suppress the material gap of the steel for large forging. In addition, strength and toughness can be secured by setting the composition of each element of the large forging steel within the above-mentioned range. Therefore, the large forging steel has excellent durability along with strength and toughness.

It is preferable that the large forging steel further has P (phosphorus) and S (sulfur), and satisfies the following formula (3). The inventor of the present invention has thus found that the large forging steel satisfies the following formula (3), whereby the hydrogen cracking resistance is excellent.

0.01 x C + 0.63 x Si + 0.1 x Mn + 3.64 x P + 4.24 x S -0.19 x Mo -0.01 x Ni ? (3)

Further, in the case of a large forging steel containing P and S, it is preferable that the P content is 0 mass% or more and 0.1 mass% or less. It is also preferable that the S content is 0 mass% or more and 0.02 mass% or less. With such a range, grain boundary fracture and increase of sulfide inclusions due to grain boundary segregation can be suppressed, and a large forging steel having better strength can be obtained.

Another aspect of the present invention is a large-sized forged part in which the large forging steel is forged. The large forged parts are manufactured by forging the large forging steel, so that they have excellent strength and toughness as well as durability. Therefore, the large-sized forged parts can be suitably used as parts for realizing improvement in output and compactness of a marine diesel engine or a diesel engine for power generation.

INDUSTRIAL APPLICABILITY As described above, the large forging steel of the present invention is excellent in strength and toughness as well as durability, and thus can be suitably used for large-sized forged parts such as marine diesel engines and power generation diesel engines.

Example

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[Examples 1 to 15 and Comparative Examples 1 to 21]

Using a high-frequency furnace, the forging steel having the components shown in Table 1 was melted and cast to produce 50 kg of a steel ingot having a diameter of 132 mm or more and 158 mm or less and a length of 323 mm.

The molten part of the obtained ingot was excluded and heated at a temperature of 1230 DEG C for not less than 5 hours and not more than 10 hours and then compressed to a height ratio of 1/2 using a free forging press machine. Next, the steel ingot was rotated by 90 DEG about the center line of the ingot for forging, and stretched to 90 mm x 90 mm x 450 mm to obtain a steel material. This steel material was allowed to cool in the air until the temperature reached room temperature. The steel material was maintained at 550 DEG C or higher and 650 DEG C or lower for 10 hours or longer before quenching and then subjected to cooling. On the other hand, in the temperature range of 500 占 폚 or more, heating was carried out at a heating rate of 50 占 폚 / hour or less to the holding temperature.

Thereafter, the steel material was subjected to quenching treatment using a small simulated furnace. In the quenching, the steel material was heated to 870 캜 at a temperature raising rate of 50 캜 / hour, held for 3 hours, and then cooled to 500 캜. At this time, a steel material having an average cooling rate of 1 占 폚 / min and a steel material having an average cooling rate of 10 占 폚 / min were prepared. Thereafter, each steel material was subjected to a tempering treatment. As the tempering treatment, a method of maintaining the temperature at 620 占 폚 for 10 hours and then cooling the furnace was adopted. Thus, large forging steels of Examples 1 to 15 and Comparative Examples 1 to 21 were obtained.

[Assessment Methods]

The following evaluations were made on the large forging steel of Examples 1 to 15 and Comparative Examples 1 to 21. The evaluation results are shown in Table 2.

<Strength Evaluation>

As the strength evaluation, a tensile test was conducted on each large forging steel. The tensile test was carried out by measuring the tensile strength (TS) based on JIS Z 2241 by using a No. 14 test piece (? 6 x G.30) of JIS Z 2201. The larger the value of the tensile strength (TS), the greater the strength of the large forging steel. On the other hand, the tensile test was conducted for both large forging steels having an average cooling rate of 1 캜 / min and for large forging steels having an average cooling rate of 10 캜 / min. As described above, when the numerical value of the tensile strength was 650 MPa or more, it was judged that the strength of the large forging steel was excellent. In addition, when the difference in tensile strength between large-forging steel having different average cooling rates is 100 MPa or less, it can be judged that the steel for large forging has excellent durability.

<Personality evaluation>

As a toughness evaluation, a Charpy impact test was performed on each large forging steel. The Charpy impact test was conducted by measuring the absorbed energy (vE) at room temperature based on JIS Z 2242 using a test piece (2 mmV notch) of JIS Z 2202. The larger the value of absorbed energy, the greater the toughness of the steel for large forgings. On the other hand, the Charpy impact test was carried out for a large forging steel having an average cooling rate of 1 캜 / min. As described above, when the absorbed energy value is 150 J or more, it can be judged that the large forging steel is excellent in toughness.

&Lt; Evaluation of cracking resistance in hydrogen &

The hydrogen cracking susceptibility of each large forging steel was evaluated by a comparative test. The hydrogen cracking resistance was evaluated in the following order. First, a round bar type test piece was taken from each large forging steel. The test specimens were machined into a dumbbell with a length of 150 mm and a distance of 10 mm between the lines. The central portion was machined to a diameter of 4 mm and the gripper portions of both ends were machined to a diameter of 8 mm, did. On the other hand, comparative evaluation of the hydrogen cracking susceptibility was carried out for a large forging steel having an average cooling rate of 1 캜 / min.

