JP3896365B2 - High strength forging steel and large crankshaft using the same - Google Patents

Description

  The present invention relates to a high-strength forging steel, and more particularly, to a high-strength forging steel for manufacturing a large crankshaft that has a high cost and high strength at a low cost, especially with an expensive alloy element, particularly Ni content. Forging steel can be effectively used as a material for a large crankshaft used as a transmission member for a marine drive source, for example. Therefore, the present invention includes a crankshaft made of such low-cost, high-strength forging steel.

  Conventionally, steel for large crankshafts used for transmitting driving force of ships and the like has been Cr-Mo steel represented by ISO standard 36CrNiMo6, DIN standard 32CrMo12, and ISO standard 42CrMo4. Of these, 42CrMo4 of ISO standard is a steel type that has a relatively high C content and a relatively low Cr and Mo content, and is relatively inferior in strength but relatively low in cost. The ISO standard 36CrNiMo6, which contains a large amount of Ni as an alloying element, is the most excellent in strength and toughness. It is used for The DIN standard 32CrMo12 has intermediate cost and strength characteristics with respect to the above two types of Cr-Mo steels, and has excellent performance especially for crankshafts where toughness is important. .

  However, recently, in order to prevent marine accidents and the like, a high Ni content ISO standard 36CrNiMo6 having a high level of strength characteristics is widely used for large-sized crankshafts of ships.

  However, the ISO standard 36CrNiMo6, which is a Ni-Cr-Mo steel, is excellent in strength and toughness, but contains a large amount of expensive Ni as an alloying element for strengthening. Compared to Cr-Mo steel, it is expensive, and is a major obstacle in promoting generalization.

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is low-cost compared to ISO standard 36CrNiMo6 and the like which are put into practical use as high-strength Ni—Cr—Mo forging steel. In addition, it has strength and toughness comparable to this, and furthermore, in terms of “hardenability”, which has been increasingly demanded by customers in recent years, it is comparable to this high-strength Ni—Cr—Mo forging steel, or this An object of the present invention is to provide a forging steel having a performance exceeding that of the above, or to provide a large-sized crankshaft using the forging steel at low cost and excellent in strength, toughness and hardenability.

The high-strength forging steel according to the present invention that was able to solve the above problems is a high-strength forging steel used for manufacturing a large crankshaft,
C: 0.36 to 0.45% (meaning mass%, the same shall apply hereinafter),
Si: 0.15 to 0.4%,
Mn: 0.8 to 1.2%
Cr: 1.5 to 2.5%,
Mo: 0.15-0.35%,
V: 0.035 to 0.17%
And at least one element selected from the group consisting of V, Nb, and Ta: 0.035 to 0 in total .35%, N: 30 to 250 ppm, the balance being Fe and inevitable impurities, further satisfying the relationship of the following formula (1), and having a gist in that it is composed mainly of bainite and martensite. .
[Total of V, Nb, Ta (mass%)] + 0.001 × Solubility N (ppm) ≧ 0.068 …… (1)

  The steel for high-strength forging according to the present invention actively uses at least one element selected from the group consisting of V, Nb and Ta and nitrogen (N) in the steel component system in which the chemical component composition is limited as described above. In addition, the relationship between the total of these (V, Nb, Ta) contents and the solid solution N is specified so as to satisfy the above formula (1), and the steel is composed of bainite and martensite-based metal structure. Thus, a forging steel having a high level of tensile strength and toughness, a very good hardenability, and a relatively low price can be provided.

  The forging steel having the above basic composition may contain a small amount of Al as a deoxidizing element, and the effect of such Al is effectively exhibited by containing 0.001% or more. Since it is saturated at 0.040%, it should be kept below that. In addition, S is often mixed as a harmful element, and it is also desirable to suppress S to 0.006% or less in order to avoid adverse effects on toughness and fatigue strength as forging steel.

