JPS6144159A - Steel for cold forging having superior suitability to carbonitriding - Google Patents

Abstract

PURPOSE:To obtain a steel for cold forging having superior suitability to carbonitriding by adding prescribed percentages of C, Si, Mn, Cr, Al and O while satisfying specified equations. CONSTITUTION:This steel for cold forging having superior suitability to carbonitriding consists of, by weight, 0.10-0.30% C, 0.05-0.35% Si, 0.30-1.50% Mn, <=2.5% Cr, 0.02-0.5% Al, <=0.0030% O and the balance Fe with inevitable impurities and satisfies equations I, II. The steel has low deformation resistance during working by cold forging, and when the steel is carbonitrided after the working, an effective hardened layer having >=800 hardness Hv is obtd. up to a large depth.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to cold forging steel with excellent carbonitriding properties, and more specifically, it has low deformation resistance during processing by cold forging, and has a surface of HV800 or higher by subsequent carbonitriding treatment. The present invention relates to a steel that provides hardness and a deep effective hardening layer depth, that is, a steel for cold forging that has excellent carbonitriding properties.

Although various surface hardening treatment methods for steel have been studied and put into practical use, gas carburizing is the most commonly employed method. However, in recent years, from the viewpoint of energy saving and reduction of heat treatment distortion, carbonitriding method in an ammonia atmosphere, which has a lower processing temperature than gas carburizing method, has been adopted. The carbonitriding method, which uses case-hardened steel as a carburizing material, has come to be widely used for small parts.

On the other hand, when it comes to forming small parts, conventionally they are formed by cutting before surface hardening treatment, but in recent years, cold forging has been used to simplify the process and improve product yield. This is becoming more common. In this case, if conventional case hardening steel is used, the tool life may be extremely shortened, so it is necessary to suppress the addition of chemical components such as C, Mn, Cr, etc. below a certain limit value.
If the amount is too small, sufficient surface hardness and effective hardened layer depth cannot be obtained by the carbonitriding treatment.

As a result of intensive research to solve the above-mentioned problems, the present inventors have determined that the main components C1Si, Mn, and Cr and the carbonitride-forming element Aj2 are added in amounts within a predetermined range in steel. By doing so, the deformation resistance during cold forging is low (furthermore, by the subsequent carbonitriding treatment, Hv8
A surface hardness of 00 or higher can be stably obtained, and a deep effective hardening layer can be obtained. , '+': It has been found that cold forging steel with low carbonitriding properties can be obtained, and the present invention has been achieved.

The cold forging steel with excellent carbonitriding properties according to the present invention contains, in weight percent, C 0.10 to 0.30%, Si 0.05 to 0.35%, Mn 0.30 to 1.50%, Or 2. 5% or less, Al 0.02-0.5%, 0.0030% or less, the balance consists of iron and unavoidable impurities, and the above elements are 1.6≦Cr ”” + 3 Al ”” ≦3 .2
, and 7.4C+1.0Si+1.4Mn+1.3Cr
It is characterized by satisfying ≦4.5.

First, the reasons for limiting the chemical components in the steel of the present invention will be explained.

In the steel of the present invention, it is necessary to add at least 0.10% of C in order to improve the core hardness of the carbonitrided parts and to deepen the effective hardness of the layer.

However, when adding too much C, the toughness and machinability of the steel deteriorate, and the heat treatment distortion that occurs during carbonitriding treatment increases.

In addition, when forming parts by cold forging before carbonitriding, as will be described later, as Cfi increases, deformation resistance during processing increases and tool life decreases.
Furthermore, since deformability decreases, in the steel of the present invention, (
The upper limit of Jl is 0.30%.

Si is a necessary element as a deoxidizing element during ingot manufacturing, and in the present invention it is necessary to add at least 0.05%, but if it is added in excess, the deformability during cold forging will decrease. However, since it causes cracking, the upper limit is set at 0.35%.

Mn is necessary for deoxidation and desulfurization during melting, and also improves the core hardness of carbonitrided parts by increasing the hardenability of steel.
This is necessary to increase the effective hardening layer depth. In the present invention, in order to fully express the above effects, 0.30
It is necessary to add % or more. However, when adding too much, the core hardness becomes too high or machinability deteriorates, and the deformation resistance during cold forging increases, leading to a decrease in tool life. , the maximum -F limit of Mn addition is 1.
It shall be 50%.

In the steel of the present invention, Cr is added to H in carbonitrided parts.
It is an essential element in order to maintain a surface hardness of V800 or higher and a predetermined effective hardened layer depth.

However, if too large a quantity is added, these effects will be saturated, the core hardness will become too high, and the deformation resistance during cold forging will also increase, so the upper limit is set at 2.5%. do.

A! is an essential element for refining austenite grains during carbonitriding treatment and maintaining a specified surface hardness of 6'. To obtain these effects at the same time,
Requires addition of at least 0.02%. However, 0
．． When added in a large amount exceeding 5%, not only does the effect of increasing surface hardness become saturated, but also cracks occur during blooming during the production of raw steel. Therefore, in the present invention, the upper limit of Affi is set to 0.5%.

