JP2005344149A - High strength steel for large-sized steel forging, and crankshaft - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide high strength steel for a large-sized steel forging which is inexpensive compared with 3.5NiCrMo steel proposed as high strength steel for an Ni-Cr-Mo steel forging and further has excellent strength and toughness compared with DIN standard 34CrNiMo6 or the like which has been put to practical use, and to provide an inexpensive large-sized crankshaft having excellent strength and toughness obtained by using the high strength steel for a large-sized steel forging. <P>SOLUTION: The high strength steel for large-sized steel forging contains, by mass, 0.30 to 0.50% C, >0.15 to 0.40% Si, 0.80 to 1.20% Mn, 0.80 to 2.5% Ni, 1.0 to 3.0% Cr, 0.35 to 0.70% Mo and 0.10 to 0.25% V, respectively, and the balance Fe with inevitable impurities. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to high-strength steel and crankshafts for large forged steel products for manufacturing large-sized crankshafts for one-piece and forged steel assemblies, which are used in marine engines and diesel engines used in generators, and the like. The present invention relates to a high-strength steel for large forged steel products having a low Ni content, which is a small alloy element, and having a high strength at a low cost, and a large crankshaft that exhibits these characteristics.

In order to improve the output and compactness of marine diesel engines and power generation diesel engines, it is necessary to increase the strength of steel materials used for these parts. Such parts are generally manufactured by forging steel, but the upper limit of the tensile strength of such forging steel is about 950 N / mm ^{2. In} order to meet the above requirements, 1000 N / There is a demand for forging steel having a strength of mm ² or more.

As a steel for large forgings having a strength of 1000 N / mm ² or more, 3.5NiCrMo steel used in a rotor is known (for example, Non-Patent Document 1). Since this steel material is most excellent in strength and toughness, it is used as a rotor (rotating shaft) for a generator that receives a high load.

  However, the above steel types contain a large amount of expensive Ni as an alloying element for strengthening, and also perform a two-stage tempering process to ensure toughness or a special pre-quenching process for grain size refinement. It is preferable that there is a problem that the cost becomes high.

On the other hand, Cr-Mo steel represented by DIN standard 34CrNiMo6, 32CrMo12, ISO standard 42CrMo4 and the like has been conventionally used as steel for large crankshafts used for driving force transmission in ships and the like. Although these steel types have the advantage that they have a lower Ni content than the 3.5NiCrMo steel and have a relatively low cost, they are more recent in terms of strength and toughness than the above steel types. The fact is that it cannot meet the demand.
“Iron and Steel” vol. 89 (2003) No. 6, Table 2, etc.

  The present invention has been made by paying attention to the above-described circumstances, and its purpose is lower in cost than 3.5NiCrMo steel proposed as steel for high-strength Ni—Cr—Mo forged steel. In addition, it provides high-strength steel for large forgings that has superior strength and toughness compared to DIN standard 34CrNiMo6, etc. that have been put to practical use. Is to provide a large crankshaft excellent in

  The high-strength steel for large forged steel products according to the present invention that has solved the above problems is C: 0.30 to 0.50% (meaning mass%, the same shall apply hereinafter), Si: more than 0.15 to 0.40%, Mn: 0.80 to 1.20%, Ni: 0.80 to 2.5%, Cr: 1.0 to 3.0%, Mo: 0.35 to 0.70%, V : 0.10 to 0.25%, respectively, with the remainder being composed of Fe and inevitable impurities.

  In the high-strength steel for large-sized forged steel products of the present invention, the crystal grain size of the metal structure is preferably 2 to 6 in terms of the particle size number according to ASTM. Moreover, it is preferable that the high-strength steel for large forged steel products of the present invention is obtained by quenching austenitic steel materials to 200 ° C. or less and then tempering them.

  By forging the high-strength steel for large forged steel as described above, a large crankshaft having desired characteristics can be obtained. Such a crankshaft is useful as a crankshaft for a diesel engine used for ships or generators.

