JPH04160135A - Steel for carburization - Google Patents

Abstract

PURPOSE:To improve pitting resistance of steel by specifying the chemical component compsn. of Cr steel and the growing state of carbide to precipitate fine spherical carbide on the surface part. CONSTITUTION:This steel for carburization contains, by weight %, 0.1-0.5 C, between >=0.05 and <0.5 Si, 0.3-1.5 Mn, <=0.03 S, <=0.02 P, 2-8 Cr, 0.015-0.06 Al, 0.005-0.020 N, <=0.002 O and the balance Fe. This steel is carburized to have >=2.0% surface carbon concn. and precipitation of fine spherical carbide of <=5mu average grain diameter by >=30% area proportion on the surface part. This steel has >=800HV surface hardness and largely improved pitting resistance.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to carburizing steel with excellent pitting resistance, and in particular, fine spherical carbides are precipitated in the surface layer by high carbon carburizing treatment, resulting in surface hardness and This invention relates to carburizing steel with significantly improved pitting resistance.

[Prior Art] Mechanical structural parts such as gears, shafts, and bearings used in automobiles and various industrial machines are required to have excellent fatigue resistance and wear resistance, and these are exemplified by JIS G 4104 and G 4105. Using chromium steel, chromium molybdenum steel, etc.
Manufactured by immersion R treatment.

In recent years, efforts have been made to make the above-mentioned parts smaller and lighter, and to increase their strength in order to cope with the increase in engine output. among them,
For mechanical structural parts such as gears and shafts, improvements in bending fatigue strength and pitting resistance are particularly required.

In order to improve the bending fatigue strength, it is effective to increase the compressive residual stress in the surface layer, as described in JP-A-1-30642.
Publication No. 1 states that by adjusting the chemical composition of the carburizing steel used to suppress the formation of an incompletely hardened layer on the surface layer after carburizing, and then applying strong shot peening, the bending fatigue strength is significantly increased. A method for improving this is disclosed.

On the other hand, increasing surface hardness is effective in improving pitting resistance, and high carbon carburizing treatment is a means of increasing surface hardness. In other words, if the carbon content in the surface layer of the carburized material is increased from the usual 0.8 to 1.0% to a high concentration of about 2 to 3%, a large amount of carbide will be generated in the martensite matrix in the surface layer of the carburized material. By dispersing it, surface hardening of the carburized material can be expected.

However, when high-carbon carburization is performed using the above-mentioned JIS standards such as chromium steel or chrome-molybdenum steel, the carbides in the carburized layer are not formed into a spherical shape that is effective for surface hardening, but are precipitated in a network shape. There are problems in that quench cracking is more likely to occur and pitting resistance is reduced.

[Problems to be Solved by the Invention] The present invention has been made in view of the above circumstances, and is a carburizing process that can significantly improve pitting resistance by precipitating fine spherical carbides on the surface layer through high carbon carburizing treatment. The aim is to provide steel for industrial use.

[Means for Solving Problem A] The present invention that achieves the above object is as follows: C: 0.1 to 0.5% Si: 0.05% or more and less than 0.5% Mn:
0.3-1.5% S: 0.03% or less P: 0.02% or less Cr: 2-8% AI: 0. (115~0.(16%N: 0
．． 005-0.020% 0: Contains 0.002% or less, the remainder consists of Fe and unavoidable impurities, and by carburizing so that the surface carbon concentration is 2.0% or more, the surface layer has an average grain size. The gist is that fine spherical carbides with a diameter of 5 μm or less are precipitated in an area ratio of 30% or more, and in addition to the above components, Ni: 0.5 to 4% Mo: 0.05 to 1.0 % V: 0.1 to 2.0% Nb: 0.01 to 0.5%, a greater improvement in pitting resistance can be expected.

[Function] As a result of intensive research, the present inventors have found that fine spherical carbides can be precipitated on the surface layer simply by high carbon carburizing and tempering treatment.
We have invented a carburizing steel that has a surface hardness of HV800 or higher and has significantly improved pitting resistance.

The reasons for limiting each component will be explained below.

C: It is necessary to add 0.1% or more of C in order to ensure the hardness of the steel material, but if it exceeds 0.5%, the hardness becomes excessive), the toughness decreases, and the Since the rigidity also decreases, the C content was limited to 0.1 to 0.5%.

