KR20160098412A - Marine steel forging - Google Patents

Abstract

The present invention relates to a steel comprising at least 0.13 mass% to 0.25 mass% of C, at least 0.15 mass% and at most 0.45 mass% of Si, at least 0.3 mass% and at most 1.0 mass% of Mn, at least 1.2 mass% V: 0.05 to 0.15 mass%, N: 0.02 mass% or less, S: 0.002 mass% to 0.015 mass%, Ti: 0.1 mass% to 0.9 mass% And Al: 0.003 mass% or more and 0.05 mass% or less in total, and the remainder being Fe and inevitable impurities, satisfies 14 x (Ti / 48 + Al / 27)? N, and ferrite- Ferrite-pearlite-bainite having a total area ratio of ferrite structure and pearlite structure of 70% or less on the surface thereof.

Description

{MARINE STEEL FORGING}

The present invention relates to a forging steel for ships.

From the viewpoint of reducing the fuel consumption of the ship, the ship member is required to be lightweight, and therefore, the strength of the steel used for the ship member is required to be increased. Generally, the strengthening of the steel material can be achieved by accelerating the cooling after the austenitization in the heat treatment step to produce hard bainite or martensite. However, in the case of a large-sized thick member having a weight of several tons to several tens of tons, if the steel is cooled for quenching after austenitization, cracks may be generated due to thermal stress or transformation stress during cooling. Therefore, cooling after austenitization can not be achieved by air cooling with a slow cooling rate, and it is difficult to obtain a high strength with a large thickness member.

On the other hand, as a large-sized low-melting-point member having high strength, (1) a cast steel which can increase the hardenability even when the cooling rate is low, such as air cooling, by controlling the addition amount of the alloy element in an appropriate range, (Japanese Patent Laid-Open Publication No. 2009-91649) which realizes excellent fatigue strength by reducing the S content in order to reduce non-metallic inclusions, paying attention to the control of non-metallic inclusions, Has been developed.

However, in the cast steel of (1), casting defects such as microporosity are unavoidably caused by casting, and it is difficult to realize a preferable fatigue strength. The cast steel of (1) can be reduced in casting defects by forging, but the grain size becomes finer than cast steel. As is generally known, the hardenability of a steel is deteriorated when the crystal grain size becomes small. Therefore, it is difficult to obtain sufficient strength even when the above-described control (1) is applied to steel forgings.

On the other hand, in forged steel products, segregation in the steel ingot becomes remarkable as the steel member becomes larger, and hydrogen tends to be thickened in the segregation portion. When the S content is reduced as in the forging steel of (2) above, the fatigue strength is improved, but the amount of MnS that becomes the hydrogen trap site decreases, and hydrogen cracks are likely to occur in the segregation portion where hydrogen is concentrated. Therefore, the forged steel of (2) is difficult to apply to a large member.

Japanese Patent No. 3509634 Japanese Patent No. 5229823 Japanese Patent Application Laid-Open No. 2009-91649

The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide a forging steel for a ship which has high strength and is also suitably used for a large size bottom member.

One aspect of the present invention is a method for producing a silicon carbide comprising 0.13 mass% or more and 0.25 mass% or less of C (carbon), 0.15 mass% or more and 0.45 mass% or less of Si (silicon), 0.3 mass% or more and 1.0 mass% (Ni): not less than 1.2 mass% and not more than 2.6 mass%, Cr (chromium): not less than 0.4 mass% and not more than 0.9 mass%, Mo (molybdenum): not less than 0.15 mass% and not more than 0.8 mass%, V (vanadium) At least one element selected from Ti (titanium) and Al (aluminum): not more than 0.15 mass%, N (nitrogen): not more than 0.02 mass%, S (sulfur): not less than 0.002 mass% Ferrite-ferrite-ferrite-ferrite-ferrite-ferrite-ferrite-ferrite-ferrite-ferrite-ferrite-ferrite-ferrite-ferrite ferrite-ferrite-ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite ferrite non- Which is a composite structure of bainite and has a total area ratio of ferrite structure and pearlite structure on the surface of 70% or less will be.

14 占 (Ti / 48 + Al / 27)? N (1)

1 is a graph showing the relationship between the V content and the tensile strength in Examples,
2 is a graph showing the relationship between the area ratio of ferrite and pearlite and the tensile strength in the examples.