  In the high-strength steel for forged steel of the present invention, the Ni content is reduced with respect to the 3.5NiCrMo steel proposed as a high-strength Ni—Cr—Mo forged steel, and the cost is reduced by reducing the Ni content. By containing a predetermined amount of Cr or the like, it is possible to increase the strength, and it is possible to provide low-cost and high-performance steel for forged products. Moreover, this steel for forgings is very excellent in hardenability, and it can be effectively used as a material for large forgings by taking advantage of its excellent hardenability, and is especially used for ships and generators. It is extremely useful as a material for large crankshafts such as those for diesel engines.

  Under the problems as described above, the present inventors have targeted 3.5NiCrMo steel, which is known as a steel for high strength forged steel of Ni-Cr-Mo system, and reduced the amount of Ni contained as an alloy element. Forging steel that has the strength and toughness comparable to these while reducing costs, and that can also exhibit excellent properties in terms of hardenability, which is important when manufacturing high-strength large-forged steel products. We have been diligently working on the development of steel for commercial use.

  As a result, in the Ni-Cr-Mo steel forgings as described above, the amount of Ni as a strengthening element is suppressed as much as possible, and an appropriate amount of elements such as Si, Mn, and Cr is contained. It has been conceived that the present invention has been realized because it has been found that steel for forgings can be obtained that can sufficiently improve strength and toughness to compensate for insufficient strength due to the reduction, and that is extremely excellent in hardenability.

  That is, Ni is an element that is extremely effective for improving the strength and toughness of Cr-Mo steels that are widely used as steels for forgings as described above, and also for improving hardenability. For high-grade Cr-Mo forgings It is a useful element for steel. However, since Ni is an expensive element, excessively increasing its content causes an increase in the cost of steel for forgings, making it impossible to satisfy the customer's price requirements. Therefore, the present invention has been developed with the greatest challenge to ensure strength characteristics and hardenability comparable to conventional Ni-Cr-Mo steel forgings while reducing the amount of Ni as much as possible. In order to achieve the purpose of reducing the cost by reducing the Ni content, it is desirable to keep the Ni content to 2.5% or less.

  The steel for forgings according to the present invention is characterized in that the Ni content is limited as described above, and an appropriate amount of alloy elements such as Si, Mn, and Cr is contained instead. The reasons for limiting the chemical component composition defined in the present invention including the components are as follows.

C: 0.30 to 0.50%
C is an element that enhances hardenability and contributes to improvement in strength. To ensure sufficient strength and hardenability, it is necessary to contain 0.30% or more. However, when the C content is excessive, the toughness is extremely lowered and, in a large ingot, reverse V segregation is promoted.

Si: more than 0.15 to 0.40%
Si acts as a strength-enhancing element and needs to be contained in an amount exceeding 0.15% in order to ensure sufficient strength. However, if the amount is too large, reverse V segregation becomes remarkable and it becomes difficult to obtain a clean steel ingot. Therefore, it is necessary to make it 0.40% or less.

Mn: 0.80 to 1.20%
Mn is also an element that enhances hardenability and contributes to strength improvement, and it is necessary to contain 0.80% or more to ensure sufficient strength and hardenability. However, if the Mn content is excessive, reverse V segregation is promoted, so it is necessary to make it 1.20% or less.

Cr: 1.0-3.0%
Cr is an effective element that enhances hardenability and improves toughness, and in order to exhibit these effects effectively, it is necessary to contain 1.0% or more. However, if contained excessively, reverse V segregation is promoted and it becomes difficult to produce highly clean steel, so it is necessary to make it 3.0% or less.

Mo: 0.35-0.70%
Mo is an element that effectively works to improve all of the hardenability, strength, and toughness, and it is necessary to contain 0.35% or more in order to exhibit these functions effectively. It is also undesirable because it will promote However, when the Mo content is excessive, micro segregation in the steel ingot is promoted, and Mo is a heavy element and weight segregation is likely to occur. Therefore, it is necessary to set the content to 0.70% or less. .

