JP2006199993A - Steel material to be case-hardened superior in cold forgeability and temper softening resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material to be case-hardened superior in cold forgeability and temper softening resistance. <P>SOLUTION: The steel material to be case-hardened superior in cold forgeability and temper softening resistance comprises, by mass%, 0.1-0.3% C, 0.45-0.9% Si, 0.2-0.5% Mn, 0.005-0.05% S, 1.0% or more but less than 1.5% Cr, 0.3% or more but less than 1.5% Mo, 0.001-0.2% Al, 0.003-0.03% N, P limited to 0.03% or less, and the balance iron with unavoidable impurities, while controlling 12Si(%)+25Mn(%)+Cr(%)+2Mo(%) to 25 or less; and has a spheroidized structure. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention has an excellent cold forgeability by performing a spheroidizing annealing treatment, and has excellent temper softening resistance after surface hardening treatment such as carburizing and carbonitriding, and is an automobile, construction vehicle, industrial machine. It is related with the steel for case hardening excellent in the cold forgeability and temper softening resistance used as a raw material of components, such as.

  For car transmissions, etc., JIS SCr420, SCM420 and other case-hardened steels are mainly used as materials, and after normalizing, annealing, or in some cases spheroidizing softening, cold forging into a gear shape A gear subjected to surface hardening treatment by carburizing, quenching and tempering after molding is used. In such gears, there is a strong demand for weight reduction and gear strength enhancement in order to increase the output and fuel consumption of automobiles. Conventionally, technology for improving the gear root bending fatigue strength has been developed to increase the strength of the gear, but in recent years, with the practical application of hard shot peening, the emphasis on increasing the strength of the gear is Shifting from tooth root bending fatigue strength of gears to pitching strength.

By the way, it is said that it is effective to improve the temper softening resistance in order to improve the pitching strength. Conventionally, as a means for improving the temper softening resistance, the component of steel which is a material of the gear is improved. Several technologies have been proposed. For example, Patent Document 1 discloses steel in which the contents of Si, Cr, and Mo included in steel are optimized in order to improve temper softening resistance. According to this document, elements such as Si, Cr, and Mo are considered to be useful for improving the temper softening resistance. When these elements are contained in steel, the content of Si is set to 0. 7% or more, Cr content of 0.1% or more, Mo content of 0.05% or more, and the total content of these elements exceed a certain value Is said to be necessary. However, when the content of these elements increases, the deformation resistance increases, cold forging cannot be performed, and hot forging is forced, resulting in a disadvantage in manufacturing costs. Conventionally, it has been reported that when the Si content exceeds 0.15%, deformation resistance is increased (see, for example, Patent Document 2). In this way, the effects of steel components (especially Si) on temper softening resistance and cold forgeability are contradictory, and there is a demand for the development of steel that satisfies both characteristics. The current situation is.
JP 2003-231943 A JP-A-6-299241

  In view of the above circumstances, an object of the present invention is to provide a case-hardening steel material excellent in both cold forgeability and temper softening resistance.

In order to solve the above-mentioned problems, the present inventors diligently studied the application of the spheroidizing annealing treatment and the component adjustment suitable for the spheroidizing annealing treatment. As a result, the present inventors have found the following matters and completed the present invention.
(1) Si and Mn have little effect on the improvement of cold forgeability by spheroidizing annealing.
(2) Cr and Mo have a great effect on improving cold forgeability by spheroidizing annealing.
However, when these elements are added in a certain amount or more, the temper softening resistance is lowered.
(3) The limit of cold forgeability must be determined by the total amount of the above four elements (Si, Mn, Cr, and Mo) taking into account the respective deformation resistance increasing effects.
(4) Si is added to the steel within a predetermined range and considering the balance with other components (for example, Mn, Cr, Mo, etc.), and carbides in the steel are obtained by spheroidizing annealing. Excellent cold forgeability can be obtained by spheroidizing.