The steel forging according to the present invention is characterized in that the steel forging comprises 0.13 mass% or more and 0.25 mass% or less of C (carbon), 0.15 mass% or more and 0.45 mass% or less of Si (silicon), 0.3 mass% or more and 1.0 mass% (Nickel): not less than 1.2 mass% and not more than 2.6 mass%, Cr (chromium): not less than 0.4 mass% and not more than 0.9 mass%, Mo (molybdenum): not less than 0.15 mass% and not more than 0.8 mass%, V (vanadium) 0.002 mass% or more and 0.015 mass% or less; Ti (titanium) and Al (at least one element of aluminum: 0.003 mass or less in total); N (nitrogen): more than 0 mass% And a ferrite-ferrite-ferrite-bainite ferrite-ferrite-bainite ferrite-ferrite-bainite ferrite-ferrite-bainite ferrite-ferrite-bainite ferrite- Which is a ship steel forgings having a total area ratio of ferrite structure and pearlite structure on the surface of 70% or less It characterized.

14 占 (Ti / 48 + Al / 27)? N (1)

It is preferable that the content of each composition of the steel satisfies the above-mentioned formula (1) in the above-mentioned range, and the metal structure is a complex structure of ferrite-bainite or ferrite-pearlite-bainite, By setting the total area ratio of the pearlite structure to be not more than the upper limit, it is possible to secure sufficient strength without increasing the cooling rate after the austenitization in the heat treatment step. In addition, the forged steel for ship has a structure in which nitrogen (N) is fixed by titanium (Ti) or aluminum (Al) having a high affinity for nitrogen (N) by satisfying the formula (1) , V nitride) is suppressed. That is, since the consumption amount of vanadium (V) by the bond with nitrogen (N) is reduced, the reduction of vanadium carbide (hereinafter abbreviated as V carbide) is suppressed and even if the cooling rate after the austenitization is slow, The strength of the forging steel forgings can be increased by the strengthening ability (ability to precipitate hard particles simultaneously with the transformation during cooling).

The forged steel for ship according to the present invention has high strength and is also used suitably for a large-sized back member.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of marine forgings according to the present invention will be described.

<Metal structure>

The steel structure for marine forgings in the present embodiment is a complex structure of ferrite-bainite or ferrite-pearlite-bainite, and the total area ratio of the ferrite structure and the pearlite structure on the surface is 70% or less. If the ferrite structure and the pearlite structure increase, it becomes difficult to secure sufficient strength. Therefore, if the composite structure of ferrite-bainite or ferrite-pearlite-bainite is used as the metal structure and the total area ratio of the ferrite structure and the pearlite structure is set to be not more than the upper limit, the ship steel forgings have high strength. As a method of measuring the area ratio of the ferrite structure and the pearlite structure, for example, a test piece for microstructure observation is cut out from forged steel products, and the parallel side of the test piece is mirror-polished in the forging stretching direction. It can be carried out by etching with a nital and observing with an optical microscope.

<Composition>

The steel forging according to the present embodiment contains 0.13 mass% or more and 0.25 mass% or less of C, 0.15 mass% or more and 0.45 mass% or less of Si, 0.3 mass% or more and 1.0 mass% or less of Mn, 1.2 mass% V: not less than 0.05 mass% and not more than 0.15 mass%, N: not less than 0 mass% and not more than 0.02 mass%, S: not less than 0.1 mass% 0.002 mass% or more and 0.015 mass% or less, at least one element selected from Ti and Al: 0.003 mass% or more and 0.05 mass% or less in total, the balance being Fe and inevitable impurities, ).

14 占 (Ti / 48 + Al / 27)? N (1)

In the present embodiment, the lower limit of the C content in marine forgings is 0.13 mass%, preferably 0.15 mass%. On the other hand, the upper limit of the C content of marine forgings is 0.25 mass%, preferably 0.23 mass%. If the C content of the steel forgings is less than the above lower limit, sufficient quenchability and strength may not be secured. On the other hand, if the C content of the ship forged steel exceeds the upper limit, susceptibility to welding cracks increases, and welding cracks tend to occur. By setting the C content of the steel forgings to the above-mentioned range, it is possible to properly secure the hardenability and strength of marine forgings.