  Furthermore, the forging steel of the present invention preferably has a DI value (critical diameter at which the central part is 50% martensite hardness by water cooling) of 30 mm or more in order to make use of its excellent hardenability. The forging steel of the present invention exhibits extremely excellent performance as a material for large crankshafts and the like for ships.

  In the present invention, for Ni-Cr-Ni-Mo based forging steel containing a relatively large amount of Ni, the Ni content is suppressed to reduce costs, and a very small amount of V, Nb or Ta and N is added. By containing a predetermined amount, the strength can be increased, and a high-performance forging steel can be provided at a low cost. Moreover, this forging steel is very excellent in hardenability, and can be effectively used as a material for large forging products by taking advantage of its excellent hardenability, especially for large crankshafts for ships. It is extremely useful as a material for.

  Under the above-mentioned problems, the present inventors have targeted the “ISO standard 36CrNiMo6”, which is known as a high strength forging steel of the Ni—Cr—Mo system, and the amount of Ni contained as an alloy element. While reducing costs and reducing costs, it has comparable strength and toughness, and even exhibits excellent properties for hardenability, which is important when manufacturing large-sized forged products with high strength. We have been working hard to develop forging steels.

  As a result, in the steel forging as described above in which the amount of Ni as a strengthening element is suppressed to 0.7% or less in the Cr—Mo based forging steel as described above, at least the element selected from the group consisting of V, Nb, and Ta One containing N and a very small amount of N, and those containing these (total of V, Nb, Ta) and solid solution N so as to satisfy the relationship of the above formula (1) are insufficient in strength due to Ni content reduction. As a result, the inventors have found that a forging steel can be obtained that has an excessive improvement in strength and toughness and is also excellent in hardenability.

  In other words, Ni is an element that is extremely effective for improving the strength and toughness of Cr-Mo steels that are widely used as forging steels as described above, and for improving hardenability. Is a useful element. However, since Ni is an expensive element, excessively increasing its content causes an increase in the cost of forging steel and cannot satisfy the price requirements of consumers. Therefore, the present invention has been developed with the greatest challenge of ensuring strength characteristics and hardenability comparable to conventional Ni-Cr-Mo forging steels while reducing the amount of Ni as much as possible. In order to achieve the purpose of reducing the cost by reducing the amount of Ni, it is desired that the Ni content is at most 0.7%, preferably 0.5% or less, more preferably 0.3% or less. .

  However, the reduction of the Ni amount means that it is difficult to obtain the effect of improving the strength, toughness, and hardenability due to Ni, and it becomes impossible to satisfy the demands of consumers from the performance aspect. Therefore, in the present invention, as a result of earnest research for the purpose of developing a forging steel that makes up for the performance deficiency due to the reduction of the Ni amount by other elements and meets the requirements in terms of both cost and performance. In a Cr—Mo based forging steel with a low content, at least one element selected from the group consisting of V, Nb, and Ta, and N, which is a trace element conventionally recognized as a harmful element, are included. Forging steel that satisfies both the cost and performance if it is contained and the above-mentioned (V, Nb, Ta) and solid solution N out of N are contained so as to satisfy the relationship of the formula (1). It was confirmed that

  In order to effectively exhibit the effects of such V, Nb, Ta and N, or even solute N on a practical scale, the forging steel in which the Ni content is limited to 0.7% or less as described above, , Nb, Ta, and at least one element selected from the group consisting of 0.035% or more, more preferably 0.045% or more, and 0.35% or less, more preferably 0.15% or less. , N is contained in a range of 30 ppm or more, more preferably 40 ppm or more and 250 ppm or less, more preferably 100 ppm or less. Further, the above (total of V, Nb, Ta) and solid solution N are represented by the formula (1). It is an essential requirement to contain them so as to satisfy the relationship.

  By the way, when the content of V, Nb, Ta and the amount of N, and the requirement of the above formula (1) are not satisfied, not only the intended strength and toughness of the present invention can be obtained, but also the hardenability. I can't get a satisfactory one.

