JPS59182952A - Case hardening steel - Google Patents

Abstract

PURPOSE:To provide a case hardening steel consisting of specified amounts of C, Si, Mn, Ni and Mo and the balance Fe with impurities and having superior properties of preventing the formation of an abnormal surface layer during carburizing or carbonitriding. CONSTITUTION:This case hardening steel consists of, by weight, 0.1-0.4% C, 0.02-0.15% Si and <=2% Mn as essential components, 0.3-5% Ni and/or 0.05- 0.5% Mo and the balance Fe with impurities. The steel has superior properties of preventing the formation of an abnormal surface layer during carburizing or carbonitriding, and case hardened parts having high quality and high fatigue strength can be obtd. In order to improve the cold workability or to increase the strength furthermore, it is preferable to regulate the amounts of said impurities to <= about 0.015% S, <= about 0.015% P, <= about 0.01% N and <= about 0.002% O.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a case hardening steel that is excellent in preventing abnormal surface layer formation during carburizing and carbonitriding.

BACKGROUND ART Conventionally, structural parts such as gears, connecting rods, pinions, and racks are often subjected to a case hardening treatment such as carburizing or carbonitriding in order to increase the wear resistance and fatigue strength of their surfaces. Materials for such parts include SC material, SCr material, SCM material, and SM material specified in JIS.
Structural steels such as n-material, SNC material, and SNCM material are normally used, but when parts made of conventional structural steel are carburized or carbonitrided, an abnormal layer occurs on the surface. However, there was a problem in that the fatigue strength could be significantly reduced. This abnormal surface layer is thought to be caused by grain boundary oxidation on the surface. When this grain boundary oxidation occurs, the hardenability improving elements present near the grain boundaries are oxidized, reducing the hardenability near the grain boundaries. This reduces surface residual stress and reduces fatigue strength.

Therefore, in conventional cases, after surface hardening treatments such as carburizing and carbonitriding, surface treatments such as lapping are performed to remove the abnormal surface layer, or parts are cooled during quenching after carburizing and carbonitriding. The speed was increased to prevent the formation of an abnormal layer on the surface.

However, in the former case, the number of steps required for surface processing such as lapping increases, and it is difficult to apply to parts with complex shapes.In the latter case, heat treatment distortion occurs due to the high cooling rate. This has resulted in problems such as increased size and reduced dimensional accuracy.

Therefore, measures have been taken to prevent the formation of an abnormal surface layer by performing vacuum carburizing and surface hardening in an atmosphere that does not contain oxygen during the nascent stage.

However, vacuum carburizing or vacuum carbonitriding must be a batch process due to the type of furnace, and continuous production is not possible, resulting in a problem of low production efficiency. Therefore, it has been desired to develop a case hardening steel that is less likely to produce an abnormal surface layer during normal carburizing or carbonitriding treatment.

This invention was made by focusing on such conventional problems, and even when surface hardening treatment such as gas carburization or gas carbonitriding treatment is performed, the occurrence of an abnormal layer on the surface is small. The purpose is to provide case-hardened steel.

The case hardening steel according to the present invention has basic content in weight percent of C: 0.1% or more and 0.4% or less, Si: 0.02% or more and J2 less than 0.15%, and Mn: 2% or less. And furthermore, Ni:
0.3% or more and 5% or less.

Contains one or two Mo: 0.05% or more and 0.5% or less, and if necessary, cr: 0.3 in order to control hardenability and improve strength. % or more”24% or less,
B:O,0O05% or more and 0.005% or less, one or two types, Pb:O,01% to improve machinability
0.3% or less, S: 0.03% or more and 0.2% or less, Cu: 0.03% or more and 0.2% or less, Cu:
0.5% or more and 3% or less, ca: 0.0005% or more and 0.0005% or more to improve machinability by controlling the form of inclusions.
005% or less, Zr: 0.02% or more and 0.5% or less, S
e: 0.01% or more and 0.3% or less, Te: 0.001%
In order to improve strength by grain refinement and precipitation hardening, V: 0.02% or more, J2O, 5% or less, Ti: 0.03% or less. 01
% or more and 0.3% or less, Nb+Ta: 0.01% or more and 0
．． One or more types of 3% or less; A statement: 0.01% or more; 0.01% or more; in order to improve strength by controlling grain size;
1% or less, N: 0.01% or more and 0.03% or less, and the remainder is Fe and impurities. Furthermore, in this impurity, in order to improve cold workability or further improve strength, S: 0.015% or less, P: 0.0
15% or less, N: 0.01% or less, (0): 0.002
It is characterized by being appropriately regulated below %.

