JPS634048A - Case-hardening steel for warm forging - Google Patents

Abstract

PURPOSE:To manufacture a case-hardening steel excellent in strength, toughness, and machinability, by incorporating specific trace amounts of Te so as to inhibit coarsening of grains due to carburizing treatment after warm forging at the time of manufacturing a case-hardening steel as a warm forging stock. CONSTITUTION:As the case-hardening steel as warm forging stock, a steel which has a composition containing, by weight, 0.10-0.30% C, 0.02-0.35% Si, 0.2-2.0% Mn, 0.003-0.06% sol. Al, <0.025% N, and 0.003-0.05% Te or further containing 1 or >=2 kinds among 0.3-3.0% Ni, 0.3-2.0% Cr, and 0.05-0.5% Mo or, under the condition of <0.020% S, 1 or >=2 kinds among 0.03-0.15% Pb, 0.03-0.15% Bi, and 0.003-0.0050% Ca independently or in combination is used. The warm-forged product of this steel causes no coarsening of grains due to surface treatment by means of carburizing, so that warm-forged material free from deterioration in mechanical properties and occurrence of heat treatment strain can be obtained.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to case hardening steel, and particularly to case hardening steel suitable as a material for warm forging. (Prior Art) Case hardened steel is widely used in power transmission parts for automobiles and various machines in order to obtain the mechanical properties required for the purpose. This is to obtain the required properties by subjecting the molded parts to surface hardening treatment such as carburizing or carbonitriding to improve surface strength, wear resistance, pitting resistance, and core strength and toughness. . By the way, the above-mentioned parts are manufactured by processing case-hardened steel,
Hot forging and cold forging are generally known as typical processing methods. Hot forging has a small forming load and excellent deformability, but it has problems with scale generation, decarburization, and finishing accuracy.On the other hand, cold forging has a large forming load and poor deformability. There is a problem that Therefore, in recent years, warm forging, which combines the advantages of hot forging and cold forging, has been used. However, if the conventionally used case hardened steel is used as is,
There is a problem that crystal grains tend to become coarse during carburizing treatment after warm forging. When the crystal grains become coarser in this way, the mechanical properties of the parts, especially the toughness and fatigue strength, deteriorate.
When mixed grains or abnormal growth in which crystal grains are partially coarsened occur, the problem of increased heat treatment strain also occurs, which is superimposed on the above problem. For this reason, it is said that it is preferable that the crystal grains after carburization be finer than &7. (Object of the Invention) The present invention was made to solve the problems of the prior art described above, and effectively prevents coarsening of crystal grains due to surface hardening of parts in a processing method using warm forging. The object of the present invention is to provide a case-hardened steel that can be improved in strength, toughness, rigidity, etc., if necessary. (Structure of the Invention) In order to achieve the above object, the present inventor reexamined conventional case hardening steel from the viewpoint of chemical composition, analyzed the causes of grain coarsening, and worked diligently to find countermeasures. As a result of repeated research, we obtained the knowledge that coarsening of crystal grains can be prevented by containing an appropriate amount of Te, and by studying the chemical components in more detail, we have developed the present invention. That is, the present invention provides c:o, i0-0.30%, si
:o, o2~0.35%, Mn:0.2~2.0%, T
e: O, OO3~0.05%, 5oflAI2: 0.0
03 to 0.060% and N: 0.025% or less, with the balance consisting of Fe and inevitable impurities, and if necessary, Ni: 0.3 to 3.0%, Cr: ．．
3-2.0% and Mo: 1 of 0.05-0.5%
species or two or more species, and/or S: under the regulation of 0.020% or less, P: 0.03 to 0.15%, Bi: 0.0
3-0.15% and Ca:O, OOO3-0.0050
The object of the present invention is to provide a case hardening steel for warm forging which has excellent grain coarsening resistance and is characterized by containing one or more of the following. The present invention will be explained in detail below based on examples. First, the reason for limiting the chemical components in the case hardening steel of the present invention will be explained. C is required to be at least 0.10% to ensure the core strength of the part, but too much C deteriorates the toughness of the core, so
The upper limit is 0.30%. Si requires 0.02% or more for deoxidation, but 0.02% or more is required for deoxidation.
．． If it exceeds 35%, cracks will occur during warm forging and warm workability will deteriorate significantly, so the upper limit is set at 0.35%. Mn is an element necessary to control the morphology of inclusions such as MnS and to ensure hardenability.
% or more is required. However, if the content is too large, machinability, especially stiffness, will deteriorate, so the upper limit is set at 2.0%. 5oQAPI is necessary for deoxidation, but if too much, the toughness deteriorates, so it is added in a range of 0.003 to 0.060%. Since a large amount of N causes blowholes, it is necessary to limit it to 0.025% or less in order to ensure the integrity of the steel ingot or slab. Te is an element effective in preventing coarsening of crystal grains during surface hardening treatments such as carburizing and carbonitriding performed after warm forging, and for this purpose, 0.003% or more is added. However, if it is too large, the warm workability will be greatly degraded.
The content shall be 0.05% or less. In addition to the above-mentioned essential components, the following elements can be added as necessary. Ni, Cr, and Mo are elements that improve strength or toughness, and when added, one or more of them are added to Ni.
:0.3~3.0%, Cr:0.3~2.0%, Mo:
It is added in a range of 0.05 to 0.5%. Adding a large amount of each element exceeding the upper limit causes deterioration of workability such as rigidity and warm workability, and increases cost, which is not preferable. Pb, Bi, and Ca can be added when it is desired to ensure machinability. When adding one or more of them, Pb: 0.03 to 0.15%, Bi: 0.03 to 0
．． 15%, Ca:O, OOO may be added in the range of 3 to 0.0050%. However, in that case, in order to obtain the above effect, it is preferable to limit S to 0.020% or less, and in particular, PB and If Bi coexists with S, warm workability will deteriorate, so care must be taken not to use more Bi than necessary. Note that if each element is present in too large a quantity, warm workability deteriorates. Case-hardening steel having the above chemical composition is subjected to warm forging as a steel ingot or slab, and then subjected to surface hardening treatments such as carburizing and carbonitriding, but the crystal grains do not become coarse and are mixed grains. It is possible to obtain a structure with fine grains. Although warm forging depends on the upsetting rate, it is usually 500 to 1
000°C, preferably at a temperature of 700 to 350°C. Next, examples of the present invention will be shown. (Example) Various case-hardening steels having chemical components (vt%) shown in Table 1 were melted using a 50 kg induction furnace and then heated to 1250°C.
It was forged and elongated to 35+a+++φ. Next, after processing the test piece, warm forging was performed at 750°C. After forging, the material at the negative part was heated at 750°C for 8 hours, then cooled to 600°C at a cooling rate of 20°C/hr, and air-cooled. Thereafter, a test piece was processed and cold forged. In addition, both the warm forging and cold forging mentioned above are 25m+
mφ, 39m+ah test piece was 12m+ with an upsetting rate of 69%.
Forged to ++h. After the above forging, carburizing and quenching was carried out at 925°C for 7 hours.
Austenite crystal grain size was measured in accordance with JIS-GO551. The results are shown in Table 2.

