KR20060126375A - Forging steel excellent in hydrogen crack resistance and crank shaft - Google Patents

Abstract

A forging steel excellent in hydrogen crack resistance is provided to effectively reduce hydrogen crack sensitivity of such forging steel by noticing that high clean steel in which light weight and fatigue strength required in a large steel forging such as a crank shaft are further improved highly efficiently damages hydrogen crack resistance, and a crank shaft manufactured by forging the forging steel is provided. A forging steel excellent in hydrogen crack resistance contains, by weight percent, 0.2 to 0.6% of C, 0.1 to 0.4% of Si, 0.7 to 1.5% of Mn, 1.0% or less of Ni, 1.2 to 3.5% of Cr, 0.1 to 0.6% of Mo, 0.01 to 0.05% of Al, and 0.005% or more of at least one of Ti, Zr, Hf and Nb, wherein an average value(an average circularity) of circularities of inclusions that are contained in steel and have the maximum subtense length of 1 mum or more is 0.5 or more, the number of inclusions having the maximum subtense length of 20 mum or more is less than 40 per 100 mm^2, and the number of inclusions having an average circularity of 0.25 or more and the maximum subtense length of 1 to 10 mum or more is 100 or more per 100 mm^2.

Description

FORGING STEEL EXCELLENT IN HYDROGEN CRACK RESISTANCE AND CRANK SHAFT}

1 is a schematic diagram of a hydrogen cracking susceptibility test apparatus for forging steel, (1) is a test piece, and (2) is an environment of H ₂ SO ₄ + KSCN aqueous solution.

2 is an electron micrograph of inclusions in steel.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to forged steel used in large crankshafts for power transmission such as ships and generators, and more particularly relates to high-clean steel having excellent fatigue strength and hydrogen cracking resistance.

As forging steels used in large crankshafts for power transmission of ships and generators, etc., so-called Cr-Mo steels conventionally represented by 36CrNiMo6 of ISO standard, 32CrMO12 of DIN standard or 42CrMo4 of ISO standard have been used.

Since the crankshaft is exposed to severe cyclic stress during its use, even if it is manufactured using these Cr-Mo steels, high-clean steels with very few inclusions such as MnS, which are the starting point of fatigue failure, are required. On the other hand, the crankshaft is natural, but in the future, it is increasingly lighter and lighter, increasing the demand for high-performance quality, that is, higher fatigue strength.

By the way, in the case of Cr-Mo steel, hydrogen cracking tends to occur as a result of purifying high cleanliness and high fatigue, so that countermeasures are considered from both the material surface of the steel and the refining technology. For example, steel having good hydrogen crack resistance is manufactured by regulating the upper limit of the amount of hydrogen during refining of molten steel and dehydrogenating when exceeding it.

Alternatively, Japanese Patent Laid-Open No. 2003-268438 discloses a conventional method of increasing the S content in steel for the purpose of improving the hydrogen cracking resistance of large forged steel products by increasing the MnS inclusions, reducing the cleanliness of the steel and suppressing fatigue strength. I want to improve it. The patent proposes a method of improving the hydrogen cracking resistance by adjusting the amount of S in the molten steel in terms of the amount of hydrogen without excessive deficiency. Japanese Laid-Open Patent Publication No. 2003-183722 discloses that a conventional method for improving the cleanliness of crank steel is to improve the reduction of inclusions by secondary refining, and to remove the condensation during RH vacuum degassing. We propose a method for removing molten steel by refluxing molten steel between degassing tanks.

As described above, the dehydrogenation treatment has a limitation in reducing the amount of hydrogen in terms of time and treatment cost, and is generally manufactured and managed at a level of 1 to several ppm, but since hydrogen cracks are generated by trace amounts of hydrogen, Quality control does not completely prevent hydrogen cracking. In addition, the method of forming sulfides while adjusting the above-mentioned S concentration causes rather deterioration in cleanliness, mechanical properties, and fatigue strength of the product steel, and it is not easy to expect further high cleanness and fatigue fatigue strength.

