JP2017222924A - Steel for shaft bearing excellent in machinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for shaft bearing excellent in machinability in a structure at an annealing state or a spheroidizing annealing state by optimizing components such as C or Cr.SOLUTION: There is provided a steel containing, by mass%, C:0.60 to 0.90%, Si:0.10 to 0.35%, Mn:0.80 to 2.00%, P:0.030% or less, S:0.030% or less, Cr:0.70 to 1.20% and the balance Fe with inevitable impurities, having a structure after annealing, which is a spheroidized structure where a spheroidized carbide is dispersed in a perlite structure or a ferrite structure, or the perlite structure and a spheroidized structure, area percentage of carbide in the steel of 30% or less, lamellar spacing of perlite of 0.40 μm or more in the case of the perlite structure, carbide spacing of 0.30 μm or more in the case of the spheroidized structure and satisfying both in the case of a mixed phase thereof and excellent in machinability after annealing.SELECTED DRAWING: None

Description

  The present invention relates to a bearing steel having excellent machinability in an annealed state.

  Machine parts of many industrial machines such as automobiles are manufactured by a cutting process from the viewpoint of manufacturing efficiency, and the required accuracy is satisfied. In particular, in the processing of bearing parts, JIS G 4805 SUJ2 can be cited as a representative bearing steel. When processing SUJ2 into a bearing component, for example, a bearing race, a process of starting production of a raw steel material from hot forging or warm forging and a process of starting from direct cutting are common.

  By the way, in the process starting from this direct cutting, the microstructure and hardness of the material delivered from the steelmaker to the machining maker have a great influence on the machinability. For example, since SUJ2 is a hypereutectoid steel, the structure after hot rolling becomes cementite + lamellar pearlite structure, and the hardness is very hard. When cutting or drilling with a lathe, the tool life is shortened. It is short and has a problem in manufacturability in mass production.

  Therefore, as a method for improving the machinability of the steel material, addition of free-cutting elements such as Pb and S can be mentioned. Pb exists as a low-melting-point metal inclusion, makes it easy to break up the chips due to the molten metal embrittlement action, and further shows a lubricating action at the interface between the chips and the tool surface, reducing cutting resistance and extending tool life. Extend (see, for example, Patent Document 1). Furthermore, MnS becomes a stress concentration source and exhibits a notch effect, which also improves chip disposal and tool life (see, for example, Patent Document 2). However, in the methods of Patent Document 1 and Patent Document 2, these free-cutting materials such as Pb and S are used in parts that are used in a high strength region such as bearing steel and require a rolling fatigue life. And the like exist, the part acts as a defect, and there is a problem that the life of the part is reduced.

JP 05-255810 A JP 2009-120955 A

  In general, bearing steel is higher in carbon than machine structural steel, so it is hard even after annealing, and machinability often becomes a problem. Therefore, the problem to be solved by the present invention is to provide a bearing steel that is excellent in machinability in a structure in an annealed state or a spheroidized annealed state by optimizing components such as C and Cr. is there.

  Therefore, the problem to be solved by the present invention is a bearing steel excellent in machinability by adding a free cutting element such as Pb, S, etc., and optimizing the chemical composition of the steel material and controlling the dispersion state of carbides. Is to provide.

  The first means for solving the above problems is mass%, C: 0.60 to 0.90%, Si: 0.10 to 0.35%, Mn: 0.80 to 2.00%. , P: 0.030% or less, S: 0.030% or less, Cr: 0.70 to 1.20%, with the balance being Fe and inevitable impurities, the structure after annealing is pearlite A spheroidized structure in which spheroidized carbides are dispersed in a structure or a ferrite structure, or a pearlite structure and a spheroidized structure thereof. In the case of 0.30 μm or more and these mixed phases, both are satisfied, and the steel is excellent in machinability after annealing.

  In addition to the first means, the second means includes one or more of Ni: 0.10 to 2.00% and Mo: 0.05 to 0.50% in mass%, The remainder is steel composed of Fe and inevitable impurities, and the annealed structure is a spheroidized structure in which spheroidized carbides are dispersed in a pearlite structure or a ferrite structure, or its pearlite structure and spheroidized structure. In the case of a pearlite lamellar spacing of 0.40 μm or more, in the case of a spheroidized structure, the carbide spacing is 0.30 μm or more, and in the case of these mixed phases, both are satisfied, and the steel is excellent in machinability after annealing.

A bearing steel having excellent machinability in annealed state, if the pearlite structure lamellar spacing D _r of pearlite than 0.40 .mu.m in the case of spheroidized structure is the average distance between particles D _theta carbide 0 In the case of these mixed phases of 30 μm or more, both are satisfied, and the steel for bearings is excellent in machinability after annealing.

  Prior to the description of the mode for carrying out the invention of the present application, first, components such as chemical components and tissues of the present invention will be described. In addition,% in a chemical component is the mass%.