Next, the reason for limiting the composition range (wt%) of the case hardening steel which is excellent in preventing the formation of an abnormal layer on the surface during carburizing or carbonitriding according to the present invention will be explained.

C is an element added to ensure the strength required for structural parts or products, especially the core strength, but 0.
If it is less than 1%, such effects cannot be sufficiently obtained, and if it exceeds 0.4%, the toughness deteriorates, so the range is set to be 0.1% or more and 0.4% or less.

St is an element that acts as a deoxidizing agent during steel manufacturing, and in order to obtain such an effect, it is necessary to contain it in an amount of 0.02% or more. However, if it is too large, it may cause an abnormal surface layer to be generated during carburizing or carbonitriding treatment, so it is necessary to keep it below 0.15%.

Mn acts as a deoxidizing agent and desulfurizing agent during steel manufacturing, and is an effective element for improving the hardenability of steel and improving its strength, but if it is present in too much, it impairs cold workability. Therefore, it is necessary to keep it below 2%.

Nf and Mo are elements that have the effect of preventing the generation of a surface abnormal layer during carburizing or carbonitriding treatment,
In order to obtain such an effect, the Ni content must be 0.3%.
Second, it is necessary to contain Mo in an amount of 0.05% or more. However, if Ni is too large, retained austenite will be generated, so it must be kept at 5% or less, and M
If there is too much o, the toughness deteriorates, so it needs to be kept at 0.5% or less.

Cr and B are effective elements for controlling the hardenability of steel and increasing its strength, and one or two of these may be added depending on the purpose of use.

In this case, if Cr is less than 0.3%, the above-mentioned effects will not be sufficient and strength cannot be expected to be secured very much, and if it exceeds 4%, toughness will deteriorate, so it should be in the range of 0.3% to 4%. It's good to do that. On the other hand, if B is less than 0.0005%, the above-mentioned effects will not be sufficient and strength cannot be expected to be secured very much, and even if it exceeds 0.005%, the effect will not increase so much, so 0.0005% or more The range is preferably 0.005% or less.

Since both Pb and S are effective elements for improving the machinability of steel, it is also good to add one or two of these as appropriate. In this case, in order to obtain such an effect, it is preferable to add 0.01% or more of Pb and 0.03% or more of S. However, if Pb exceeds 0.3%, hot workability deteriorates, which is undesirable, and if S exceeds 0.2%, hot workability deteriorates, which is undesirable.

Cu is an element that contributes to improving corrosion resistance and weather resistance, and in order to obtain such effects, it is good to include it in an amount of 0.5% or more. However, if it exceeds 3%, toughness and hot workability deteriorate, which is not preferable.

Ca, Zr, Se, and Te are all effective elements for controlling the morphology of inclusions and improving machinability, so it is also good to add one or more of these as appropriate. . In this case, in order to obtain such an effect, Ca must be 0.
．． 0005% or more, Zr is 0.02% or more, and Se is 0.005% or more.
0.01% or more, and Te is preferably added in an amount of 0.001% or more. However, if Ca exceeds 0.005%, toughness deteriorates, which is undesirable. If Zr exceeds 0.5%, toughness deteriorates, which is undesirable. If Se exceeds 0.3%, toughness deteriorates. If Te exceeds 0.03%, hot workability deteriorates, which is not preferable.

Since V, Ti, Nb, and Ta are all effective elements for improving strength by refining crystal grains and precipitation hardening, it is also good to add one or more of these as appropriate. In this case, in order to obtain such an effect, ■ should be 0.02% or more, Ti should be 0.01% or more, and Nb+
It is preferable to add Ta in an amount of 0.01% or more. However, if ■ exceeds 0.5%, it is not preferable because the toughness deteriorates.
If Ti exceeds 0.3%, toughness deteriorates, which is undesirable, and if Nb+Ta exceeds 0.3%, toughness deteriorates, which is not preferable.