In addition, the following patent document discloses an improvement of this type of forging steel. Japanese Patent Application Laid-Open No. 1995-150289 discloses that in order to improve the high temperature strength of Cr-based heat resistant steel, the particle diameters and shapes of oxides such as Ta and Nb in the steel are adjusted and dispersed. Further, Japanese Patent Application Laid-Open No. 1998-194946 is a method of improving the high temperature creep strength and low temperature toughness of CrMoV-based heat resistant steels, thereby improving the heat treatment of the product. In addition, Japanese Patent Application Laid-Open No. 2002-241892 discloses that in order to improve the strength, toughness and hardenability of a 36CrNiMo6-based crank, not only the cost reduction by low Ni and the addition of V, Nb and Ta, but also the strength by solid solution N are improved. Is trying to. On the other hand, suppression of hydrogen embrittlement by external factors such as corrosion or suppression of hydrogen diffusion using precipitated carbonitride by tempering is known as a method for inhibiting hydrogen embrittlement in the field of cold rolled steel production. However, these have different behaviors of hydrogen from hydrogen cracks that occur in a short time than corrosion occurs, such as during cooling or at room temperature.

The present invention focuses on the fact that high-clean steels, which have higher performance, such as lighter weight and fatigue strength required for large forged steel products such as crank shafts, impair hydrogen cracking resistance, and thus, hydrogen cracking sensitivity of this type of forging steel. The effective task is to reduce this problem effectively.

The present invention, in order to solve the above problems,

(1) C: 0.2 to 0.6% (weight% or less), Si: 0.1 to 0.4%, Mn: 0.7 to 1.5%, Ni: 1.0% or less, Cr: 1.2 to 3.5%, Mo: 0.1 to 0.6 %, Al: 0.01 to 0.05% and a steel containing 0.005% or more in total of at least one of Ti, Zr, Hf or Nb, and the average value of the circularity of inclusions having a maximum chord length of 1 μm or more included in the steel (hereinafter, Average circularity) is at least 0.5, the number of inclusions having a maximum chord length of 20 μm or more is less than 40 per 100 mm ² , the average circularity of 0.25 or more, and the number of inclusions having a maximum chord length of 1-10 μm is 100 mm the hydrogen cracking than 1OO per ^second excellent forging steel,

(2) Forging steel excellent in hydrogen crack resistance as described in said (1) containing 0.10% or less in total and 1 or more types of Ti, Zr, Hf, or Nb, and 0.02% or less of N,

(3) The above (1) or (1) wherein the inclusions occupying 10% or more of the inclusions having a maximum chord length of 1 to 10 μm contained in the steel contain at least 1% of Ti, Zr, Hf or Nb in total; Forging steel with excellent hydrogen cracking resistance as described in 2),

(4) forging steels having excellent hydrogen crack resistance as described in the above (1), (2) or (3) containing 0.005% or less of S;

(5) V: steel forging as described in said (1), (2), (3) or (4) containing 0.035 to 0.30%,

(6) Hydrogen cracking forging steel as described in said (1), (2), (3), (4) or (5) which uses manufacture of a large crankshaft, and

(7) A crankshaft manufactured by forging the steel for forging according to the above (1), (2), (3), (4) or (5).

The forging steel excellent in the hydrogen crack resistance of the present invention is characterized by relatively adjusting characteristic values such as shape, size and number of inclusions in the steel while specifying the composition of the steel.

First, the component composition of steel is demonstrated.

By containing C: 0.2 to 0.6%, the hardenability and strength of steel can be improved, and for this purpose, 0.2% or more is required, but when it exceeds 0.6%, toughness of steel will deteriorate and it will promote reverse V segregation. More preferably, 0.3 to 0.5% of range is preferable.

