JP2010222697A - Steel for machine structural use having excellent toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel being a steel for a machine structural use for a large-sized component, the steel being used after carburizing treatment or carbonitriding treatment by increasing hardenability and the steel having toughness equal to or above that of the conventional Ni-Mo-added steel and adding neither Ni nor Mo and therefore has the reduced cost. <P>SOLUTION: The steel for carburizing or carbonitriding has a composition comprising, by mass, 0.13 to 0.33% C, 0.1 to 1.0% Si, 1.2 to 2.0% Mn, ≤0.030% P, ≤0.030% S, 1.8 to 3.5% Cr, ≤0.050% Al and ≤0.0020% O, and the balance Fe with inevitable impurities, wherein the contents of C, Si, Mn and Cr satisfy (0.04+0.35C)×(1.00+0.70Si)×(0.70+3.96Mn)×(1.0+2.16Cr)≥3.0. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to steel for use in various machine structural parts that are used by carburizing or carbonitriding a machine structural steel, and particularly relates to a machine structural part having excellent impact resistance and toughness.

  Conventionally, applications requiring impact resistance include, for example, JIS standard SNCM420 (mass% contains C: 0.20%, Ni: 1.6%, Mo: 0.15%) and SNCM815 (mass% , C: 0.15%, Ni: 4.0%, Mo: 0.15% included), etc., steel with 1.6 to 4.0% Ni content and about 0.15% Mo added is used. Has been. However, these elements have high raw material costs and are rare, and in recent years, resource saving has been demanded. By the way, SNCM with 1.6% to 4.0% Ni content and about 0.15 to 0.30% Mo content is added to large structural gears and rolling parts that require toughness. The system Ni-Mo steel has been used. When further improvement in toughness is required, as a countermeasure, the toughness is enhanced by reducing the impurity elements such as P and S as much as possible and adding alloy elements such as Ni, Mo and V (for example, Patent Documents). 1, see Patent Document 2).

  However, when higher toughness is required as described above, it is possible to reduce impurity elements, increase the amount of alloy elements such as Ni, Mo and V, and reduce unavoidable impurities such as P, S and Cu. Is required. The addition of alloy elements such as Ni, Mo, and V cannot achieve further toughness improvements required for recent large parts, and the addition of further alloy elements ensures toughness. In some cases, there is a problem that the material cost increases.

  Generally, steel parts that have been subjected to carburizing or carbonitriding are used for parts that require impact resistance. By the way, many large parts such as large gears are difficult to obtain a sufficient cooling rate to obtain a martensite structure in the quenching stage during carburizing or carbonitriding, and are partially incompletely quenched including a bainite structure. A phase may be formed, resulting in a reduction in core toughness. Surface strength and hardness are increased by performing a carburizing quenching process or a carbonitriding quenching process. Because these large parts have large mass, the core structure, which is an unhardened layer after carburizing and quenching or carbonitriding and quenching, contains a bainite structure that is an incompletely quenched structure in the martensite structure due to insufficient hardenability. There is a case. In this case, if a bainite structure is present in the core in an environment that is subjected to an impact load, the crack progress is facilitated and sufficient toughness cannot be ensured. I can not cope. Therefore, steel with good hardenability is used as a large structural material, and after carburizing or nitriding and annealing, the surface hardness of HRC 60 or higher and the core strength of HRC 30 to 48 are obtained by quenching and tempering. An invention to be obtained has been proposed (see, for example, Patent Document 3).

  Further, in a large part, due to impact addition, a crack may be transmitted from a defect generated on the surface to the core part, resulting in destruction, and it is necessary to increase the core part toughness. In that case, Ni and Mo are added for the purpose of suppressing shock destruction and ensuring hardenability, but there is a problem that the material cost increases. Further, as described above, in the method of reducing inevitable impurities such as P and Cu, the raw material cost and the manufacturing cost are similarly increased due to restrictions on raw materials and steelmaking operations. In addition, since many large parts are hardened in actual carburizing or carbonitriding, the cooling rate of the core during carburizing or carburizing and quenching changes. It is difficult to provide large parts with the same structure and hardness. For example, when a normal quenching process is performed, if the cooling rate is slow and a lot of bainite structure is present, the core toughness is reduced, and a large component having a certain toughness can be provided. There is a problem that cannot be done.