The gist of the present invention is as follows.
1)% by mass
C: 0.1 to 0.3%
Si: 0.45 to 0.9%,
Mn: 0.2-0.5%
S: 0.005-0.05%
Cr: 1.0 to less than 1.5%,
Mo: 0.3 to less than 1.5%,
Al: 0.001 to 0.2%,
N: 0.003 to 0.03%
Containing
P: 0.03% or less, the balance is iron and inevitable impurities,
12Si (%) + 25Mn (%) + Cr (%) + 2Mo (%) is 25 or less,
A steel for skin hardening excellent in cold forgeability and temper softening resistance, characterized by having a spheroidized structure.

2) The steel material is further mass%,
Nb: 0.2% or less,
Ti: Steel material for case hardening excellent in cold forgeability and temper softening resistance as described in 1), including one or two of 0.2% or less.

  As described above, it is possible to provide a case-hardening steel material that is excellent in both cold forgeability and temper softening resistance. By using these, the manufacturing cost of gears becomes advantageous, and automobiles and construction vehicles are provided. In addition, it is possible to make a significant contribution to higher output and improved fuel consumption of industrial machinery.

As a result of the study by the present inventors, when the total amount of Si, Mn, Cr and Mo in the steel is 25 or less in the formula: 12Si (%) + 25 Mn (%) + Cr (%) + 2 Mo (%), it is excellent. However, when the total amount of the four elements exceeds 25 in the above formula, it has been clarified that the cold forgeability is poor. From this, Si and Mn have little effect on the improvement of cold forgeability even when subjected to spheroidizing annealing treatment, and Cr and Mo give the improvement of cold forgeability by applying spheroidizing annealing treatment. It was suggested that the effect is great.
The reason for this is that Si and Mn are elements that dissolve in iron, so-called solid solution strengthening elements, so that even when subjected to spheroidizing annealing, most of them remain in solution in ferrite iron and are not softened. It is believed that there is. In addition, since Cr and Mo are carbide forming elements, they are dissolved in cementite by applying spheroidizing annealing treatment, or by forming carbides themselves, most of them are discharged from iron and no solid solution strengthening occurs. It is thought that. The mechanism by which the cold forgeability is originally improved by spheroidizing annealing is due to the fact that cementite existing in a lamellar shape is divided into spheres to reduce the deformation resistance, but at the same time, Cr, Mo itself It is also considered that solid solution strengthening is eliminated by forming a spherical carbide.
From the above, it became clear that the cold forgeability is greatly influenced by the Si and Mn contents.

  Further, as a result of the study by the present inventors, it has been clarified that the temper softening resistance is lowered when the Si content is below a certain amount. From this, it was suggested that the content of Si is required to be a certain amount or more.

  Furthermore, as a result of the study by the present inventors, it has been clarified that the temper softening resistance is lowered when the Cr and Mo contents are below a certain amount. The reason for this was thought to be that during surface hardening treatment such as carburizing, quenching and tempering, Cr carbide, Mo carbide and the like were dissolved in steel, and Cr and Mo effectively acted as alloying elements.

  Further, as a result of the study by the present inventors, it has been clarified that the temper softening resistance is lowered when the Cr content exceeds a certain amount. The reason is that the carbide formed when adding a certain amount or more of Cr and subjected to the spheroidizing annealing process is coarsened and stabilized, and then the Cr remains in the steel even after surface hardening treatment such as carburizing quenching and tempering. This is considered to be due to the fact that the solid solution cannot be completely dissolved. The same can be said for Mo.

  In summary, it is necessary to limit the total amount of Si, Mn, Cr, and Mo in order to ensure cold forgeability. Specifically, 12Si (%) + 25Mn (%) + Cr (%) + 2Mo (% ) Must be limited to 25 or less. Moreover, in order to obtain temper softening resistance, it is necessary to add Si, Mn, Cr, and Mo within a certain range.