The content of Si: 0.1 to 0.4% is expected to enhance the strength of the steel. In order to secure sufficient strength, 0.1% or more is required. When too much, the reverse V segregation becomes remarkable, and the cleanliness of the steel is inhibited. % Or less, More preferably, it is 0.3% or less.

The content of Mn: 0.7 to 1.5% is because the hardenability and strength of the steel are further improved, and when it exceeds 1.5% or more, it promotes reverse V segregation, so the range of 0.8 to 1.2% is more preferable.

Ni: 1.0% or less is because Ni is dissolved in steel to improve the hardenability of the steel and imparts toughness. Expensive Ni is preferably 1.0% or less.

It is because the content of Cr: 1.2 to 3.5% improves the hardenability and toughness of the steel, and if too large, it leads to the promotion of reverse V segregation, more preferably 1.5 to 2.5%.

Although Mo: 0.1-0.6% containing improves all the hardenability, strength, and toughness of steel effectively, More preferably, 0.15% or more is preferable. However, since the equilibrium distribution coefficient of Mo is small and it is easy to produce micro segregation (normal segregation), it is preferable to suppress it to 0.6% or less, more preferably 0.35% or less.

In addition, since the presence of S forms sulfides such as MnS in the steel and causes the fatigue properties of the steel to deteriorate, 0.005% or less, and more preferably 0.003% or less.

The forging steel of the present invention is essentially characterized by containing the following components in addition to constituting the basic composition of this kind of steel by essentially containing the above-described components.

First of all, the Al content of 0.01 to 0.05% is expected to deoxidize Al, and requires 0.01% or more, and when the Al ₂ O ₃ -based inclusions formed are fine, the starting point of hydrogen cracking becomes. It captures hydrogen and contributes to the improvement of the hydrogen cracking resistance of the product steel. This is because the Al ₂ O ₃ tends to have a larger circularity to be described later than the sulfide. On the other hand, Al mainly fixes N in the form of AlN, hinders the reinforcing action of the steel by the combination of N and V, and also combines with many other elements to form nonmetallic inclusions or intermetallic compounds. Since it adversely affects the toughness, the limit is 0.05%.

Next, one or two or more of Ti, Zr, Hf or Nb is contained in 0.005% or more. These metal elements form fine inclusions such as Ti ₄ C ₂ S ₂ in the steel, are dispersed in the steel, occlude and capture excess hydrogen in the steel exceeding the solid solution limit, and have a great effect of improving the hydrogen cracking resistance of the steel. have. Moreover, the chemical bond energy when these inclusions, such as Ti, capture | acquire hydrogen is strong, and suppresses the hydrogen diffusion which arises at the time of cooling of steel from high temperature. This means that these inclusions, such as Ti, are more excellent in inhibiting hydrogen cracking than inclusions such as MnS and Al ₂ O ₃ .

In order to anticipate the above effects, Ti and the like are required to be 0.005% or more. However, since Ti and the like are active elements, they may react with impurities and refractory components in molten steel to make huge inclusions. Since the inclusions may significantly degrade the mechanical properties and hydrogen cracking resistance of the steel, the inclusions are preferably controlled at 0.10% or less and more safely at 0.03% or less.

On the other hand, the amount, size, or type of the inclusions such as Ti may be governed by the refining state such as stirring of molten steel, the drawing speed or the cooling rate during casting, independently of the content of Ti and the like. It does not specifically limit.

In addition, in the present invention, it is preferable to control the content of N to 0.02% or less together with the content of Ti or the like to form nitrides or carbonitrides in combination with Ti to contribute to suppressing hydrogen cracking susceptibility of steel. Because. When the content of N exceeds 0.02%, the resulting nitride or the like coarsens and deteriorates the mechanical properties of the steel. Therefore, it is more preferable to suppress the content to 0.01% or less.