Next, as shown in Fig. 4, the test piece 1 was set in the test apparatus 2 and immersed in an aqueous solution (3) in which 0.5 mol / L of H ₂ SO ₄ and 0.01 mol / L of KSCN were mixed . The negative electrode was electrolyzed at a current density of 0.5 A / dm ² while adding hydrogen in this state. An SSRT (low strain rate test) was conducted in which a tensile load N in the major axis direction was applied to the specimen 1 having been prepared as described above and its breaking stress S1 (elongation) was measured. At this time, the tensile speed of the crosshead of the test apparatus 2 was set to 2 10 ^-3 mm / min.

On the other hand, SSRT (low deformation rate test) was performed under the same conditions as those described above except that the immersion in the aqueous solution 3 was omitted, that is, in the atmosphere, and the fracture stress S0 of the large forging steel was measured.

The measurement value obtained in these measurements was substituted into the following equation (6) to calculate the hydrogen cracking susceptibility S value. When the S value of the hydrogen cracking susceptibility was 50% or less, it was judged that the large forging steel had excellent hydrogen cracking resistance.

S value = (1 - S1 / S0) x100 ... (6)

On the other hand, in Table 2, the function H means a function of the following equation (7) on the left side of the equation (3).

H = 0.01 × C + 0.63 × Si + 0.1 × Mn + 3.64 × P + 4.24 × S -0.19 × Mo -0.01 × Ni (7)

[Evaluation results]

As shown in Table 2, when the content ratios of C, Si, Mn, Ni, Cu, Cr, Mo, V, Al, N and O were within the range of the present invention, , It is found that the large forging steels of Examples 1 to 15 are excellent in strength, durability and toughness due to the difference in tensile strength between large forging steels having different tensile strengths and average cooling rates, and high absorbed energy .

On the other hand, in the large forging steels of Comparative Examples 1, 4 and 8, since the value of the function G is too small, it is considered that the tensile strength of a large forging steel having an average cooling rate of 1 캜 / It can be seen that it is insufficient.

It is considered that the large forging steels of Comparative Examples 2, 9 and 19 have an excessively large value of the function F, so that the difference in tensile strength between large forging steels having different average cooling rates is increased, Able to know.

The large forging steels of Comparative Examples 3, 5 to 7, and 11 to 17 were found to have a large amount of Si, Mn, Ni, Cu, Mo, V, Al, N and O, It is thought that the energy is lowered, and it is understood that the personality is lacking.

Further, since the large forging steel of Comparative Example 10 had less Mo than the scope of the present invention, it was considered that the tensile strength of a large forging steel having an average cooling rate of 1 占 폚 / min was reduced, .

For the comparative examples 18, 20 and 21, although the mechanism thereof is not clear, since the value of the function F or the function G is out of the scope of the present invention, the difference in tensile strength between large forging steels It is thought that it can not be done, and it is understood that the durability is insufficient.

In Examples 1 to 14 satisfying the formula (3), the sensitivity to hydrogen cracking was lower than that in Example 15 in which the formula (3) was not satisfied. From this, it can be seen that, by further satisfying the formula (3), the large forging steel has excellent hydrogen cracking resistance.

This application is based on Japanese Patent Application No. 2015-022402 filed on February 6, 2015, the content of which is incorporated herein by reference.

In order to express the present invention, although the present invention has been adequately and fully explained through the embodiments with reference to the drawings in the foregoing, it should be appreciated by those skilled in the art that modifications and / or improvements of the above-described embodiments can be easily made. It is therefore to be understood that the type of modification or mode of modification encompassed by the scope of the claims is not intended to be limited by the scope of the claims as amended by the person skilled in the art.

INDUSTRIAL APPLICABILITY As described above, the large forging steel of the present invention is excellent in strength and toughness as well as durability, and thus can be suitably used for large-sized forged parts such as marine diesel engines and power generation diesel engines.

Claims (4)

C: 0.18 mass% or more and 0.35 mass% or less,
Si: 0 mass% or more and 0.3 mass% or less,
Mn: 1 mass% or more and 2.7 mass% or less,
Ni: 0 mass% or more and 1 mass% or less,
Cu: 0 mass% or more and 1 mass% or less,
Cr: 1.5% by mass or more and 2.5% by mass or less,
0.35 mass% or more and 0.55 mass% or less of Mo,
V: 0 mass% or more and 0.15 mass% or less,
Al: 0.015 mass% or more and 0.05 mass% or less,
N: 30 mass ppm or more and 100 mass ppm or less,
O: more than 0 mass ppm and not more than 30 mass ppm,
The balance being Fe and inevitable impurities,
The metal structure is mainly composed of bainite,
A steel for large forgings characterized by satisfying the following formulas (1) and (2).
1.15? C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14? (One)
0.53? C + Si / 30 + Mn / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10? (2) The method according to claim 1,
Wherein the large forging steel further contains P and S,
A large forging steel satisfying the following formula (3).
0.01 x C + 0.63 x Si + 0.1 x Mn + 3.64 x P + 4.24 x S -0.19 x Mo -0.01 x Ni ? (3) 3. The method of claim 2,
P: more than 0% by mass, not more than 0.1%
S: Steel for large forgings having a content of 0 mass% or more and 0.02 mass% or less. A large-sized forged part forged with a large forging steel according to any one of claims 1 to 3.

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