In the marine forging steel of the present embodiment, the lower limit of the Si content is 0.15 mass%, preferably 0.16 mass%. On the other hand, the upper limit of the Si content is 0.45 mass%, preferably 0.30 mass%. If the Si content is less than the above lower limit, there is a fear that deoxidation can not be performed sufficiently or strength can not be secured. On the other hand, when the Si content exceeds the upper limit, there is a fear that the reverse V segregation is promoted. By setting the Si content in the above-described range, the strength of the forging steel for ship can be appropriately secured.

In the marine forging steel of the present embodiment, the lower limit of the Mn content is 0.3% by mass, preferably 0.31% by mass. On the other hand, the upper limit of the Mn content is 1.0% by mass, preferably 0.95% by mass. If the Mn content is less than the above lower limit, there is a possibility that sufficient strength and hardenability can not be secured. On the other hand, if the Mn content exceeds the upper limit, there is a risk of promoting tempering embrittlement, and there is a fear of deteriorating the weldability. By setting the Mn content in the above range, it is possible to secure adequately the hardenability, strength and weldability of marine forgings.

In the marine forging steel of the present embodiment, the lower limit of the Ni content is 1.2% by mass, preferably 1.4% by mass. On the other hand, the upper limit of the Ni content is 2.6% by mass, preferably 2.5% by mass. If the Ni content is less than the above lower limit, there is a possibility that sufficient strength and hardenability can not be secured. Further, since Ni is an expensive element, when the Ni content exceeds the upper limit, the effect of improving the strength and hardenability reaches the limit, and the manufacturing cost is increased, which is not preferable from an industrial point of view. By setting the Ni content to the above-mentioned range, it is possible to appropriately secure the hardenability and strength of marine forgings.

In the marine forging steel of the present embodiment, the lower limit of the Cr content is 0.4% by mass, preferably 0.41% by mass. On the other hand, the upper limit of the Cr content is 0.9% by mass, preferably 0.85% by mass. If the Cr content is less than the lower limit described above, sufficient quenching and temper softening resistance may not be secured. On the other hand, if the Cr content exceeds the upper limit, there is a possibility that the weldability is deteriorated and the reverse V segregation is promoted. By setting the Cr content in the above range, it is possible to properly secure the hardenability, temper softening resistance and weldability of marine forgings.

In the marine forging steel of the present embodiment, the lower limit of the Mo content is 0.15 mass%. On the other hand, the upper limit of the Mo content is 0.8% by mass, preferably 0.7% by mass. If the Mo content is less than the above lower limit, sufficient quenching and temper softening resistance may not be secured. On the contrary, if the Mo content exceeds the upper limit, the weldability may be lowered, and micro-segregation in the steel ingot may be promoted, or gravity segregation may easily occur. By setting the Mo content in the above range, the hardenability, temper softening resistance and weldability of marine forgings can be appropriately secured.

V is an element that forms a fine V carbide and increases the strength by precipitation strengthening. When cooling after austenitization is slow as in air cooling, soft ferrite is generated and the strength is lowered. However, by precipitating V carbide in ferrite, the ferrite can be hardened and the strength can be increased.

In the marine forging steel of the present embodiment, the lower limit of the V content is 0.05% by mass, and the upper limit of the V content is 0.15% by mass. If the V content is less than the lower limit, the strength becomes insufficient. On the contrary, if the V content exceeds the upper limit, the effect of improving the strength is not only reduced but also the weldability may be deteriorated. By setting the V content in the above range, strength and weldability of marine forgings can be appropriately secured.

N combines with V in the steel to produce V nitride. Since the V-nitride has a higher melting temperature than the V-carbide, it sometimes remains unused for austenitization, and the precipitation strengthening ability by carbide precipitation is reduced. Therefore, the content of N is preferably low. However, since N is inevitably mixed as an impurity, the content of N can not be zero. Therefore, the lower limit of the N content of the forged steel for marine of the present embodiment is more than 0% by mass. On the other hand, the upper limit of the N content is 0.02 mass%, preferably 0.015 mass%, and more preferably 0.012 mass%. If the N content exceeds the upper limit, there is a possibility that the precipitation strengthening ability is reduced and sufficient strength can not be ensured. By setting the N content to the above-mentioned range, the strength of the ship forging steel can be appropriately secured.