Regarding O, when oxide inclusions A1□03 exist in steel, it causes cracks to occur during cold forging, reduces deformability, and also reduces machinability and fatigue strength of parts. Cause. On the other hand, as described above, Al1 is essential for maintaining a predetermined surface hardness after carbonitriding. Therefore, in the present invention, it is necessary to reduce the oxygen content (■) in the steel as much as possible (■), which is the cause of AhOi formation.
The upper limit is set to 0.0030%.

The steel according to the present invention must have its chemical components within the above-mentioned range, and in particular must satisfy the following formula (1) for Cr and AN, which are elements for improving surface hardness. Here, each element symbol indicates its content in steel (% by weight). That is, according to the present invention,
Stable surface hardness of HV800 on carbonitrided steel parts
In order to find out the above, we need to go to the surface! C, which is an element that improves
It is necessary that r and Al content (2) satisfy the value of the above formula (1) (hereinafter referred to as A value). However, even if excessive amounts of both elements are added, the surface hardness will not increase commensurately, so the upper limit of the A value is set at 3.2 in consideration of economic efficiency.

Next, in order for the steel of the present invention to have excellent cold forgeability, the following (
2) It is necessary to satisfy the formula.

7.4 C+ 1. OS i n-1, 4Mn + 1
3 Cr≦4.5 +21 That is, as a result of focusing on the deformation resistance of steel during cold forging and quantitatively examining the effects of each alloying element, the above A value (hereinafter referred to as H value) is satisfied [ 1 of the tool by adjusting the alloying element ■ in the U concave circle! This allows the deformation resistance to be reduced so that virtually no wear occurs.

Furthermore, in the steel of the present invention, in addition to the above-mentioned elements, N
By adding at least one element selected from the group consisting of i and Mo, it is possible to improve the hardenability of steel and increase the core hardness of carbonitrided parts. In the present invention, the amount of these elements added is determined in consideration of economical efficiency.
3.0% for i and 0.50 for Mo
% respectively.

In this way, when the steel contains Ni and/or MO, which are elements that improve hardenability, in order for this steel to have good cold forgeability, the addition of alloying elements (■) is determined by the formula (3) below. It is necessary to have a Δ value of .

7,4 C+ 1. OS i + 1.4M n
+ 1.3 Cr+ 0.6 N i + 2.5 M
o≦4.5 F31 In addition, for cases where forming processing by cutting is required before surface hardening treatment by carbonitriding, the steel of the present invention contains s, pb, and Z.
The machinability of steel can also be improved by adding at least one element selected from the group consisting of r. For each element, in order to improve the machinability of 1.21, in the present invention, S 0.03% or more, P b 0.05%
It is necessary to have at least 0.05% or more of Zr. However, if the amount added is too large, cracks may occur during cold forging.
This causes deformation of the steel and deteriorates the fatigue strength of the part. Therefore, the upper limit for the addition of each element is 0.30% for S and 0.1% for PB.
0%, and 0.10% for Zr.

As described above, in the present invention S4, the amount of alloying elements added is set within a predetermined range, and in particular, the amounts of Cr and 7B, which are elements for improving surface hardness, satisfy a predetermined relationship. Since the amounts of the main alloying elements including these elements also satisfy the predetermined relationships, the deformation resistance during cold forging is low, and furthermore, the subsequent carbonitriding treatment provides a stable surface hardness of Hv800 or higher. A deep effective case depth can be obtained.

The present invention will be explained below with reference to Examples.

Example (1) Relationship between the surface hardness of carbonitrided steel and the amount of Cr and Al The surface hardness of carbonitrided steel and the amount of alloying elements, especially C
In order to investigate the relationship between r and the amount of Al, various steels with different amounts of these alloying elements were melted in a small high-frequency furnace. The composition of each component is shown in Table 1.

These steel materials are hot-forged into a round bar with a diameter of 30 mm, and after normalizing, it is made into a round bar with a diameter of 25 m5 and a length of 100 mm.
Machined into am round bar. Next, carbonitriding treatment was carried out at 830°C for 6 hours in an atmosphere in which approximately 5% ammonia gas was added to a mixed gas of RX residue and butane gas, and after applying oil υε, heating at 150°C for 2 hours. It was tempered by air cooling. The thus treated round bar was cut at the center in the longitudinal direction, and the hardness at a minute depth of 0.02 u from the surface was determined as surface hardness J. The results are shown in Table 1.

Effect of Cr and ANli on surface hardness after carbonitriding treatment)
In order to quantitatively investigate the effect of Table 1, the hardness of Steel 1 in Table 1 was defined as convex hardness, and the difference between it and the surface hardness of other steels was determined. This value is shown in Table 1 as the surface hardness increase value (Hv).

Furthermore, the relationship between this surface hardness increase value, Δp, and Cr appearance is shown in FIG. From Figure 1, the more Cr and ΔpM, the more
Although the increase in surface hardness increases, the effect is saturated even if it is added in excess. Therefore, considering economic efficiency, Cr and A
The i-view only needs to satisfy the above A value. By satisfying these conditions, the present invention ensures that the surface hardness Hv
You can get more than 800.