Since All and N are effective elements for controlling the crystal grain size and improving the strength, it is also good to add one or two of these as appropriate. In this case, in order to obtain such an effect, it is preferable that AM be 0.01% or more and N be 0.01% or more. However, if An exceeds 0.1%, toughness deteriorates, and if N exceeds 0.03%, blowholes are likely to occur, which is not preferable.

Since 0 increases the amount of inclusions in the steel and deteriorates strength properties such as transfer fatigue and rotary bending fatigue, it is more desirable to limit it to 0.0020% or less.

Since P is an element that tends to cause segregation in steel ingots or continuous slabs, it is more desirable to limit it to 0.015% or less.

Since S increases the amount of inclusions in the steel and has a negative effect on cold plastic formability, it is more desirable to limit it to 0.010 or less.

Examples of the present invention will be described below along with comparative examples.

Steel with the chemical composition shown in the table below is melted in a 2.5 ton electric furnace, then formed into ingots, then bloomed and rolled into products.
Two types of rolling stock with diameters of 30 mm and 55+++n were produced. Subsequently, each rolled material was normalized at 925°C, and then the 30 mm diameter material was machined to a diameter of 25 mm, and the 55 mm diameter material was machined to a diameter of 50 m+n. Next, for each processed material, carburizing gas (Rx 10 enriched gas)
Heating in an atmosphere at 925°C for 5 hours → 100°C oil quenching → Carburizing and quenching at 180°c x 1180°c x 1 condition,
A tempering treatment was performed, and the abnormal layer depth of each treated material was measured by microscopic observation, and retained austenite (retained γ) M was measured by an X-ray method. These results are also shown in the table.

As shown in the table, in N091 to 4, which have too much St content, the occurrence of abnormal surface layer is large, and in N009, which has too much Ni content, the amount of retained austenite is large, resulting in a decrease in strength, and the amount of retained austenite is large, resulting in a decrease in strength. In cases where No. 11 is too high, the occurrence of abnormal surface layer becomes large. On the other hand, it is clear that in No. 5 to No. 8.10, which are within the chemical composition range of the present invention, the occurrence of surface abnormal layers is small and the amount of retained austenite is small.

As explained above, in the case hardening steel of the present invention, in weight percent, C: O is 1% or more and 0.4% or less, Si: 0.0
2% or more J-0.15% or less, Mn: 2% or less as basic components, and Ni: 0.3% or more and 5% or less, M
o: 0, Contains 0.5% or less of one or two of O'rb, and further contains Cr, B, Cu,
Pb,S.

Ca, Zr, Se, Te, V, Ti, Nb.

Contains elements such as Ta, AJlj, N, O, P.

Since we have regulated the content of N, S, etc., even when surface hardening treatments such as carburizing or carbonitriding3 are performed in a gas atmosphere, there is little occurrence of an abnormal layer on the surface, and the surface can be hardened in a vacuum. Since the generation of surface abnormal layers can be suppressed without any treatment, it is possible to significantly improve the performance of parts, reduce manufacturing costs of parts, and increase productivity. There is no need to perform surface processing such as lapping to remove the abnormal surface layer, and the generation of the abnormal surface layer can be suppressed without increasing the quenching speed after carburizing or carbonitriding, so heat treatment distortion can be suppressed. It is possible to reduce the occurrence of
It has the remarkable effect that various bare parts such as connecting rods, gears, and spindles can be obtained as parts with high quality and high fatigue strength.

Patent applicant: Daido Steel Co., Ltd. Representative Patent Attorney Yutaka Shio 4 =269

Claims (1)

[Claims] (1) In weight%, C: O, 1% or more and 0.4% or less, Si
: 0.02% or more J-0.15% or less, Mn = 2% or less, Ni: 0.3% or more and 5% or less, Mo: 0.05% or more and 0.5% or less A case hardening steel having an excellent property of preventing the formation of an abnormal layer on the surface during carburizing and carbonitriding treatment, which is characterized by containing one or two of these elements, with the remainder being Fe and impurities.