Si: Si is an effective element for deoxidizing molten steel, and is 0.05
% or more, but if it exceeds 0.5%, the carburizability is inhibited, the surface carbon concentration decreases, the formation of carbides is inhibited, and the grain boundary oxidation layer becomes deep, resulting in a decrease in bending fatigue strength. Therefore, the Si content was limited to 0.05% or more and less than 0.5%.

Mn: An element effective in deoxidizing molten steel and improving hardenability, and is added in an amount of 0.3% or more, but if it exceeds 1.5%, the hardenability becomes excessive and the hardness increases. It reduces the toughness and stiffness. Therefore, the Mn content is 0
．． It was limited to 3-1.5%.

S: S exists as a sulfide inclusion in steel and is an effective element for improving stiffness. However, S content is 0.0
If it exceeds 3%, the MnS acts as a starting point to cause pitting and deteriorate toughness, so the upper limit was set at 0.03%.

P: P is an element that reduces toughness, and its content needs to be reduced as much as possible. Therefore, the P content was limited to 0.02% or less.

Cr: Cr is an effective element for improving the hardness and forming spherical carbides to ensure high surface hardness when high carbon carburizing treatment is performed. However, Cr
If the Cr content is less than 2%, the above effect is insufficient, while if it is added in excess of 8%, the effect is saturated, so the Cr content was limited to 2 to 8%.

Al: Al is an element effective in deoxidizing and refining crystal grain size. If the A1 content is less than 0.015%, this effect is insufficient, and if it exceeds 0.06%, the effect of refining the grain size is saturated. Therefore, the A1 content is 0.015~
It was limited to 0.06%.

NUN is an element that combines with A1, V, etc. to form nitrides and refines the crystal grain size, and is required to be 0.005% or more. However, the effect is saturated even if it is added in excess of 0.020%, so the N content was limited to o, oos to 0.020%.

o: o combines with A1 and SL to form oxide inclusions (
A1205.5in2) is a harmful element that produces
When the O content increases, hard oxide-based inclusions are generated, which can become the starting point for fatigue failure and have a negative impact on rigidity.
Therefore, the upper limit of the 0 content was set to 0.002%.

Ni: Ni is an element that is effective in increasing the toughness of the carburized layer and ensuring the hardenability of the carburized layer and the 6 parts, and it is desirable to add 0.5% or more, but there is no effect even if it is added in excess of 4%. is saturated, the Ni content is preferably 0.5 to 4%.

Mo: Mo is an effective element for greatly increasing the hardenability of a carburized layer and suppressing the formation of an incompletely hardened layer, and is also a carbide-forming element, forming a composite carbide with Cr to form a carbide. This increases the hardness of the carburized material and further increases the surface hardness of the carburized material. Therefore, it is desirable to add 0.05% or more, but the effect is saturated even if it is added in excess of 1.0%, so the Mo content is preferably 0.05 to 1.0%.

V, Nb: V and Nb combine with C and N in steel to form carbonitrides, and are effective elements for refining grains and increasing toughness. Since it forms carbides and increases surface hardness, ■ is 0.1% or more, N
It is desirable to contain b in an amount of 0.01% or more. However, if the V content exceeds 2.0% and the Nb content exceeds 0.5%, these effects will be saturated, so the V content should be between 0.1 and 0.1%.
2.0%, Nb content is 0. It is desirable that O is 1 to 0.5%.

Next, the state of carbide formation in high carbon carburized materials will be explained.

If the surface carbon concentration is less than 2.0%, the amount of carbide produced is small and a high hardness layer cannot be obtained. In addition, the generated carbides precipitate in a mesh pattern, making it easier to cause quench cracks and lowering the spalling resistance. Therefore, the surface carbon concentration was limited to 2.0% or more.

If the average grain size of the carbide in the surface layer portion exceeds 5 μm, fatigue properties will be reduced, so the average grain size of the carbide was limited to 5 μm or less.

Furthermore, if the area ratio of carbides is less than 30%, the surface hardness will not increase and no improvement in pitting resistance will be observed.
% or more.

[Example] As shown in Table 5-5, various steel materials with different chemical compositions were melted in a small furnace, hot forged and normalized, then machined to 25+++IlφX100mm, and high carbon immersion was performed. After charcoal quenching and IA treatment, a test piece was obtained. Regarding the test piece, carbon concentration in the surface layer, hardness, and 0.0 from the surface.
A pitting test was conducted to investigate the formation of carbides on the 5m+++.