The steel of the present invention may further contain V: 0.035 to 0.30%. V has an effect of refining austenite particles in the steel by generating carbonitrides during quenching of the steel, and more preferably in the range of 0.007 to 0.25%.

In addition to the composition described above, the present invention is characterized by controlling characteristic values such as the shape, number or size of inclusions present in steel as follows.

Average circularity of inclusions with a maximum chord length of 1 μm or more, 0.5 or more,

Less than 40/100 mm ² inclusions having a maximum chord length of 20 μm or more, the average circularity of 0.25 or more, and

100 inclusions / OOmm ² or more with a maximum chord length of 1 to 10 μm.

The various inclusions dispersed in steel vary in shape and size depending on the composition, production process, and the like. However, when the shape is noticed, the elliptic sphere is more likely to be the starting point of the stress concentration crack than the spherical shape. In addition, it is thought that the strain concentrates on the stress field to cause the concentration of hydrogen, which is likely to cause hydrogen cracking of copper. In order to examine this situation quantitatively, the steel cross section was observed and the inclusions were grasped in two dimensions, and the shape thereof was measured by the circular length represented by the length of the elliptic sphere and the following equation (1), and the relationship between the hydrogen cracking properties of the steel It was examined by the analysis of experimental data. As a result, it was found that the smaller the circularity, the more prone to hydrogen cracking.

In other words, when the average circularity of inclusions having a maximum chord length of 1 μm or more was less than 0.5 and the average circularity of inclusions having 20 μm or more was less than 0.25, the steel was easily prone to hydrogen cracking. More safely, by controlling the average circularity of inclusions having a maximum chord length of 1 μm or more to 0.6 or more and the average circularity of inclusions of 20 μm or more to 0.4 or more, hydrogen cracking of steel can be effectively suppressed.

It has also been found that some of the number of these inclusions in the steel affects the hydrogen cracking susceptibility of the steel. That is, when the maximum chord length is 20 µm or more, the number of the inclusions has a large influence on the hydrogen cracking property, and hydrogen cracks originating from the inclusions are likely to occur at 40 or more per 100 mm ² . For more safety, it is better to control less than 30.

It has been found that inclusions having a maximum chord length of 1 to 10 탆 act to allow the interface to capture hydrogen, thereby inhibiting hydrogen diffusion and making it difficult to become a starting point of hydrogen cracking. On the other hand, inclusions of less than 1 μm have the same function, so that their inclusion is not disturbed.

Although the present invention contains 0.005% or more of Ti, Zr, Hf, or Nb or two or more thereof in order to efficiently realize hydrogen trapping performance in steel, these active elements form inclusions as described above. Dispersed in the river. And when the maximum chord length of this inclusion is fine to 10 micrometers or less, it will be easy to be disperse | distributed and the trapping | capacitance of excess hydrogen beyond the solid solution limit will improve. Moreover, the strong chemical bonding energy of this inclusion is particularly excellent in the capturing action of hydrogen, and has a great advantage of suppressing the diffusion of hydrogen from cooling at a higher temperature during cooling of the steel, and this effect has an effect of inclusions such as MnS and Al ₂ O ₃ . Greater than

In addition, the effect on the hydrogen of inclusions derived from Ti and the like is remarkable when the maximum string length is 10 μm or less, and moreover, the number of the fine inclusions is 10% or more, more preferably 20% or more in the same inclusions. Remarkable Moreover, the above-mentioned effect cannot be expected as a trace amount which the quantity which occupies in all other interference | inclusions of these inclusions, such as Ti, in total is 1% or less, More preferably, it is effective to control to 20% or more.