  Thus, the steel material which adjusted the component of steel and performed the spheroidizing annealing process and formed the spheroidized structure is excellent in cold forgeability, and can be processed at low cost. In addition, after forging using the steel material of the present invention, the surface-hardened member (tools, cutting tools, and dies, and parts such as automobiles, motorcycles, aircrafts, and machines (for example, gears, bearings, etc.)) Since it has excellent resistance to temper softening, it is possible to save resources. In particular, the gear manufactured using the steel material according to the present invention has a surface near the tooth surface due to frictional heat generated by contact between the driving surface and the driven surface of the gear with high surface pressure while sliding. The temperature rises to about 300 ° C. and has resistance to temper softening as a result, and it is considered that it can greatly contribute to higher output and improved fuel consumption of automobiles, construction vehicles, industrial machines, etc. . Examples of the surface hardening treatment include known treatments such as carburizing treatment and carburizing and nitriding treatment, but are not limited thereto, and are not limited to these, and further include shot peening treatment, sub-zero treatment, WPC treatment, WJP, and the like. Processing such as processing may be performed. Thereby, it becomes possible to further increase the temper softening resistance.

  Next, the mass% range of each chemical component included in the steel of the present invention (skin-hardened steel) will be described.

C: 0.1 to 0.3%
C is an element essential for maintaining the strength of the steel, and its content determines the hardness of the core and also affects the effective hardened layer depth. Therefore, in the present invention, the lower limit of the C amount is set to 0.1% to ensure the core hardness. However, if the content is too large, the toughness decreases, so 0.3% was made the upper limit.

Si: 0.45-0.9%
Si is an element effective for improving the temper softening resistance, and an effect can be obtained by adding 0.45% or more. Therefore, in the present invention, the lower limit of the Si amount is set to 0.45%. However, if its content exceeds 0.9%, cold forgeability deteriorates, so 0.9% was made the upper limit.

Mn: 0.2 to 0.5%
Mn is an element effective for improving hardenability, and is also an element effective for improving temper softening resistance. Furthermore, it has the effect | action which makes it harmless by fixing S which is an impurity element inevitably mixed in steel as MnS. Therefore, it is considered that 0.2% or more is necessary as the amount of Mn. Therefore, in the present invention, the lower limit of the amount of Mn is set to 0.2%. However, if the content exceeds 0.5%, cold forgeability deteriorates, so 0.5% was made the upper limit.

S: 0.005-0.05%
S is an impurity element inevitably mixed in, but it is necessary to contain 0.005% or more from the viewpoint of machinability. Therefore, in the present invention, the lower limit of the amount of S is set to 0.005%. However, if its content exceeds 0.05%, forgeability is impaired, so 0.05% was made the upper limit.

Cr: Less than 1.0 to 1.5% Cr is an element effective for improving the temper softening resistance, and is important in the present invention because the cold forgeability is improved by application of the spheroidizing annealing treatment. It is an element. In order to sufficiently obtain the effect of temper softening resistance, 1.0% or more is necessary. Therefore, in the present invention, the lower limit of the Cr amount is set to 1.0%. However, if the content is 1.5% or more, the carbides produced during the spheroidizing annealing process become coarse and stable, and are completely solidified in the steel even during the subsequent surface hardening treatment such as carburizing quenching and tempering. On the contrary, the temper softening resistance deteriorates due to remaining without being dissolved, so the content was made less than 1.5%.

Mo: 0.3 to less than 1.5% Mo is an element effective for improving the temper softening resistance, and is important in the present invention because the cold forgeability is improved by application of the spheroidizing annealing treatment. It is an element. In order to sufficiently obtain the effect of temper softening resistance, 0.3% or more is necessary. Therefore, in the present invention, the lower limit of the Mo amount is set to 0.3%. However, if the content is 1.5% or more, the carbides produced during the spheroidizing annealing process become coarse and stable, and are completely solidified in the steel even during the subsequent surface hardening treatment such as carburizing quenching and tempering. On the contrary, the temper softening resistance deteriorates due to remaining without being dissolved, so the content was made less than 1.5%.