(2) Hardening properties and cold forgeability after carbonitriding In order to investigate the hardening properties and cold forgeability after carbonitriding, steel with the chemical composition shown in Table 2 was melted in a small high-frequency furnace. did.

First, in order to investigate the cold forgeability of these steels, they were hot-forged into round bars with a diameter of 25 mm, normalized, and annealed to make them spheroidized. It was processed into a 30am cylinder. Next, using a concentric grooved pressure plate, restraint compression deformation was performed until the rolling reduction ratio reached 60%, and the deformation resistance of each steel material was determined based on the load and restraint coefficient at that time. This is shown in Table 2.

Regarding the hardening characteristics when carbonitriding is performed, refer to JIS G 0557 for the surface hardness at the longitudinal center section and the distance from the surface to Hv550 for the depth of the effective hardened layer after processing the steel in the same process as above. ) was sought. The results are shown in Table 2.

Comparative steels 20.21 and 22 have Cr and A1
They do not satisfy the conditions regarding the value, and neither has sufficient surface hardness. Furthermore, a sufficient effective hardened layer depth cannot be obtained. Comparative steels 23 and 24 do not satisfy the conditions according to the present invention for the A value and also do not satisfy the conditions according to the present invention for the H value, so their deformation resistance during restrained compressive deformation is large. Such steels significantly reduce tool life during cold forging. Comparative steels 25, 26 and 27 are C
r and /l/! fi does not satisfy the conditions regarding the A value and does not have sufficient surface hardness. On the other hand, in the steel of the present invention, a surface hardness of Hv800 or more and a sufficient effective hardening layer depth are obtained, and the deformation resistance during cold forging is also small.

(3) Relationship between oxygen content and cold forgeability of steel Regarding cold forgeability, in particular, in order to investigate the relationship between the deformability of Button 1 and the oxygen content, we used copper having the chemical composition shown in Table 3. were melted in a small high-frequency furnace and some in a small vacuum furnace.

These steels were machined into round bars with a diameter of 20mm and a length of 301mm using the same processing steps as described above, and the critical compression ratio for cracking when this was subjected to restrained compressive deformation was determined. Table 3 shows the results. The steel of the present invention, which has a reduced oxygen content, has a higher compressibility at cracking limit than comparative steels, and has excellent deformability during cold forging.

[Brief explanation of the drawing]

The drawing is a graph showing the relationship between the AA content and Cri in steel and the increase in surface hardness after carbonitriding. Ai'-! -(weight%) -;301

Claims (1)

[Claims] (1) C 0.10-0.30%, Si 0.05-0.35%, Mn 0.30-1.50%, Cr 2.5% or less, Al 0.02 to 0.5%, O 0.0030% or less, the balance consists of iron and unavoidable impurities, and the above elements are 1.6≦Cr^1^/^2+3Al^1^/^2≦3.
2, and 7.4C+1.0Si+1.4Mn+1.3C
A cold forging steel with excellent carbonitriding properties, which satisfies r≦4.5. (2) In weight% (a) C 0.10-0.30%, Si 0.05-0.35%, Mn 0.30-1.50%, Cr 2.5% or less, Al 0.02 ~0.5%, O 0.0030% or less, and (b) at least one member selected from the group consisting of Ni 3.0% or less and Mo 0.50% or less, the balance consisting of iron and inevitable impurities, and , the above element is 1.6≦Cr^1^/^2+3Al^1^/^2≦3.
2, and 7.4C+1.0Si+1.4Mn+1.3C
A cold forging steel with excellent carbonitriding properties, which satisfies r+0.6Ni+2.5Mo≦4.5. (3) In weight% (a) C 0.10-0.30%, Si 0.05-0.35%, Mn 0.30-1.50%, Cr 2.5% or less, Al 0.02 ~0.5%, O 0.0030% or less, and (b) S 0.03-0.07%, Pb 0.05-0
．． 10% and at least one selected from the group consisting of Zr 0.05 to 0.10%, the balance consisting of iron and inevitable impurities, and the above elements are 1.6≦Cr^1^/^2+3Ar^1^ /^2≦3.
2, and 7.4C+1.0Si+1.4Mn+1.3C
A cold forging steel with excellent carbonitriding properties, which satisfies r≦4.5. (4) In weight% (a) C 0.10-0.30%, Si 0.05-0.35%, Mn 0.30-1.50%, Cr 2.5% or less, Al 0.02 ~0.5%, O 0.0030% or less, (b) Ni 3.0% or less, and Mo 0.50% or less, and (c) S 0.03-0. 07%, Pb 0.05-0
．． 10% and at least one selected from the group consisting of Zr 0.05 to 0.10%, the balance consisting of iron and inevitable impurities, and the above elements are 1.6≦Cr^1^/^2+3Al^1^ /^2≦3.
2, and 7.4C+1.0Si+1.4Mn+1.3C
A cold forging steel with excellent carbonitriding properties, which satisfies r+0.6Ni+2.5Mo≦4.5.