V: 0.10 to 0.25%
V is an element that effectively acts to improve hardenability and strength in a small amount, and in order to exert such effects, it is necessary to contain 0.10% or more. However, since V has a low equilibrium partition coefficient, microsegregation (normal segregation) is likely to occur if it is excessively contained. Therefore, V needs to be 0.25% or less.

  The preferred basic components of the steel for forging used in the present invention are as described above, and the remaining component is substantially Fe, but a small amount of inevitable impurities (for example, P, O, etc.) are present in the steel for forging. , N, Al, etc.) are allowed to be included, and forging steel containing other elements can be used as long as it does not adversely affect the operation of the present invention. It is. Examples of other elements that are allowed to be positively added include Ti, Ca, Mg, S, etc., but in terms of suppressing the formation of coarse inclusions, the total should be suppressed to about 0.5% or less. desirable.

  By the way, it is well known that the toughness of a steel material is improved by reducing the crystal grain size. In order to refine the crystal grain size, an austenitizing treatment is necessary as a precondition. However, in the austenitizing treatment of a large forged steel product, a long holding time is required for austenite transformation to the inside. For this reason, the larger the size, the more easily the difference in particle size occurs inside, and there are cases where special treatments such as multi-stage austenitization of two or more stages are performed in order to refine the particle size.

  On the other hand, it is also known that it is useful to contain Ni as a means for improving the toughness of the steel material. However, when the Ni content is too high, the crystal grains tend to become coarse and austenite during quenching. Tends to remain. There is a case where a large amount of retained austenite is present during quenching, martensitic transformation occurs during subsequent tempering, and a hard and brittle structure is mixed, resulting in deterioration of toughness. For these reasons, when severe toughness is required, the martensite generated during tempering needs to be tempered again, and a two-stage tempering process may be performed.

  From these viewpoints, the present inventors can ensure sufficient toughness even if the crystal grains are coarse, and suppress the retained austenite during quenching, and do not require special treatment such as two-stage quenching to ensure toughness. The study was repeated with the aim of developing a steel material that would As a result, in the high strength steel for forgings having the chemical composition as described above, sufficient toughness is ensured even if the crystal grain size of the metal structure is No. 6 or less in ASTM, and a single tempering treatment is sufficient. It became clear that. However, when the grain size number of ASTM is less than 2, coarsening of the crystal grains becomes remarkable and the toughness of the forged steel product is deteriorated. Therefore, it is preferable that the crystal grain size is at least 2 or more.

  A crankshaft for a diesel engine used for a ship or a generator has a journal diameter of about 150 mm, which is very large compared to a crankshaft for an automobile (for example, about 15 mm). Further, in large forged steel products, there is a risk of cracking when water quenching is performed. Therefore, quenching of a large crankshaft is generally performed by oil cooling, polymer quenching, air cooling, or the like. In the large crankshaft targeted by the present invention, the cooling rate at the time of quenching varies depending on the diameter of the crankshaft, but in the case of oil quenching, it is 50 ° C./min or less, and about 20 ° C. in the 500 mm diameter class. When the diameter becomes larger (for example, 1000 mm), the cooling rate becomes even smaller.

  In order to achieve both strength and toughness in high-strength steel for large forged steel products, it is preferable to control to a structure composed of bainite and martensite, and a quenching cooling rate of 20 ° C. for application to a large crankshaft having a diameter of 150 mm or more. As a result of studying the conditions for realizing such a structure even in the case of oil quenching (in the case of oil quenching), the chemical component composition as described above was reached.

  In the case of a larger crankshaft, the desired quenching cooling rate (20 ° C./min) may not be achieved by oil quenching. In such a case, it is recommended to perform the above “polymer quenching”. In this polymer quenching, an organic solvent such as glycols (for example, diethylene glycol, polyethylene glycol, etc.) dissolved in water is used as a cooling medium for cooling. In such polymer quenching, rather than oil quenching. A high cooling rate can be realized. Therefore, polymer quenching is useful cooling to achieve a large cooling rate and a cooling rate greater than 20 ° C./min for a relatively large large steel product while preventing cracking due to water quenching. It is a method.