On the other hand, the method of quantifying various characteristic values of an inclusion as mentioned above is not fixed, but can be obtained by the following method, for example. After controlling Al in the scope of the present invention, the slag basicity (CaO / SiO ₂ ) is 3.0 or more, whereby the number of oxides can be significantly suppressed to form fine inclusions centering on the spinel. In addition, by controlling the amounts of S and N forming inclusions within the scope of the present invention, not only the size of the sulfide and the nitride but also the composition can be controlled. These inclusions are unlikely to cause coagulation or coarsening of the whole body, and are easily controlled by the characteristic values of the present invention. On the other hand, more precise inclusion control can be performed by adjusting the injection speed, the stirring method / speed, the cooling rate, the quenching and tempering temperatures for each steel type. Moreover, you may contain Mg, Ca, etc. as an element which controls an inclusion. In addition, the above-mentioned chord length, circumferential length, area, or number of inclusions, etc. can be easily carried out by image analysis in SEM-EPMA (JXA.8900 RL, XM.Z0043T, XM.87562). I can measure it.

In the steel of the present invention, it is acceptable to add and contain other elements depending on the use. For example, in order to control the form of B or MnS in order to improve the hardenability of steel, Ca, Mg, Te, etc. can be contained in about 0.03% or less in total.

The present invention includes a large crankshaft for ships manufactured by forging a steel material having the chemical composition described above and containing inclusions having controlled sizes, shapes, and the like, but the crankshaft includes hydrogen cracking resistance as described above. Needless to say, it's a lightweight, high-performance product.

[ Example ]

As the steel and comparative steel of the present invention, the forging steel containing the chemical components shown in Tables 1 and 2 and whose slag basicity was adjusted to 3.0 was dissolved in a 150 kg vacuum furnace to obtain test steel. . Each ingot was forged and quenched (870 ° C. × 1 hour) and tempered (600 ° C. × 13 hours) so as to have a tensile strength of around 950 MPa after cooling. From each test steel, the test piece of 20 mm square x 5 mm thickness size was cut out three by one steel grade, and each cross section was polished.

Each test piece is captured by an automatic EPMA (JXA, 8900 RL, XM, Z0043T, XM, 87562) manufactured by Japan Electronics Co., Ltd., at a magnification of 100 times, to hold a reflection electron image with a field of view of 10 × 10 mm ² , with a maximum length of 1 μm. All the inclusions recognized above. At the same time, the circumference, area, and circularity other than the maximum chord length of the inclusion were automatically calculated, and the center point of the inclusion was automatically analyzed by EDS for 10 seconds with respect to one point.

About each test piece, the following four types were computed based on the maximum chord length of an inclusion, this was performed three times with respect to one steel type, and each value was averaged.

Number of inclusions with a maximum chord length of 1 to 10 μm.

The proportion of inclusions containing 1% or more in total of one or two or more of Ti, Zr, Hf, and Nb therein.

Average value of the circularity of inclusions with a maximum chord length of at least 1 μm and at least 20 μm.

Number of inclusions with a maximum chord length of 20 μm or more.

On the other hand, the hydrogen crack susceptibility of each test piece was experimentally evaluated by the comparative test method of the hydrogen crack susceptibility of the forging steel shown in FIG. 1. The round bar-shaped test piece 1 was machined with a length of 150 mm and a distance between the marks in the form of a dumbbell of 10 mm, the center portion having a diameter of 4 mm, and the holding means portions at both ends having a diameter of 8 mm at a length of 15 mm. Installed screws over. The test piece was provided in the immersion test to be surrounded in _{0.5 Mol / 1H 2 SO 4 +} 0.01 Mol / 1KSCN environment consisting of an aqueous solution.

Each test piece is immersed in the aqueous solution, catalyzed by a current density of 0.5 A / dm ² , and the test piece 1 is attached to the apparatus of FIG. 1 while adding hydrogen. Then, a tensile load in the long axis direction was applied to the test piece 1 by SSRT (low strain rate test) to measure the stress S ₁ (elongation). The tensile velocity of the crosshead of the test apparatus at this time was 2 * 10 <^-3> mm / min. Apart from this, immersion in the aqueous solution was omitted, and SSRT test was carried out in the air under the same tensile conditions as described above, and the breaking stress S ₀ of the test pieces of this group was measured.