  The strength and toughness of the low Ni-based forging steel are remarkably improved by containing the total of V, Nb, Ta and N, and further including the solid solution N so as to satisfy the requirement of the above formula (1). Although the reason has not yet been clarified in spite of the pursuit of the present inventors, V, Nb, and Ta form nitrides with N, and solid solution N itself has an excellent strength improvement effect. It is thought that it demonstrates. In addition, these strength improving elements such as V, Nb, and Ta are very expensive elements and cause a considerable increase in cost even though the amount is small, but in the present invention, inexpensive N is used together with the addition of a small amount of these elements. By increasing the strength by increasing the amount of solute N, it is possible to reduce the economic burden due to the addition of a large amount of V or the like.

  And the strength improvement effect by such V, N, and also solid solution N is effectively exhibited with respect to the Cr-Mo type forging steel in which the Ni content is suppressed to 0.7% or less. In the case of high-grade forging steel exceeding 0.7%, it appears as a very unique phenomenon that it is hardly exhibited.

  Therefore, the forging steel of the present invention is intended for Cr-Mo based forging steel in which the Ni content is suppressed to 0.7% or less, and the sum of V, Nb, Ta, N (total nitrogen), and solute N are included. It is positioned as having strength, toughness and hardenability comparable to high-grade Cr-Mo forging steel, which is relatively inexpensive and has a high Ni content, by containing it at a specific content ratio so as to satisfy the above requirements. It is done.

The forging steel of the present invention is characterized in that the Ni content is limited as described above, and instead, at least one of V, Nb, Ta and N, or further, these and a solid solution N are contained in a specific amount. In order to make more effective use of the strength and toughness required for large crankshafts and the hardenability characteristic of the present invention, a Cr-Mo based forging steel having the following basic composition is selected. It is necessary to.
C: 0.36 to 0.45% (preferably 0.38 to 0.45%)
Si: 0.15-0.4% (preferably 0.20-0.4%)
Mn: 0.8 to 1.2% (preferably 0.9 to 1.2%)
Cr: 1.5 to 2.5% (preferably 1.75 to 2.5%)
Mo: 0.15 to 0.35% (preferably 0.20 to 0.35%)
The reasons for determining the content of each element are as follows.

C: 0.36-0.45%
C is an element that enhances hardenability and contributes to improvement in strength. To ensure sufficient strength and hardenability, C must be contained in an amount of 0.36% or more. C is preferably contained in an amount of 0.38% or more, but if it is too much, it deteriorates toughness and promotes reverse V segregation, so it is necessary to make it 0.45% or less, preferably 0.42% or less. It is good to keep it down.

Si: 0.15-0.4%
Si acts as a strength improving element, and it is necessary to contain 0.15% or more in order to ensure sufficient strength. Si is preferably contained in an amount of 0.20% or more, but if it is too much, reverse V segregation becomes remarkable and it becomes difficult to obtain a clean steel ingot. Therefore, it is necessary to make it 0.4% or less, preferably It is better to keep it below 0.3%.

Mn: 0.8 to 1.2%
Mn is an element that improves hardenability and contributes to strength improvement. To ensure sufficient strength and hardenability, it is necessary to contain 0.8% or more, preferably 0.9% or more is desirable. . However, if it is too much, reverse V segregation is promoted, so it is necessary to make it 1.2% or less, preferably 1.1% or less.

Cr: 1.5-2.5%
Cr is an effective element that enhances hardenability and improves toughness. Their action is effectively exhibited by containing 1.5% or more, preferably 1.75% or more. However, if it is too much, reverse V segregation is promoted and it becomes difficult to produce highly clean steel.

Mo: 0.15-0.35%
Mo is an element that effectively works to improve all of the hardenability, strength, and toughness. It is necessary to contain 0.15% or more, and preferably 0.20% or more in order to exert these functions effectively. It is desirable to contain. However, since Mo has a small equilibrium distribution coefficient and easily causes microsegregation (normal segregation), it should be suppressed to 0.35% or less, preferably 0.30% or less.