JP-A-1-247561 Japanese Patent Laid-Open No. 9-53148 JP 2007-308740 A

  The problem to be solved by the present invention is that the toughness of mechanical structural steel for parts used by carburizing or carbonitriding by using a steel component with improved hardenability is compared with the conventionally used Ni or Mo. An object of the present invention is to provide a steel having toughness equal to or higher than that of the added steel and having reduced material costs without adding Ni or Mo.

  The inventors of the present application have an incompletely quenched structure even when the cooling rate of the middle peripheral part at the time of carburizing quenching or carbonitriding quenching is 2 ° C./s or less by using a steel component having high hardenability. It was found that toughness equivalent to or better than SNCM420 and SNCM815, which are conventional steels, can be secured by suppressing a certain bainitic structure and making the steel core portion appropriate in its martensite area ratio and hardness. Moreover, since it is not necessary to reduce inevitable impurities such as P and Cu in order to improve toughness, there is no restriction on raw materials and manufacturing operations, and Ni or Mo is not added, so that material costs are reduced. Found that it would be possible.

  Therefore, the means of the present invention for solving the problem is that, in the invention of claim 1, as a steel for carburizing or carbonitriding, in mass%, C: 0.13 to 0.33%, Si: 0.1 -1.0%, Mn: 1.2-2.0%, P: 0.03% or less, S: 0.030% or less, Cr: 1.8-3.5%, Al: 0.050% Hereinafter, O: 0.0020% or less, the balance is made of steel for carburizing or carbonitriding with an alloy composition consisting of Fe and inevitable impurities, and the contents of C, Si, Mn and Cr as steel components are It is a steel for carburizing or carbonitriding that satisfies (0.04 + 0.35C) × (1.00 + 0.70Si) × (0.70 + 3.96Mn) × (1.0 + 2.16Cr) ≧ 3.0.

  In the invention of claim 2, in addition to the steel component of the means of claim 1, it contains N: 0.0250% or less in mass%, and the contents of C, Si, Mn and Cr are (0.04 + 0. 35C) × (1.00 + 0.70Si) × (0.70 + 3.96Mn) × (1.0 + 2.16Cr) ≧ 3.0.

  In the invention of claim 3, the carburizing or carbonitriding steel having the steel component of the means of claim 1 or 2 is subjected to carburizing and quenching treatment at a cooling rate of 2 ° C./s or less at the middle circumference of the steel material made of the steel. Or, the martensite area ratio of the core portion of the steel material subjected to carbonitriding and quenching treatment is 50% or more, the hardness of the core portion of the steel material is 30 to 50 HRC, and the crystal grain size number of the core portion of the steel material is 7 or more. Satisfactory carburizing or carbonitriding steel.

  The reasons for limiting the components of the steel for machine structural use of large parts used by carburizing or carbonitriding in the means of the invention will be described. In addition,% of each following component is shown by the mass%.

C: 0.13-0.33%
If C is less than 0.13%, a long time is required for carburizing or carbonitriding. On the other hand, if it exceeds 0.33%, the internal hardness is more than necessary, and the toughness is lowered and the machinability and cold workability are also lowered. Therefore, C is set to 0.13 to 0.33%.

Si: 0.1 to 1.0%
Si is necessary as a deoxidizing element when melting steel. Therefore, Si needs to be 0.1% or more. However, if it exceeds 1%, the toughness is lowered, the material hardness is further increased, the workability is deteriorated, and the carburization is inhibited and the grain boundary oxidation is promoted. Therefore, Si is 0.1 to 1.0%.

Mn: 1.2 to 2.0%
Mn is necessary as a deoxidizing element when melting steel. Furthermore, it is necessary to add 1.2% or more in order to improve hardenability. However, if it is added in an amount of more than 2.0%, the hardenability becomes excessive and the toughness is lowered, and further the machinability and cold workability are deteriorated. Therefore, Mn is set to 1.2 to 2.0%.

P: 0.030% or less, S: 0.030% or less Since P and S are elements that lower toughness, each is made 0.030% or less.

Cr: 1.8-3.5%
Since Cr is an element that improves hardenability and toughness, it is necessary to add 1.8% or more. However, if it is added in an amount of more than 3.5%, the machinability and workability are deteriorated and the carburizing property is inhibited. Therefore, Cr is set to 1.8 to 3.5%.