The results are shown in Table 2.

In the above-mentioned high carbon carburizing and quenching treatment, the machined specimen was oil quenched at 870°C for 5 hours in a carburizing atmosphere with an equilibrium carbon concentration of 2.0% or more, and then oil quenched at 180°C for 2 hours.
This was carried out by holding for a certain period of time and subjecting it to tempering treatment.

In addition, the pitching test was conducted using a cylindrical test piece (70 φ x 4 m
m), rotation speed 300 Orpm, slip rate -4
Evaluation was made based on the number of repetitions until pitching occurred under test conditions of 0% 1 surface pressure and 350 kgf/mm.

Nos. 011 to 15 are examples according to the present invention, in which the surface carbon concentration is 2% or more, spherical fine carbides with an average particle size of 5 μm or less are dispersed at an area ratio of 30% or more, and the surface hardness is It has a high HV of 800 or more, and has excellent pitting resistance.

Nos. 16 to 22 are comparative examples in which one or more of the chemical components according to the present invention are not satisfied. No
, 16 to 19 are comparative examples in which the Cr content is too low, and the surface carbon concentration is low at less than 2%, and the carbides are formed in a network shape and the area ratio is low. Therefore, the surface hardness is HV750. The pitting resistance is also less than one-fifth that of the steel of the present invention. No. 20 is a case where the Cr content is high, and the improvement effect of pitting resistance is that of invention steel N004.
is hardly recognized compared to In addition, No. 21.22 is the case where the S, 0 content is high, and the pitting resistance is the inventive steel N.
o, inferior to 1.

Comparative Examples Invention Steel 2.6, 12.13 and Comparative Steel 1 shown in Table 1
．． The six types of steel materials in 2 were used in a carburizing atmosphere with an equilibrium carbon concentration of 1.2%, except that they were oil quenched at 925°C for 3 hours and tempered at 180°C for 2 hours. A test piece was obtained in the same manner as in Example 1, and the carbon concentration and hardness of the surface layer and the state of carbide formation within 0.05 mm from the surface were examined, and a pitting test was conducted.

The results are shown in Table 3.

Nos. 31 to 36 are comparative examples in which carburizing treatment was not performed in a high carbon atmosphere of 2.0% or more, and all had surface carbon concentrations of 2% or less and low surface hardness.

Nos. 31 to 34 used the invention steels, but the pitching properties were poor because the fine spherical carbides were not dispersed and the carbide area ratio was low.

No. 35.36 uses comparative steel, and no carbides are formed and the pitting resistance is significantly inferior.

[Effects of the Invention] Since the present invention is configured as described above, it is possible to provide carburizing steel that can significantly improve pitting resistance by precipitating fine carbides in the surface layer through high carbon carburizing treatment. .

Claims (2)

[Claims] (1) C: 0.1 to 0.5% (meaning of weight %, same below) Si: 0.05% to less than 0.5% Mn: 0.3 to 1.5% S: 0.03% Contains the following: P: 0.02% or less Cr: 2-8% Al: 0.015-0.06% N: 0.005-0.020% O: 0.002% or less, the balance being Fe and unavoidable impurities By performing carburizing treatment so that the surface carbon concentration is 2.0% or more, fine spherical carbides with an average particle size of 5 μm or less are precipitated on the surface layer in an area ratio of 30% or more. Steel for carburizing. (2) C: 0.1-0.5% Si: 0.05% or more and less than 0.55 Mn: 0.3-1.5% S: 0.03% or less P: 0.02% or less Cr: Contains 2 to 8% Al: 0.015 to 0.06% N: 0.005 to 0.020% O: 0.002% or less, and Ni: 0.5 to 4% Mo: 0.05 to Contains one or more selected from the group consisting of 1.0% V: 0.1-2.0% Nb: 0.01-0.5%, and the balance is Fe.
and unavoidable impurities, and the surface carbon concentration is 2.0%.
A steel for carburizing, characterized in that it is carburized to achieve the above-mentioned characteristics, so that fine spherical carbides with an average grain size of 5 μm or less are present in the surface layer in an area ratio of 30% or more.