And each crack value was substituted into the following formula (2), and the hydrogen cracking susceptibility S value was computed.

The obtained S value was evaluated for hydrogen cracking susceptibility of the steel according to the following criteria.

× S value is 50 or more ... inferior to hydrogen crack resistance.

△ S value of 40 to 50 ... hydrogen cracking resistance is normal.

The S value is 30 to 40 ..... excellent hydrogen crack resistance.

◎ S value is less than 30. Hydrogen cracking resistance is very excellent.

According to the said criteria, although the relationship between the inclusion characteristic value of each test piece and the hydrogen cracking resistance of steel is shown in Table 3, it is clear that the Example of this invention steel is excellent in the hydrogen cracking resistance as steel. For example, as in Example A, when the Al content is relatively small, the amount of deposition of Al ₂ O ₃ -based inclusions is small, and the circularity of the fine inclusions is relatively low as a whole. In addition, as in Example B, when the content of the Ti-based component is small, there are few fine inclusions and the roundness is low.

2 shows an example of an electron micrograph of inclusions in steel. The fine inclusions Ti ₄ C ₂ S ₂ in the figure are formed by containing Al and Ti-based active elements, and it is understood that hydrogen cracking susceptibility of steel is more suppressed when a large number of these fine inclusions are produced. On the other hand, especially Ti addition is the most effective. In addition, as in Examples F to J, when the S content is lowered, hydrogen cracking susceptibility is lowered.

On the other hand, in the case of Comparative Examples K-O, since the characteristic value of an inclusion deviates from the prescription | regulation of this invention, it turns out that the hydrogen cracking sensitivity of steel is improving continuously.

This invention limits each content of C, Si, Mn, Ni, Cr, Mo, Al, and N normally contained in forging steel to a specific range according to each function, and also Ti, Zr, Hf, or Nb It was set as the composition added together, and the shape, size, and number of inclusions in steel were strictly specified on it. According to these two conditions, while maintaining high cleanliness, the hydrogen cracking resistance is very excellent, and furthermore, it is possible to provide forging steel with high fatigue strength by being light in weight, and to be effectively used for large crankshafts. There is.

Claims (7)

C: 0.2 to 0.6% (wt%, the same as below), Si: 0.1 to 0.4%, Mn: 0.7 to 1.5%, Ni: 1.0% or less, Cr: 1.2 to 3.5%, Mo: 0.1 to 0.6%, Al : Steel containing 0.01 to 0.05% and at least 0.005% of Ti, Zr, Hf or Nb in total, the average value of the circularity of inclusions having a maximum chord length of 1 μm or more (hereinafter, referred to as average circular degree). Is more than 0.5, the number of inclusions having a maximum string length of 20 μm or more is less than 40 per 100 mm ² , the average circularity is 0.25 or more, and the number of inclusions having a maximum string length of 1 to 10 μm is 100 mm ^2. Forging steel with excellent hydrogen cracking resistance, characterized in that more than 100 per sugar. The method of claim 1, A forging steel having excellent hydrogen crack resistance, characterized by containing 0.10% or less in total of one or more of Ti, Zr, Hf, or Nb, and 0.02% or less of N. The method according to claim 1 or 2, Hydrogen crack resistance, characterized in that the inclusions occupying at least 10% of the inclusions having a maximum chord length of 1 to 10 μm included in the steel contain at least 1% of Ti, Zr, Hf or Nb in total. This excellent forging steel. The method of claim 1, Forging steel with excellent hydrogen crack resistance, characterized by containing S of 0.005% or less. The method of claim 1, V: Forging steel excellent in hydrogen crack resistance, characterized by containing 0.035 to 0.30%. The method of claim 1, A hydrogen cracking resistant forging steel characterized by the purpose of producing a large crankshaft. Crankshaft, characterized in that produced by forging the forging steel according to claim 1.

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