Al: 0.001 to 0.2%
Since Al has an effect of refining crystal grains by forming a compound with N, 0.001% or more is necessary. Therefore, in the present invention, the lower limit of the Al amount is set to 0.001%. However, if its content exceeds 0.2%, the machinability is remarkably impaired, so 0.2% was made the upper limit.

N: 0.003 to 0.03%
N is an element inevitably mixed, but 0.003% or more is necessary because there is an effect of crystal grain refinement by forming a compound with Al and N. Therefore, in the present invention, the lower limit of the N amount is set to 0.003%. However, if its content exceeds 0.03%, forgeability is remarkably impaired, so 0.03% was made the upper limit.

P: Restricted to 0.03% or less P is an impurity element which is inevitably mixed, and needs to be limited to 0.03% or less in order to segregate at the grain boundaries and reduce toughness. Therefore, in the present invention, the P content is limited to 0.03% or less.

  In addition to the above chemical components, Nb, Ti, and the like may be further added to the steel of the present invention for the purpose of further refinement of crystal grains and prevention of crystal grain coarsening. In this case, these elements are preferably contained in the following ranges that do not impair productivity such as hot rolling, hot forging, and cutting.

One or two of Nb: 0.2% or less and Ti: 0.2% or less Nb and Ti have the effect of crystal grain refinement by forming a compound with N. Of these, it is preferable to contain one or two of them. However, even if each element is contained in excess of 0.2%, the effect of crystal grain refinement is saturated and the economic efficiency is impaired, so 0.2% was made the upper limit.

Next, the total amount of Si, Mn, Cr and Mo in the steel of the present invention will be described.
In the present invention, it is a necessary condition that the total amount of Si, Mn, Cr and Mo is limited to 25 or less by the following formula. As described above, as a result of intensive research and development by the present inventors, the total amount of Si, Mn, Cr, and Mo is limited to 25 or less by the following formula, and the spheroidizing annealing treatment is performed. This is because it has been found that this is a necessary condition for ensuring the forgeability. The coefficient of each element of Si, Mn, Cr, and Mo is different on the left side of the following equation because the degree of contribution to the cold forgeability varies depending on the element.
12Si (%) + 25Mn (%) + Cr (%) + 2Mo (%) ≦ 25

Next, the reason for having a spheroidized structure is described.
Steel materials that are not subjected to spheroidizing annealing treatment, such as normalization treatment materials and annealing treatment materials, have a lot of cementite that exists in a lamellar shape, and require large deformation resistance when the cementite is divided during cold forging, Cold forgeability deteriorates. Therefore, it is necessary to spheroidize the cementite existing in a lamellar shape by applying a spheroidizing annealing treatment. Preferably, the degree of the spheroidized structure is satisfied in the section (No. 1 to No. 4) shown in FIG. The deformation resistance is preferably 700 MPa or less.

  Hereinafter, the present invention will be specifically described by way of examples. These examples are for explaining the present invention, and do not limit the scope of the present invention.

  The hot-rolled material having the chemical components shown in Table 1 is held at 760 ° C. for 400 minutes, then gradually cooled to 660 ° C. over 600 minutes, and then subjected to spheroidizing annealing treatment by allowing to cool, followed by machining. φ14 mm × 21 mm deformation resistance evaluation test pieces (test Nos. 1 to 18) were produced.

<Table 1>

  The deformation resistance was measured by applying a reduction of equivalent strain 1 at a strain rate of 10 / s using each of the deformation resistance evaluation test pieces manufactured as described above. Moreover, the structure | tissue observation of each test piece was performed with the optical microscope, and the grade of the spheroidization structure | tissue was classified according to the division (No.1-No.4) shown by the attached FIG. 1 of JISG3545.