  The preferred basic components of the forging steel used in the present invention are as described above, and the remaining component is substantially Fe. However, the forging steel is allowed to contain a trace amount of inevitable impurities. Of course, it is also possible to use a forging steel that further contains other elements within a range that does not adversely affect the operation of the present invention. Examples of other elements that are allowed to be positively added include B having a hardenability improving effect, Ti having a deoxidizing effect, Ca, Mg, Ce, Zr, Te having a MnS form controlling action, etc. Can be added singly or in combination of two or more, but it is desirable to suppress them to about 0.03% or less in total.

  In addition, Al inevitably mixed in the forging steel with the above basic composition is an element contained as a deoxidizing element for reducing the oxygen content in the steel, so that the deoxidizing action can be effectively expressed. Is preferably contained in an amount of about 0.001% or more. However, Al mainly fixes N in the form of AlN and not only makes it difficult to express the strengthening effect due to the blending of N and V, but also bonds with many other elements to form non-metallic inclusions and intermetallic compounds. Is adversely affected to toughness, so it is desirable to keep it to 0.04% or less. In other words, from the viewpoint of the strengthening mechanism by V and N and N, the point is how to prevent the generation of AlN. Specifically, when the Al content is very low, about 0.001%. Even if the N content is about 30 ppm, the effect of improving the strength is produced because the amount of AlN generated is small. However, when the Al content is about 0.03%, the amount of AlN generated increases, so that the strengthening effect is effective. In order to achieve expression, it is necessary to increase the N content to about 250 ppm or increase the V content.

  In addition, S is a harmful component contained as sulfide derived from coke, which is a raw material for iron making, and in particular, forms sulfides such as MnS in steel and causes fatigue characteristics to deteriorate. Therefore, in order to prevent such a failure, it is desirable to suppress the S content to 0.006% or less, more preferably 0.005% or less.

  Furthermore, the forging steel of the present invention has an important feature in that it has a metal structure mainly composed of bainite and martensite in addition to the above-described component composition. For example, the area fraction of ferrite and pearlite is 10%. If it exceeds, the strength at the level intended by the present invention cannot be secured. Here, the main body of bainite and martensite means that most of the cross-sectional structure that can be confirmed by an optical microscope or the like is bainite and martensite structure, and the area ratio of ferrite or pearlite is about 10% or less. . Incidentally, at present, a method for quantitatively evaluating the area ratio of bainite and martensite has not been established, but even in the present situation, it can be confirmed empirically that the metal structure is mainly composed of bainite and martensite.

  And the structure mainly composed of bainite and martensite uses a steel material satisfying the above-described chemical components, and cools the temperature range of 870 to 500 ° C. at the time of quenching at an average cooling rate of about 0.5 to 30 ° C./min. Can be obtained.

  The chemical components of the forging steel according to the present invention are as described above, and despite being a steel type in which the amount of Ni is suppressed to 0.7% or less, at least one of a small amount of V, Nb or Ta and N are contained. By containing or further defining the relationship of the amount of solute N, it is possible to obtain a forging steel that is inexpensive and exhibits high strength characteristics without causing an increase in cost due to the additive alloy component. In addition, this forging steel is very excellent in hardenability, and the DI value that can be confirmed by the Jominy test method as will be clarified in the examples described later (water-cooled and the central part has a martensite hardness of 50%. The critical diameter is 30 mm or more, and the hardenability is 32 mm or more. Therefore, it is extremely effective as a material for crankshafts that are strengthened by quenching after forging, especially for large crankshafts for ships. Can be used.

  That is, the forging steel of the present invention having an increased DI value has excellent hardenability, and a forged product that requires high strength on both the surface side and the core side after quenching treatment, such as a crankshaft. For example, it is extremely useful as a raw material for obtaining a large forged product having a large mass effect such as a diameter of 150 to 1000 mm.