Al: 0.050% or less Al is necessary as a deoxidizing element when melting steel. If too much is added, the cleanliness of the steel decreases and the fatigue strength decreases. Therefore, Al is made 0.050% or less.

O: 20 ppm or less O has an upper limit of 20 ppm in order to form oxidative inclusions harmful to impact resistance.

N: 250 ppm or less N combines with Al in the steel to form a nitride, and is effective in suppressing grain coarsening. However, the effect is saturated at 250 ppm, and if it is contained more than that, the nitride becomes inclusions and adversely affects the fatigue strength. Therefore, the upper limit is set to 250 ppm, preferably to suppress coarsening of crystal grains. It is set as 100-250 ppm as 100 ppm or more which comes to have an effect.

  In the present invention, excellent toughness can be obtained by reducing the incompletely quenched structure remaining after carburizing or carbonitriding a large part of steel material made of the steel of the present invention and increasing the martensite area ratio. It is important to impart high hardenability. As an index representing the hardenability, in claims 1 and 2, calculation of (0.04 + 0.35C) × (1.00 + 0.70Si) × (0.70 + 3.96Mn) × (1.0 + 2.16Cr) is performed. It was set as a formula, and it was considered as a hardening index by applying each of these calculation formulas. The higher the quenching index value is 3.0 or higher, the higher the hardenability of the steel material, and the higher the martensite area ratio of the core. If the value of the quenching index is as low as less than 3.0, the hardenability of the steel material is low, and the martensite area ratio of the core part decreases. In addition, when the quenching index value of each of these calculation formulas is 3.0 or more, it has been clarified that particularly excellent toughness is obtained. It was set as a configuration requirement. Further, in the means of claim 3, even when carburizing and quenching or carbonitriding and quenching at a cooling rate of 2 ° C./s or less in the middle circumference of the steel, the martensite area ratio of the core of the steel is 50% or more, And the hardness of a core part is 30-50 HRC, and also the crystal grain size number of the core part is refined | miniaturized with 7 or more, the outstanding toughness can be obtained, and the high hardenability is acquired.

  The calculation method of the quenching index is obtained by approximating the values of C, Si, Mn, Ni, and Cr of each element from the values described in ASTM (America Society of Test Material) Standard 255-02.

  In the means of claim 1, even if Ni or Mo is not added to the steel composition, the core structure after cooling the steel by carburizing or carbonitriding is mainly martensite with suppressed bainite structure. For example, the middle periphery of a material obtained by oil quenching a φ90 mm round bar simulating the core of a large component has a martensite area ratio of 50% or more, and can ensure high toughness.

  According to the second aspect of the present invention, by adding the N content to the steel component of the first aspect of the present invention prescribed to 250 ppm or less, carburizing treatment or carbonitriding treatment can be performed, and excellent toughness can be ensured.

  According to the third aspect of the present invention, the cooling rate of the middle peripheral portion when the carburizing treatment or the carbonitriding treatment of the large component made of steel of the steel component of the first or second aspect is changed to the cooling rate of the core portion. Even when the temperature is equivalent to 2 ° C./s or less, the structure of the core of this large component has a martensite area ratio of 50% or more, a core hardness of 30 to 50 HRC, and a crystal grain size of the core. The number is 7 or more, and excellent toughness can be ensured.

  In the steel used for car structure or carbonitriding, which is a structural structural steel for large parts comprising the means of the present invention, the cooling rate of the middle part of the large part corresponds to the cooling rate of the core part. Even if it becomes 2 degrees C / s or less, hardenability can be improved, the martensite area ratio of a core part can be 50% or more, and the hardness of a core part can be 30-50 HRC. Furthermore, it is possible to obtain a steel for machine structural use that has excellent toughness without adding Ni or Mo as a steel component, and has a toughness equivalent to or better than the current Ni-containing steel, and the material cost. The machine structural steel for large parts with reduced is obtained.

It is a figure which shows the process process of the carburizing quenching and tempering conditions employ | adopted in experiment.