  Next, using each hot-rolled material subjected to spheroidizing annealing as described above, a small roller test piece having a cylindrical portion with a diameter of 26 mm and a width of 28 mm (test No. 1 to No. 1) for a roller pitching fatigue test. 18) And after rolling the material upside down, large roller test pieces (test Nos. 1 to 18) having a diameter of 130 mm and a width of 18 mm were produced. In addition, Test No. In 13 and 14, since cold forging could not be performed, a roller pitching test using these test pieces was not performed. In addition, the surface hardening treatment of the small roller and large roller test pieces was conducted according to Test No. 1-3 and no. In Nos. 5 to 18, quenching was performed after gas carburizing treatment at 950 ° C. for 120 minutes, followed by tempering at 150 ° C. for 90 minutes. In addition, Test No. In No. 4, a gas carburizing treatment at 950 ° C. for 120 minutes and a carburizing and nitriding treatment at 860 ° C. for 30 minutes were sequentially performed, followed by quenching, followed by tempering at 150 ° C. for 90 minutes. Using the small roller and the large roller treated in this manner, a roller pitching fatigue test was performed under the conditions of a Hertz stress of 3000 MPa, a slip rate of 40%, and an ATF oil temperature of 80 ° C., and durability evaluation of the small roller was performed. The results are shown in Table 2.

<Table 2>

  As shown in Table 2, No. of the present invention example. The test pieces 1 to 12 exhibited a deformation resistance of less than 700 MPa, and were found to be excellent in cold forgeability. In addition, No. of the present invention example. It was revealed that the roller test pieces 1 to 12 have a life of 6 million times or more and have excellent pitching strength.

  In contrast, Comparative Example No. The specimens 13 and 14 had a deformation resistance of 700 MPa or more, and it was revealed that cold forging was not possible. This was thought to be because the total amount of Si, Mn, Cr and Mo exceeded 25 in the formula: 12Si (%) + 25 Mn (%) + Cr (%) + 2 Mo (%). Comparative Example No. In No. 13, it was considered that the Mn content was relatively higher than that of the examples of the present invention.

  Comparative Example No. Although the test piece of No. 15 shows a deformation resistance of less than 700 MPa, No. 15 in the comparative example. The 15 roller test pieces were found to have a short life of 5 million cycles or less. As a result of the investigation after the test, the comparative example No. In 15 roller test pieces, many residual Cr carbides were confirmed. From this, it was considered that if the amount of Cr is too large, the lifetime may be reduced.

  Further, Comparative Example No. 16, no. 17 and no. Although the test piece of 18 also exhibits a deformation resistance of less than 700 MPa, the comparative example No. 18 It was revealed that the 16-18 roller test pieces had a short life of 5 million times or less. From these results, it was considered that if the amounts of Si, Cr, and Mo are too small, the softening resistance may be lowered.

  From the above, in order to obtain excellent cold forgeability, the total amount of Si, Mn, Cr and Mo is 25 or less in the formula, 12Si (%) + 25Mn (%) + Cr (%) + 2Mo (%) In order to obtain an excellent temper softening resistance, it is necessary to set the amount of Mn within a certain range (at least 0.2 to 0.5%). It must be within a certain range (at least Si is 0.45 to 0.9%, Cr is 1.0 to less than 1.5%, Mo is less than 0.3 to 1.5%) It has been shown.

Claims (2)

% By mass
C: 0.1 to 0.3%
Si: 0.45 to 0.9%,
Mn: 0.2-0.5%
S: 0.005-0.05%
Cr: 1.0 to less than 1.5%,
Mo: 0.3 to less than 1.5%,
Al: 0.001 to 0.2%,
N: 0.003 to 0.03%
Containing
P: limited to 0.03% or less, the balance is iron and inevitable impurities,
12Si (%) + 25Mn (%) + Cr (%) + 2Mo (%) is 25 or less,
A steel for skin hardening excellent in cold forgeability and temper softening resistance, characterized by having a spheroidized structure. The steel material is further in mass%,
Nb: 0.2% or less,
The steel for case hardening excellent in cold forgeability and temper softening resistance according to claim 1, comprising one or two of Ti: 0.2% or less.