  The method for producing the forging steel according to the present invention is not particularly limited, and may be cast after adjusting to a predetermined chemical component using a high-frequency melting furnace, electric furnace, converter or the like according to a conventional method. It is also effective to perform a vacuum treatment after adjusting the components. As for casting, ingot casting is mainly used in the case of large-sized forging steel, but in the case of relatively small forging, it is also possible to adopt a continuous casting method. In the present invention, it is necessary to strictly control the N content in the forging steel, and preferable means for that purpose include adding a nitrogen-containing metal such as manganese nitride or chromium nitride or blowing nitrogen gas into the N-containing metal. When the content is increased and the N content is decreased, a method of appropriately controlling the conditions by performing vacuum degassing treatment may be employed.

  Also, for example, a method of manufacturing a crankshaft or the like using the forging steel is not particularly limited. For example, a process of melting steel having a predetermined component composition in an electric furnace or the like → vacuum refinement or the like such as S or the like The process of removing gas components such as → The process of ingot making → The process of forging the material after heating the steel ingot → The process of heating after intermediate inspection and forging into the crankshaft shape → Homogenizing and quenching by heat treatment Then, the process of hardening and the process of finishing machining may be performed sequentially.

  The forging method for the crankshaft includes a free forging method (a method in which a crank arm and a crankpin are forged as a single block and finished into a crankshaft shape by gas cutting and machining), and R.I. R. And T. R. Forging method (forging so that the axis of the steel ingot is the center of the crankshaft, and forging so that the parts that tend to deteriorate characteristics due to center segregation become all the axes of the crankshaft In particular, if the latter forging method is employed, the shaft surface layer side can be occupied by a portion with a high degree of cleanliness, and a crankshaft excellent in strength and fatigue characteristics is easily obtained, which is preferable. .

  However, the forging steel of the present invention takes advantage of its excellent strength characteristics and low cost. Besides the crankshaft, the forging shaft, propulsion shaft, assembled crankshaft throw, hollow material Thus, it can be effectively used as a material for forging a high-strength product that is not welded.

  Next, the present invention will be described in more detail with reference to experimental examples. However, the present invention is not limited by the following examples, but is appropriately modified within a range that can meet the purpose described above and below. It is also possible to carry out and they are all included in the technical scope of the present invention.

Experimental Example Forging steels having the components shown in Tables 1 and 2 were melted and cast using a high frequency furnace to produce a steel ingot (50 kg) having a diameter of 158 to 132 × length of 323 mm. At this time, the amount of manganese in the steel is adjusted by adjusting the amount of manganese nitride added and the atmosphere gas, and a predetermined amount of V, Nb, Ta is added to the steel by externally adding a predetermined amount of V, Nb or Ta. Yield Ta.

  After cutting the hot metal part of each steel ingot obtained and heating it at 1230 ° C. for 5 to 10 hours, it was compressed to 1/2 in height ratio using a free forging press machine, and the steel ingot center line was 90 ° After rotating and forging to 90 mm × 90 mm × 450 mm, it was allowed to cool in the atmosphere. Each material that was allowed to cool to room temperature was then subjected to austenitization using a small simulated furnace. In the austenitizing treatment, each material was heated to 870 ° C. at a heating rate of 40 ° C./hr and held for 1 hour, and then the temperature range of 870 to 500 ° C. was cooled at an average cooling rate of 20 ° C./min. As a treatment, a method of holding at 610 ° C. for 13 hours and then cooling the furnace was adopted.

  The amount of N in the test material indicates the total amount of solute N and compound type N, and an inert gas dissolution method was adopted for the analysis. As shown in FIG. 5, the amount of solute N is determined by the above-mentioned method after measuring precipitates in steel by electrolytic extraction and then measuring the amount of N such as a compound by adopting indophenol absorptiometry. It was obtained by subtracting from the total N amount.

  In addition, after the austenitizing treatment and tempering treatment, the cross section of each test material was etched with nital, and then two or more fields of view were photographed with a 100 × optical microscope. The metal structure was examined by determining the rate. As a result, the area fraction of ferrite and pearlite was zero in any of the examples and comparative examples, and it was confirmed that the structure was mainly composed of bainite and martensite.