In the marine forging steel of the present embodiment, the lower limit of the S content is 0.002 mass%, preferably 0.003 mass%. On the other hand, the upper limit of the V content of the steel forgings of the present embodiment is 0.015% by mass, preferably 0.01% by mass. S bonds with Mn in the steel to form MnS, and MnS becomes a hydrogen trap site in the steel to prevent hydrogen cracking. Therefore, if the S content is less than the lower limit, hydrogen cracking may occur. On the other hand, if the S content exceeds the upper limit, there is a possibility that the ductility and toughness in the vertical direction with respect to the main forging direction are lowered. By setting the S content in the above range, it is possible to appropriately prevent hydrogen cracking of marine forgings.

In the marine forging steel of the present embodiment, the lower limit of the total content of at least one element of Ti and Al is 0.003 mass%, preferably 0.005 mass%. On the other hand, the upper limit of the total content of Ti and Al is 0.05% by mass, preferably 0.045% by mass. In order to sufficiently exhibit the precipitation strengthening ability by V carbide, it is necessary to suppress the formation of V nitride. However, by adding Ti or Al higher in affinity to N than N, N can be fixed, Can be suppressed. Therefore, when the total content of Ti and Al is less than the above lower limit, the formation of V nitride can not be sufficiently suppressed. On the other hand, Ti and Al also bond with other elements. Therefore, when the total content of Ti and Al exceeds the upper limit, non-metallic inclusions and intermetallic compounds may be generated to cause internal defects. By setting the total content of Ti and Al within the above range, the precipitation strengthening ability by V carbide can be sufficiently exhibited and the strength of the forging steel for ship can be appropriately secured.

The marine forging steel of the present embodiment contains Fe and inevitable impurities in the remainder in addition to the basic components described above. As the inevitable impurities, incorporation of elements such as P (phosphorus), Sn (tin), As (arsenic), Pb (lead), etc., which are mixed according to the conditions of raw materials, materials, . In addition, it is also effective to positively contain other compositions, and the properties of the single steel are further improved according to the type of the contained composition.

The upper limit of the content rate of P, which is an unavoidable impurity in marine forgings of the present embodiment, is preferably 0.1% by mass, more preferably 0.05% by mass, and even more preferably 0.01% by mass. If the P content exceeds the upper limit, there is a fear of promoting grain boundary fracture by grain boundary segregation.

<Relation of each composition>

In the marine forging steel of the present embodiment, the content of each element satisfies the following formula (1).

14 占 (Ti / 48 + Al / 27)? N (1)

The left side of the above formula (1) represents N amount stoichiometrically consumed as TiN or AlN. As shown in the above formula (1), when the amount of consumed N becomes equal to or more than the N content, all of N is consumed as TiN or AlN, generation of V nitride can be suppressed and precipitation strengthening ability by V carbide can be suppressed It can be exercised enough.

It is preferable that the forging steel for ship according to the present embodiment satisfies the following expression (2) as the content of each element.

C + Mn / 6 + Ni / 15 + (Cr + Mo + V) /5?

The left side of the above formula (2) is a carbon equivalent (Ceq) equivalent to the effect of alloy elements on the hardenability and weldability of a steel in terms of carbon amount, and is used as an index of the weldability of the steel (for example, Japanese Patent No. 3863413). When Ceq is increased, the hardness of the weld heat affected zone (HAZ: Heat-Affected Zone) is increased, and cracks are generated. Generally, it is known that there is a correlation between Ceq and HAZ maximum hardness. When Ceq is high, it is necessary to increase the preheating temperature at the time of welding. However, in the case of a large forged steel product, it is difficult to preheat at a high temperature. Therefore, Ceq is set to 0.8 or less as a weldability index for enabling the welding at a preheating temperature of 50 캜 or lower in marine forgings of this embodiment. As a result, the forging steel for ship according to the present embodiment is excellent in weldability and can be suitably used as a steel forgings requiring welding.

<Mechanical Properties>

The lower limit of the tensile strength TS of the steel forging ship of the present embodiment is preferably 600 MPa. If the tensile strength is not less than the lower limit, the strength required for the large-sized backing member for a ship can be satisfied. The tensile strength can be evaluated, for example, by a tensile test according to JIS-Z2241 (1998).