  Further, in quenching, from the viewpoint of complete transformation, it is preferable that the austenitic steel is quenched to 200 ° C. or lower and then tempered. When this quenching temperature (that is, tempering start temperature) exceeds 200 ° C., untransformed retained austenite remains, which causes variation in characteristics.

  The method for producing steel for forgings 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 adopted in the case of large-sized forging steel, but in the case of relatively small forged steel products, it is also possible to adopt a continuous casting method.

  Also, for example, a method of manufacturing a crankshaft using steel for forged products 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 step of hardening and then the step of finishing machining, etc. may be carried out 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. . The crankshaft thus obtained is useful as a crankshaft for a diesel engine used for ships or generators.

  Hereinafter, the effects of the present invention will be described more specifically by way of examples. However, the following examples are not intended to limit the present invention, and should be implemented with appropriate modifications within a range that can meet the purpose described above and below. Are all possible and are within the scope of the present invention.

Example 1
Steel for forgings having the chemical composition shown in Table 1 below was melted using a high frequency furnace, and a steel ingot (40 kg) having a diameter of 158 to 132 × length of 323 mm was cast. 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 × 600 mm, it was allowed to cool in the atmosphere.

  After the temperature of the material reached room temperature, a quenching process was performed that simulated a heating / cooling rate corresponding to a position 50 mm (1/10 of the diameter) below the surface of the crankshaft having a diameter of 500 mm. Specifically, using a small simulated furnace, the temperature is increased to 870 ° C. at a temperature increase rate of 40 ° C./hour, held at that temperature for 1 hour, and then the average cooling rate in the temperature range of 870 to 500 ° C. is 20 ° C. The austenitizing treatment was performed at a cooling rate of 1 min. Moreover, as a tempering process, after holding at 580-630 degreeC for 13 hours and furnace-cooling, the mechanical property of each forged steel was measured by the following method. The crystal size in the metal structure was determined based on ASTM (crystal grain size number).

[Measuring method of mechanical properties]
(A) The tensile test was implemented about each steel material based on JISZ2241. At this time, the shape of the test piece was a JIS Z2201 No. 14 test piece of φ6 × G. The mechanical properties [yield stress (YS), tensile strength (TS), elongation (EL), drawing (RA)] were measured.

(B) In addition, a Charpy impact test was performed on the steel types having a tensile strength TS of 1000 N / mm ² or more. The Charpy impact test was performed based on JIS Z2242, and the absorbed energy vE was measured. The test piece shape at this time was a 2 mmV notch of JIS Z2202. In order to evaluate the toughness at the same strength, determined absorbed energy vEat (1000N / mm ²⁾ at 1000 N / mm ² by the calculation method described below was evaluated for toughness.

The absorbed energy at a tempering temperature of about 1000 N / mm ² is Eu and Ed, and the tensile strengths are Su and Sd.
Eu: Absorbed energy at a tempering temperature (Td) at which a strength of 1000 N / mm ² or more is obtained Su: Tensile strength Ed: Absorbed energy at a tempering temperature (Td) at which the strength is 1000 N / mm ² or less Sd: Tensile strength vEat (1000 N / mm ² )
= Eu + (Ed-Eu) / (Su-Sd) * (Su-1000)
These results are shown in the following Table 2 together with the quenching temperature and the tempering temperature, and can be considered as follows from these results. First, test no. In the cases of 1 to 3, 13, and 14, any of the chemical components defined in the present invention is out of the specified range, and none of the tensile strength TS has reached 1000 N / mm ² (strength not achieved). I understand that.

On the other hand, the test N.1 satisfying the range specified in the present invention. It can be seen that the tensile strength of 1000 N / mm ² or more was obtained with the materials of 4 to 12. Moreover, in these things, it was confirmed that the Charpy absorbed energy vE tends to depend on the Ni content.