  It melts in a 100 kg vacuum melting furnace and contains steel components of Examples 1 to 12 and Comparative Examples 1 to 5 shown in Table 1, Examples 13 to 24 and Comparative Examples 6 to 10 shown in Table 2. And the steel which the remainder consists of Fe and an unavoidable impurity was manufactured. Table 1 shows an example and a comparative example of claim 1, and Table 2 shows an example and a comparative example of claim 2. In these Tables 1 and 2, the calculated values of the quenching index of the examples and comparative examples of the present invention are (0.04 + 0.35C) × (1.00 + 0.70Si) × (0.70 + 3.96Mn) × (1 0.0 + 2.16Cr). The values of this quenching index are 3.4 to 10.4 in the examples in both Tables 1 and 2, and these are all 3.0 or more. However, in the comparative example of Table 1, the value of the quenching index is 0.6 to 4.5, and in the comparative example of Table 2, the value of the quenching index is 1.5 to 3.9. All except Example 10 are less than 3.0. In Tables 1 and 2, shaded values indicate that they are outside the scope of the claims.

  As described above, Table 1 shows an example and a comparative example of claim 1, and Table 2 shows an example and a comparative example of claim 2. The steel of the example and the comparative example was formed into a steel bar having a diameter of 90 mm by hot forging. Furthermore, after normalizing these, the carburizing quenching tempering process shown in FIG. 1 was performed. In this case, the quenching was oil quenching. In order to evaluate the Charpy impact test, fracture toughness test, martensite area ratio of core, hardness of core and grain size from these steel bars subjected to carburizing and tempering treatment, A specimen was cut out.

  The reason why the above-mentioned φ90 mm steel bar is oil-quenched is that the cooling rate of 2 ° C / s, which is the cooling rate of large actual parts, is substantially equal to the cooling rate of the middle circumference of φ90 mm oil-quenched material, and therefore the micro- This is because the hardness of the structure and the core is almost the same even in large actual parts. This makes it possible to easily evaluate the core toughness of carburizing steel or carbonitriding steel for large parts in a laboratory.

Table 3 shows the martensite area ratio when φ90 mm steel bars were cut out from the middle circumference after oil quenching as Examples and Comparative Examples of Claim 1. The area ratio of each tissue was calculated by measuring images of photographs taken by SEM. In the embodiments of claims 1 and 2, the hardenability is improved, and both have a martensite area ratio of 50% or more. On the other hand, in the comparative example, since the addition amount of Si, Mn and Cr is small despite the addition of Ni and Mo, the hardenability is low, and the martensite area ratio is lower than that of the example except for the comparative example 5. The value is lower than 50%.

Further, Table 3 shows the results of the Charpy impact test. The test piece is 10 × 10 × 55 mm, and has a JIS standard 2 mmV notch. The impact value of an Example is 56-98 (J / cm < ² >), and the impact value is falling with the raise of hardness. On the other hand, in Comparative Example 1 and Comparative Example 4, the result was equal to or better than that of the Example. The shaded areas in Table 3 indicate that the impact value and fracture toughness value at room temperature are within the range of the examples.

Further, Table 3 shows the results of the fracture toughness test. The specimen shape is a 1 inch size specimen having a thickness of 25.4 mm. In the fracture toughness test, the propagation resistance of the material to the crack is known by first introducing and testing the crack in advance. The toughness value of the examples is 67 to 123 (MPa · m ^1/2 ), and the impact value decreases as the hardness increases, as in the result of the Charpy impact test. On the other hand, in Comparative Example 1 and Comparative Example 4, the result was the same as or better than that of the Example as in the Charpy impact test result. However, in this comparative example, Ni is added in a relatively large amount of 1.90 to 3.28% and Mo is 0.31 to 0.52%, which increases the material cost. Therefore, even if these comparative examples have high toughness, they are different from the invention of claim 1 of the present application.

  Next, Table 4 shows the martensite area ratio when a φ90 mm steel bar was cut out from the middle circumference after oil quenching as an example and a comparative example of claim 2. The area ratio of each tissue was calculated by measuring images of photographs taken by SEM. In the embodiment of claim 2, the hardenability is improved, and all have a martensite area ratio of 50% or more. On the other hand, in the comparative example, despite the addition of Ni or Mo, the addition amount of Si, Mn and Cr is small, so the hardenability is low, and the martensite area ratio is lower than 50% except for Comparative Example 10. It is a value.

  Further, Table 4 shows the core hardness measurement results. No. In the examples of 13 to 24, the core hardness is 30 to 50 HRC by using an appropriate steel component with improved hardenability. On the other hand, in Comparative Examples 6 to 9, since the hardenability was low and the martensite area ratio was 50% or less, the core hardness was 30 HRC or less. However, in Comparative Example 10, the martensite area ratio was 52% and the C content was as high as 0.40%, so the core hardness was 49 HRC.