The lower limit of the 0.2% proof stress (YS) of the ship steel forgings of the present embodiment is preferably 400 MPa. If the 0.2% proof strength of the ship steel forgings is above the lower limit, the strength required for large shipyards for ships can be satisfied. The evaluation of the 0.2% proof stress can be carried out, for example, by a tensile test according to JIS-Z2241 (1998).

<Manufacturing Method>

The ship forged steel of the present embodiment is manufactured by the following dissolution step, casting step, heating step, forging step and heat treatment step, for example.

(Dissolution step)

In the melting 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 dissolved steel after the component adjustment 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 process, ingot (ingot) casting is employed mainly for large forging strength. In the case of relatively small steel forgings, a continuous casting method may be employed.

(Heating process)

In the heating process, the ingot is heated at a predetermined temperature for a predetermined time. When the temperature becomes low, the deformation resistance of the material increases. Therefore, the heating temperature should be 1150 DEG C or higher in order to perform processing in a good range of deformability of the material. In addition, a predetermined heating time is required for uniformizing the temperature of the surface and inside of the steel ingot, and the heating time is set to 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 and holding time is, the larger the material is.

(Forging process)

In the forging process, a steel ingot heated to a temperature of 1150 占 폚 or more is forged in the heating process. In order to squeeze casting defects such as shrinkage cavities and micropores, 3S or more is preferable as the rough molding ratio.

(Heat treatment process)

In the present embodiment, since the constitution of marine steel forgings is attained not only by chemical composition but also by appropriately controlling microstructure, heat treatment is performed to obtain a predetermined microstructure. In the heat treatment process, after the normalizing process is performed, a tempering process is performed. In the annealing treatment, the austenitizing treatment is first carried out, and then the cooling treatment is carried out after the austenitization.

In the steaming treatment, austenitization of the steel is first carried out. The austenitization is carried out at a heating rate of 30 to 70 占 폚 / hr at a temperature not lower than the Ac3 transformation point (830 占 폚) and held for a predetermined time (for example, 1 hour or more). From the viewpoint of restraining the old austenite grain coarsening, the austenitization is preferably carried out at 940 占 폚 or lower. Further, in the case of a large-sized product, since a temperature difference occurs between the inside and the outside of the material at the time of heating, it is necessary to gradually heat to the austenitizing temperature and to keep the surface temperature and the inside temperature uniform for a certain period of time. This retention time depends on the steel diameter, and it is necessary to lengthen the retention time as large as possible.

Next, in the steaming treatment, the steel material is cooled after the temperature of the steel material becomes homogeneous by austenitization. In the case of a large-sized subcell having a weight of several tons to several tens of tons, since a crack occurs due to a thermal stress or a transformation stress during cooling, the cooling after the austenitization is cooled by air cooling or the like slower than water cooling . For example, the cooling rate of the air cooling at the D / 4 position is about 300 DEG C / hr at? 200 mm, about 150 DEG C / hr at? 500 mm,? 1000 Mm to about 70 ° C / hr. Further, the steel is cooled to 200 DEG C or less to complete the transformation completely. If the cooling is insufficient, untransformed residual austenite remains, which causes a characteristic deviation.

After cooling, tempering treatment is carried out to obtain steel forgings for the ship. The tempering of the steel material is gradually heated to a predetermined temperature at a heating rate of 30 to 70 占 폚 / hr and held for a predetermined time (for example, 5 to 20 hours). Tempering is carried out at 550 캜 or higher to adjust the balance of strength, ductility and toughness, and to eliminate the internal stress (residual stress) caused by the phase transformation. However, at high temperatures, the steel is softened by coarsening of carbide, recovery of dislocation structure, etc., and sufficient strength can not be ensured.

<Machining>

If necessary, after the heat treatment, the steel for the ship can be made by performing finish machining including grinding at least a part of the surface layer of the ship forged steel of the present embodiment.