C: 0.60-0.90%
C is an element that improves hardness, wear resistance and fatigue life after quenching and tempering. However, if C is 0.55% or less, sufficient hardness cannot be obtained, and C needs to be 0.60% or more. By the way, when C increases, the hardness of the material increases, the amount of carbide in the structure increases more than necessary, and the area ratio of carbide increases, so that workability such as machinability and forgeability is hindered. Therefore, C needs to be 0.90% or less. From the above, C is 0.60 to 0.90%, preferably C is 0.60 to 0.80%.

Si: 0.10 to 0.35%
Si is an element effective for deoxidation of steel, and is an element that imparts the hardenability necessary for steel and increases strength. In order to obtain these effects, Si needs to be 0.10% or more. Desirably, Si is preferably 0.20% or more. On the other hand, when Si increases, the hardness of the material increases and the workability such as machinability and forgeability is hindered. Therefore, Si needs to be 0.35% or less. Desirably, Si is 0.30% or less. Therefore, Si is 0.10 to 0.35%, preferably Si is 0.20 to 0.30%.

Mn: 0.80 to 2.00%
Mn is an element effective for deoxidation of steel. Further, it is added to impart necessary hardenability to the steel and increase strength. For this reason, Mn is required to be 0.80% or more, desirably 0.90% or more. On the other hand, when Mn increases, the material hardness increases and the workability such as machinability and forgeability is hindered. Therefore, Mn needs to be 2.00% or less, preferably 1.60% or less. Therefore, Mn is 0.80 to 2.00%, preferably 0.90 to 1.60%.

P: 0.030% or less P is an impurity element inevitably contained in the steel, and segregates at the grain boundaries to deteriorate toughness. Therefore, P is set to 0.030% or less, preferably 0.015%. The following is good.

S: 0.030% or less S is an element that combines with Mn to form MnS and improves machinability. However, excessive addition of S coarsely forms MnS and lowers the rolling fatigue life. Therefore, S is set to 0.030% or less, preferably 0.010% or less.

Cr: 0.70 to 1.20%
Cr is an element that improves hardenability. Further, it is an element that facilitates spheroidization of carbides by spheroidizing annealing. In order to obtain the above effects, Cr needs to be 0.70% or more, preferably 0.80% or more. However, when Cr is added excessively, the amount of carbide increases and machinability is lowered. Therefore, Cr needs to be 1.20% or less, preferably 1.10% or less. Therefore, Cr is made 0.70 to 1.20%, preferably 0.80 to 1.10%.

Ni: 0.10 to 2.00%
Ni is an element effective for improving hardenability and toughness. In order to obtain the above effects, Ni needs to be 0.10% or more. However, when Ni is increased, the material hardness increases, and the workability such as machinability and forgeability is hindered. Furthermore, Ni is an element that increases the cost. Therefore, Ni is made 2.00% or less. Therefore, Ni is set to 0.10 to 2.00%.

Mo: 0.05 to 0.50%
Mo is an element effective for improving hardenability and toughness. To obtain the above effect, Mo needs to be 0.05% or more. However, when Mo is increased, the material hardness increases, and workability such as machinability and forgeability is hindered. Furthermore, Mo is an element that increases costs. Therefore, Mo is 0.50% or less. Therefore, Mo is set to 0.05 to 0.50%.

Area ratio S _θ of carbide in steel: 30% or less As the amount of carbide in steel increases, material hardness increases and machinability decreases. Therefore, the area ratio S _theta carbides in the steel was 30% or less.

Lamella spacing D _r (in the case of pearlite structure): 0.40 μm or more The lamellar spacing of pearlite has a correlation with the cooling rate of pressure support or the cooling rate of normalization. The slower the cooling rate, the larger the pearlite lamella spacing. Tend to be. Moreover, machinability improves as the lamella spacing increases. For this reason, the lamella spacing is set to 0.40 μm so that sufficient machinability can be obtained.

Average interparticle distance D _θ of carbides in steel (in the case of spheroidized structure): 0.30 μm or more Spheroidizing annealing is performed to cementite spheroidize, and the structure becomes ferrite + spheroidized cementite. The machinability can be improved by lowering. However, if the spheroidized carbide is too finely dispersed and the average particle spacing of the spheroidized carbide is reduced, the machinability is deteriorated. Therefore, the average distance between particles D _theta carbide, was more than 0.40μm obtained sufficient machinability.

In the case of a mixed phase of a pearlite structure and a spheroidized structure: a lamellar spacing D _{r of} 0.40 μm or more and an average interparticle distance D _{θ of} carbide of 0.30 μm or more are the same as the reasons for the two structures immediately above, and the lamellar spacing D _r is A sufficient machinability of 0.40 μm or more was obtained, and the average interparticle distance D _θ of the carbide was 0.30 μm or more of sufficient machinability.