  Further, Table 4 shows the measurement results of the crystal grain size. No. In the examples of Nos. 11 to 20, the grain size number is 7.0 or more. On the other hand, no. 6 and no. In the comparative examples 8 to 9, all have a grain size number of less than 7.0, and the mass ratio between the amount of Al and the amount of N for forming AlN that is effective in suppressing grain coarsening is appropriate. It is thought that there was not. However, the crystal grain size numbers of Comparative Examples 7 and 11 were 7.0 or higher.

Further, Table 4 shows the results of the Charpy impact test. The test piece is 10 × 10 × 55 mm, and is a test piece having a JIS-defined 2 mmV notch. No. The impact values of Examples 11 to 20 are 56 to 98 (J / cm ² ), and the impact value decreases as the hardness increases. On the other hand, in Comparative Example 6 and Comparative Example 9, the result was equal to or better than that of the Example. In all other comparative examples, the impact value did not exceed that of the example. The reason why the impact value is inferior to that of the examples is that the martensite area ratio contributing to the improvement of toughness is low and the crystal grain size is lower than that of the examples.

Table 4 shows the results of the fracture toughness test. The shape of the test piece is a 1 inch size test piece having a thickness of 25.4 mm. In the fracture toughness test, by first introducing a crack into a test piece in advance and testing it, the propagation resistance of the material to the crack can be determined. No. The toughness values of Examples 13 to 24 are 67 to 123 (MPa · m ^1/2 ). On the other hand, in Comparative Example 6 and Comparative Example 9, the result was equal to or better than that of the Example. In the other comparative examples, none of the fracture toughness values exceeded that of the examples. The reason why the fracture toughness value is inferior to that of the examples is that the martensite area ratio contributing to the improvement of toughness is the same as the result of the Charpy impact test, and that the crystal grain size is lower than that of the examples. In Comparative Examples 6 and 9, Ni is added in a large amount of 2.10 to 3.32% and Mo is 0.33 to 0.50%, which increases the material cost. Therefore, even if these comparative examples have high toughness, they are different from the invention of claim 1 of the present application.

  As is apparent from the above description, machine structural parts manufactured from machine structural steel obtained by carburizing or carbonitriding the steel for carburizing or carbonitriding according to the present invention have improved hardenability, The core toughness can be secured by increasing the site area ratio. Furthermore, the steel used for the machine structural component of the present invention has high hardenability and a core martensite area ratio of 50% or more, and the core has a hardness of 30 to 50 HRC grain size number of 7.0 or more. Therefore, the core toughness of the steel for structural parts for large parts used by carburizing or carbonitriding is made equal to or higher than that of conventionally used steel with added Ni or Mo, and Ni or Steel which does not require addition of Mo and whose material cost is reduced can be obtained. Therefore, it can be set as the steel for machine structure which reduced the material cost.

Claims (3)

  In mass%, C: 0.13-0.33%, Si: 0.1-1.0%, Mn: 1.2-2.0%, P: 0.030% or less, S: 0.030 %, Cr: 1.8-3.5%, Al: 0.050% or less, O: 0.0020% or less, with the balance being Fe and unavoidable impurities, C, Si, Mn and Cr Content of (0.04 + 0.35C) × (1.00 + 0.70Si) × (0.70 + 3.96Mn) × (1.0 + 2.16Cr) ≧ 3.0 Steel for nitriding.   In addition to the steel component according to claim 1, N: 0.0250% or less is contained by mass%, and the contents of C, Si, Mn, Ni and Cr are (0.04 + 0.35C) × (1 0.000 + 0.70Si) × (0.70 + 3.96Mn) × (1.0 + 2.16Cr) ≧ 3.0, a steel for carburizing or carbonitriding characterized by the following.   The steel for carburizing or carbonitriding having the steel component according to claim 1 or 2 is a steel material obtained by carburizing or carbonitriding and quenching the middle circumference of the steel material made of the steel at a cooling rate of 2 ° C / s or less. For carburizing, characterized in that the martensite area ratio of the core is 50% or more, the hardness of the core of the steel is 30 to 50 HRC, and the crystal grain size number of the core of the steel is 7 or more Carbonitriding steel.

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