&Quot; Example &

Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

[Preparation of test sample]

The steels A to P having the composition shown in Table 1 were dissolved. The steels A, B, C, E, F, G, H, I, J, K and M are dissolved by the high frequency melting furnace and the steels D, L, N, O and P are dissolved by the vacuum melting furnace, ~ 150kg of ingot (ingot) was cast. The obtained ingot was heated at 1230 占 폚 for 3 hours and then subjected to forging elongation at a temperature of 3 占 to 6 占 at the shaping forming ratio and then cooled to room temperature in the air. Thereafter, test pieces of 20 mm x 20 mm x 150 mm were cut out from each forging elongation member. The thus-cut test pieces were subjected to a heat treatment (adiabatic treatment and tempering treatment) in order to secure mechanical properties. As for the condition of the soaking, a heat treatment simulating the heating rate and the cooling rate of a large steel forgings such as a rudder stock or an intermediate shaft was performed. Specifically, the temperature was raised to 40 ° C / hr up to the austenitizing temperature (850 to 920 ° C) using a small heat treatment furnace, and the temperature was held for 1 hour or more at that temperature. Thereafter, cooling was carried out such that the average cooling rate in the temperature range of 800 to 500 ° C was 30 to 300 ° C / hr. The tempering treatment was conducted at a temperature of 580 to 640 DEG C for 10 hours or longer, followed by furnace cooling. Thus, test specimens of steel forgings of Examples 1 to 19 and Comparative Examples 1 to 8 shown in Table 2 were prepared.

In Table 1, "-" represents the measurement limit or less. The strengths A to P were designed such that the carbon equivalent Ceq shown on the left side of the above formula (2) was 0.8 or less, considering weldability. In Table 2, "N-14 x (Ti / 48 + Al / 27)" indicates the amount of N remaining without consuming Ti or Al by reducing the left side of the formula (1) If this value is less than 0, it can be said that N is consumed as TiN or AlN.

(Examples 1 to 19)

The test specimens of Examples 1 to 19 were prepared so that the contents of C, Si, Mn, Ni, Cr, Mo, V, N, S, Ti and Al were within the range of the present invention, And is created by the above-described method using the strengths A to J. The test samples of Examples 6 to 8 were prepared by using a steel (F) having the same composition and different cooling rates in the denitration treatment as shown in Table 2. Likewise, the test samples of Examples 9 to 12, Examples 13 to 15, and Examples 16 to 18 were also prepared by using steels G, H and I of the same composition, respectively, at different cooling rates in the steaming treatment will be.

(Comparative Examples 1 and 2)

The test samples of Comparative Examples 1 and 2 were prepared by using the steels E and F having the content ratios of C, Si, Mn, Ni, Cr, Mo, V, N, S, Ti and Al within the range of the present invention. The cooling rate in the treatment is slowed to generate a large amount of ferrite structure or pearlite structure.

(Comparative Examples 3 to 5, 7 and 8)

The test specimens of Comparative Examples 3 to 5, 7 and 8 were prepared by using steels K to M, O, and P having a content ratio of at least any one of C, Ni, Cr, Mo and V outside the range of the present invention.

(Comparative Example 6)

The composition of the steel N is such that the content of C, Si, Mn, Ni, Cr, Mo, V, N, S, Ti and Al is within the range of the present invention but does not satisfy the above formula (1).

The test sample of Comparative Example 6 was prepared using this steel N.

[Table 1]

[Observation of Microstructure]

After the heat treatment, a test piece for observing the microstructure was cut out from the test sample, and the parallel side of the test piece was mirror-polished in the forging stretching direction. Observation was carried out at a magnification of 400 times and arbitrarily observed at 4 fields to judge microstructure, and area ratios of the ferrite structure and pearlite structure of each of the four fields of view were obtained and averaged. Table 2 shows the area ratios of the metal structures obtained for each test sample. In Table 2, the ferrite structure is referred to as "F", the pearlite structure as "P", and the bainite structure as "B".

[Measurement of mechanical properties]

After the heat treatment, a test sample was processed so that the longitudinal direction of the test piece became parallel to the forging elongation direction, and a tensile test was conducted. The shape of the test piece was φ6 × G.L.30 mm as No. 14 test piece of JIS-Z2201 (1998). The tensile test was carried out on the basis of JIS-Z2241 (1998), and the tensile strength, 0.2% proof stress, elongation and sectional shrinkage were measured. B "having a tensile strength of not less than 600 MPa and a tensile strength of not less than 600 MPa and a 0.2% proof stress of not less than 400 MPa as the total strength" A " . The results of these measurements are shown in Table 2.

[Table 2]

[Measurement result]

In all of the test samples of Examples 1 to 19, the total area ratio of the ferrite structure and the pearlite structure was 70% or less. The tensile strength was 600 MPa or more, and the 0.2% proof stress was 400 MPa or more.