表１に示す実施例鋼１〜１０と比較例鋼１１〜１８の化学組成の鋼を電気炉にて溶製し、連続鋳造後、分解圧延、続いて熱間圧延を行い、圧延材からドリル穿孔試験用の試験片を採取した。その後、焼ならしもしくは球状化焼ならしを行った後、硬さ試験、光学顕微鏡観察、ならびにドリル穿孔試験を行った。なお、焼ならしは、加熱保持温度を８００〜８６０℃とし、かつ６００℃までの徐冷速度を３７℃／Ｈｒで冷却とした。球状化焼きなましは、加熱保持温度を７２０〜７８０℃とし、かつ６００℃までの徐冷速度を２３℃／Ｈｒ以下で冷却とした。焼なまし後は、全鋼種において全面パーライト組織になっていることを確認した。また、球状化焼なまし後は、全鋼種において球状化炭化物がフェライト中に分散する球状化組織であることを確認した。ドリル穿孔試験の条件は以下の表２に示す。

Example steels 1 to 10 and comparative steels 11 to 18 shown in Table 1 having chemical compositions are melted in an electric furnace, continuously cast, then subjected to decomposition rolling, followed by hot rolling, and drilled from the rolled material. Test specimens for the drilling test were collected. Then, after normalizing or spheroidizing normalizing, a hardness test, an optical microscope observation, and a drill drilling test were performed. In normalization, the heating and holding temperature was 800 to 860 ° C., and the slow cooling rate to 600 ° C. was cooled at 37 ° C./Hr. In the spheroidizing annealing, the heating and holding temperature was 720 to 780 ° C., and the slow cooling rate to 600 ° C. was 23 ° C./Hr or less. After annealing, it was confirmed that all steel types had a full pearlite structure. In addition, after spheroidizing annealing, it was confirmed that the spheroidized carbide was dispersed in ferrite in all steel types. The conditions of the drilling test are shown in Table 2 below.

焼なましを行ったものを焼なまし材、球状化焼なましを行ったものを球状化焼なまし材と定義して、以上の硬さ試験、光学顕微鏡観察、ドリル穿孔試験の結果として、硬さ、炭化物の面積率Ｓ_θ、パーライトのラメラ間隔Ｄ_r、炭化物の平均粒子間距離Ｄ_θ、異常音発生までの穴数について、以下の表３の焼なまし材および表４の球状化焼なまし材に記載する。

As a result of the above hardness test, optical microscope observation, drill drilling test, the annealed material is defined as the annealed material and the spheroidized material is defined as the spheroidized annealed material. , Hardness, carbide area ratio S _θ , pearlite lamellar spacing D _r , carbide average interparticle distance D _θ , and number of holes until abnormal noise occurs, annealing material in Table 3 below and spherical shape in Table 4 Described in chemical annealing material.

表３の焼なまし材において、比較例鋼のＮｏ．１１〜１８の網掛けをしている部分は、本願の請求項から外れているものである。本願の請求項を満たす実施例鋼では、いずれもドリル穿孔試験における工具寿命を示す穴数は焼なまし材では２５個以上であり、比較例鋼に対して良好な被削性を示した。

In the annealed material of Table 3, the comparative steel No. The shaded portions 11 to 18 are not included in the claims of the present application. In each of the example steels satisfying the claims of the present application, the number of holes indicating the tool life in the drill drilling test was 25 or more in the annealed material, and good machinability was exhibited with respect to the comparative example steel.

表４の球状化焼なまし材いて、比較例鋼のＮｏ．１１〜１８の網掛けをしている部分は、本願の請求項から外れているものである。本願の請求項を満たす実施例鋼では、いずれもドリル穿孔試験における工具寿命を示す穴数は球状化焼なまし材では３０個以上であり、比較例鋼に対して良好な被削性を示した。

The spheroidizing annealing materials shown in Table 4 show the No. of the comparative steel. The shaded portions 11 to 18 are not included in the claims of the present application. In all of the example steels satisfying the claims of the present application, the number of holes indicating the tool life in the drill drilling test is 30 or more in the spheroidizing annealed material, and shows good machinability compared to the comparative example steel. It was.

Claims (2)

  In mass%, C: 0.60 to 0.90%, Si: 0.10 to 0.35%, Mn: 0.80 to 2.00%, P: 0.030% or less, S: 0.030 % Or less, Cr: 0.70 to 1.20%, with the balance being Fe and inevitable impurities, the microstructure is either a pearlite structure, or a spheroidized structure in which spheroidized carbides are dispersed in a ferrite structure The area ratio of carbides in steel is 30% or less, the pearlite lamella spacing in the pearlite structure is 0.40 μm or more, and the carbide spacing in the spheroidized structure is 0.30 μm or more. Steel for bearings with excellent machinability.   In addition to the chemical component of claim 1, in mass%, Ni: 0.10 to 2.00%, Mo: 0.05 to 0.50%, or one or more of them are contained, and the balance is Fe And a steel composed of unavoidable impurities, the microstructure is pearlite structure, the spheroidized structure in which the spheroidized carbide is dispersed in the ferrite structure, or the mixed phase of two structures, and the area ratio of carbides in the steel is A bearing steel with excellent machinability, characterized by 30% or less, a pearlite lamella spacing in a pearlite structure of 0.40 μm or more, and a carbide spacing in a spheroidized structure of 0.30 μm or more.