  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 taken with a 100 × optical microscope, and the area of the region classified as ferrite and pearlite from the photograph. When the metal structure was examined by determining the ratio, the area fraction of ferrite and pearlite was substantially zero, and it was confirmed that the structure was composed of bainite and martensite.

Based on this result, the relationship between the value converted into the absorbed energy at 1000 N / mm ² and the Ni content is shown in FIG. The relationship between the Ni content and the crystal grain size number is shown in FIG. As can be seen from FIG. 1, good impact characteristics are obtained when the Ni content is 0.80% to 2.5%. In general, it is known that the Ni content increases and the toughness is improved, and it is considered that the effect of Ni is remarkably exhibited when the Ni content is 0.8% or more. On the other hand, when the Ni content is more than 2.5%, it can be seen that the toughness is decreased. As shown in FIG. 2, the crystal grain size was increased as the Ni content was increased, and it was considered that the toughness was lowered by such coarsening of the crystal grains. As is apparent from FIGS. 1 and 2, it can be seen that good toughness can be secured if the crystal grain size number is 2 or more.

Example 2
For large forged steel products, from the viewpoint of preventing cracking due to deformation capability and thermal stress in the quenching and tempering process, it is common to continuously temper after cooling to 200-300 ° C without lowering to room temperature. It is.

  From this point of view, the test Nos. 1 and 9 (steel types A and I in Table 1), tempering start temperature (quenching end temperature) is mechanical properties [tensile strength (TS), 0.2% proof stress (0.2 PS), elongation (EL ), Aperture (RA), absorbed energy (vE)] were investigated (other conditions are the same as in the example). The results are shown in Table 3 below. Based on this result, the relationship between the tempering start temperature and the tensile strength TS is shown in FIG. 3, and the relationship between the tempering start temperature and the impact value (absorbed energy) is shown in FIG.

  From these results, it can be considered as follows. Test No. In the case of No. 1, the effect of the tempering start temperature is not so much observed, but the test No. In the case of No. 9, the influence of the tempering start temperature is clearly recognized. From these results, it was confirmed that by setting the tempering start temperature (quenching end temperature) to 200 ° C. or less, the strength can be stably secured without deteriorating toughness.

  Further, in the chemical component defined in the present invention, the factor that the tensile strength is affected by the tempering start temperature is not clear, but in the component range defined in the present invention, a complete bainite structure is not achieved, and partly martensite. It is considered that the tempering start temperature affects the formation of such a transformed structure.

It is a graph which shows the relationship between the value converted into the absorbed energy in 1000 N / mm < ² >, and Ni content. It is a graph which shows the relationship between Ni content and a crystal grain size number. It is a graph which shows the relationship between tempering start temperature and tensile strength TS. It is a graph which shows the relationship between tempering start temperature and an impact value (absorption energy).

Claims (5)

  High strength steel for large forged steel products, C: 0.30 to 0.50% (meaning mass%, the same shall apply hereinafter), Si: more than 0.15 to 0.40%, Mn: 0.80 1.20%, Ni: 0.80 to 2.5%, Cr: 1.0 to 3.0%, Mo: 0.35 to 0.70%, V: 0.10 to 0.25%, respectively A high-strength steel for large-sized forged steel, characterized in that the balance is made of Fe and inevitable impurities.   The high-strength steel for large forged steel products according to claim 1, wherein the crystal grain size of the metal structure is 2 to 6 in terms of the particle size number according to ASTM.   The high-strength steel for large-sized forged steel products according to claim 1 or 2, wherein the austenitized steel is quenched to 200 ° C or lower and then tempered.   A large-sized crankshaft obtained by forging the high-strength steel for large-sized forged steel products according to any one of claims 1 to 3. The crankshaft according to claim 4 which is a crankshaft for a diesel engine used for ships or generators.

Images