On the other hand, in the test samples of Comparative Examples 1, 2 and 8, the total area ratio of the ferrite structure and the pearlite structure exceeded 70%. In the test samples of Comparative Examples 1 to 8, all tensile strengths were less than 600 MPa, and in the test samples of Comparative Examples 1, 4, 5 and 8, the 0.2% proof stress was less than 400 MPa.

In Comparative Examples 3, 4, 5, 7, and 8, since the steels K, L, M, O, and P having a composition outside the scope of the present invention were used, it can be said that the strength required as a large backing member was not obtained. Further, in the comparative example 6, since the steel N having the composition which does not satisfy the above formula (1) of the present invention was used, it can be said that the strength required as a large backing member is not obtained. Further, in Comparative Examples 1 and 2, the cooling rate after austenitization was slower than in Examples 5 to 8 using the same steels E and F, so that the area ratio of ferrite and pearlite was larger than those of Examples 5 and 8, , It can be said that the required strength as a large backing member is not obtained. Further, in Comparative Example 8, the area ratio of ferrite after austenitization was increased due to the fact that the composition of the steel P used was greatly different from the range of the present invention, and thus the strength was lowered in comparison with other comparative examples .

(Relation to V content)

(A) to (D) and K to N which are different from each other in the element (Ti, Al, N) included in the formula (1) and the basic composition (C, Si, Mn, Ni, Cr and Mo) Fig. 1 shows the relationship between the V content and the tensile strength of the test samples (Examples 1 to 4 and Comparative Examples 3 to 6) prepared by the above method. From Fig. 1, it can be seen that when the test sample having the same V content is observed, high strength is obtained and satisfactory precipitation strengthening of the V-carbide is achieved when the above formula (1) is satisfied. It is also understood that, in order to secure a tensile strength of 600 MPa or more, it is necessary to satisfy the above formula (1) and also to contain V of 0.05 mass% or more.

(Relation between Ferrite and Perlite Area Ratio)

Fig. 2 shows the relationship between the ferrite and the pearlite area ratio and the tensile strength on the surface of the test samples of the examples and comparative examples using the steels A to J satisfying the composition specified in the present invention. From Fig. 2, it can be seen that tensile strength of 600 MPa or more can be secured by setting the total area of ferrite and pearlite to 70% or less, although the strength decreases as the number of ferrite and pearlite increases.

This application is based on Japanese Patent Application No. 2013-259564 filed on December 16, 2013, the contents of which are incorporated herein.

While the present invention has been described in its preferred and fully suf fi cient manner by way of example with reference to the drawings and the like in the foregoing description, it should be appreciated by those skilled in the art that altering and / or improving the above- do. Accordingly, unless the person skilled in the art is of a type or mode of modification that is beyond the scope of the claims as set forth in the claims, the corresponding mode or mode of modification is to be construed as encompassing the scope of the claim.

[Industrial Availability]

The present invention has wide industrial applicability in the technical field of marine forgings. Particularly, it is useful as a large backing member for ships such as rudder stock, rudder plate flange, rudder bolt, shaft bolt, pintle, propeller shaft, intermediate shaft and the like.

Claims (1)

C: 0.13 mass% or more and 0.25 mass% or less,
Si: 0.15 mass% or more and 0.45 mass% or less,
Mn: 0.3 mass% or more and 1.0 mass% or less,
Ni: 1.2% by mass or more and 2.6% by mass or less,
Cr: 0.4 mass% or more and 0.9 mass% or less,
Mo: 0.15 mass% or more and 0.8 mass% or less,
V: 0.05% by mass or more and 0.15% by mass or less,
N: more than 0 mass% and not more than 0.02 mass%
S: 0.002 mass% or more and 0.015 mass% or less,
At least one element selected from the group consisting of Ti and Al: 0.003 mass% or more and 0.05 mass% or less in total,
The balance being Fe and inevitable impurities,
Satisfy the following formula (1)
The metal structure is a composite structure of ferrite-bainite or ferrite-pearlite-bainite, and the total area ratio of the ferrite structure and pearlite structure on the surface is 70% or less
Steel forgings for ships.
14 占 (Ti / 48 + Al / 27)? N (1)

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