  Further, the obtained steel materials were examined for mechanical properties and hardenability by the following methods, and the results shown in Tables 3 and 4 and FIGS.

[Mechanical property evaluation method: room temperature]
The tensile test was performed according to ISO 6892. The test piece shape was L ₀ = 5.65√S ₀ . The Charpy impact test was performed in accordance with ISO 148, and the test piece shape employed a 2 mmV notch described in ISO 148.

[Hardenability evaluation method]
The test was performed according to the Jominy test method described in ISO 642. The test piece shape was a flanged test piece, and the critical diameter (DI) at which the central part was 50% martensite by water cooling was determined. The heating temperature was 870 ° C.

  1 and 2 are graphs showing the relationship between the Ni content and the tensile strength and the relationship between the N amount and the tensile strength from the data shown in Tables 1 to 4, and FIG. V, Nb, Ta) and the effect of improving the strength by adding N is effectively exhibited for a steel type having a Ni content of 0.7% or less. From FIG. It can be confirmed that the amount is 30 ppm or more, more preferably 40 ppm or more, and more certainly 50 ppm or more. Further, such N-containing effects reach a nearly saturated state when the N amount is about 60 to 70 ppm. Therefore, containing more than that, especially containing more than 100 ppm is almost meaningless for the purpose of improving the strength. Rather, since there is a concern about toughness deterioration due to an increase in nitride, it can be seen that it is desirable to suppress the N content to 100 ppm or less, more preferably 80 ppm or less.

  FIG. 3 shows a comparison of the DI values of the example material and the comparative material in the forging steel obtained in the above experimental example. The example material also has a higher DI value than the comparative material and is hardenable. It turns out that it is excellent.

  Further, FIG. 4 is a graph showing the effects of the V content and the solute N amount on the tensile strength, and the relationship between the V amount and the solute N amount satisfies the relationship of the above formula (1). It can be seen that it exhibits a high level of strength characteristics. As is clear from this figure, when 0.068% or more can be secured in (total of V, Nb, Ta), sufficient strength characteristics are secured even when the amount of dissolved N is substantially zero. I understand that I can do it.

It is a graph which shows the relationship between Ni content and tensile strength of the forging steel used in experiment. It is a graph which shows the relationship between N content of the forging steel used in experiment, and tensile strength. It is a graph which compares and shows an Example material and a comparison material DI value. It is a graph which arranges and shows the influence which V content in steel and the amount of solute N have on tensile strength. It is a figure which shows the analysis procedure of the solid solution N.

Claims (7)

A high-strength forging steel used in the manufacture of large crankshafts,
C: 0.36 to 0.45% (meaning mass%, the same shall apply hereinafter),
Si: 0.15 to 0.4%,
Mn: 0.8 to 1.2%
Cr: 1.5 to 2.5%,
Mo: 0.15-0.35%,
V: 0.035 to 0.17%
Together with each containing a Ni content of 0.7% or less (not including 0%), at least one element and V, Nb, is selected from the group consisting of Ta: a total 0.035 to 0 .35%, N: 30 to 250 ppm, the balance being Fe and inevitable impurities, further satisfying the relationship of the following formula (1), and consisting of a structure mainly composed of bainite and martensite Steel.
[Total of V, Nb, Ta (mass%)] + 0.001 × Solubility N (ppm) ≧ 0.068 …… (1)   The steel for high-strength forging according to claim 1, wherein the steel contains Al: 0.001 to 0.040% as another component.   The steel for high-strength forging according to claim 1 or 2, wherein the S content of the steel is 0.006% or less.   The steel for high-strength forging according to any one of claims 1 to 3, wherein a DI value (a critical diameter at which a central portion becomes 50% martensite by water cooling) is 30 mm or more.   The high-strength forging steel according to any one of claims 1 to 4, which is used for manufacturing a large crankshaft for a ship.   A large crankshaft obtained by forging the steel according to any one of claims 1 to 5.   The crankshaft according to claim 6, which is a large crankshaft for a ship.

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