[Left below]

Table 2 Ratios are for cold forged materials. As is clear from Table 2, the case hardening steels of the comparative examples all have mixed grains and coarse grains, whereas the case hardening steels of the present invention all have fine grains. In addition, Comparative Example 3.
4 and invention examples C and D are cold forged materials,
It was also confirmed that the present invention is also effective for cold forging. In addition, when we investigated the influence of the amount of S on the warm workability of Examples A, H, A', and H' of the present invention, we found that the machinability improving element p
When B was added, cracks occurred during warm forging in H', in which the S content was not regulated to 0.20% or less, but no cracks occurred in the other steels. (Effects of the Invention) As described in detail above, according to the present invention, coarsening of crystal grains can be extremely effectively prevented in the warm forging processing method. It is possible to eliminate problems such as deterioration of mechanical properties of parts and heat treatment distortion, which occur in parts, and it is also possible to improve strength, toughness, or machinability as necessary.

Claims (4)

[Claims] (1) In weight% (the same applies hereinafter), C: 0.10 to 0.3
0%, Si: 0.02-0.35%, Mn: 0.2-2
．． 0%, Te: 0.003-0.05%, solAl:
A case-hardened steel for warm forging with excellent coarse graining resistance, containing 0.003 to 0.060% and 0.025% or less of N, with the remainder consisting of Fe and unavoidable impurities. (2) C: 0.10-0.30%, Si: 0.02-0
．． 35%, Mn: 0.2-2.0%, Te: 0.003
~0.05%, solAl: 0.003~0.060%
and N: 0.025% or less, and further Ni: 0.3
~3.0%, Cr:0.3~2.0% and Mo:0.0
A case hardening steel for warm forging with excellent grain coarsening resistance, characterized in that it contains one or more of 5 to 0.5%, and the remainder consists of Fe and unavoidable impurities. (3) C: 0.10-0.30%, Si: 0.02-0
．． 35%, Mn: 0.2-2.0%, Te: 0.003
~0.05%, solAl: 0.003~0.060%
and N: 0.025% or less, and further S: 0.02
Pb: 0.03-0.15% under the regulation of 0% or less,
Bi: 0.03~0.15% and Ca: 0.0003~
A case hardening steel for warm forging with excellent grain coarsening resistance, characterized in that it contains one or more of 0.0050% and the remainder consists of Fe and unavoidable impurities. (4) C: 0.10-0.30%, Si: 0.02-0
．． 35%, Mn: 0.2-2.0%, Te: 0.003
~0.05%, solAl: 0.003~0.060%
and N: 0.025% or less, and further Ni: 0.3
~3.0%, Cr:0.3~2.0% and Mo:0.0
One or more of 5 to 0.5% and S: 0.0
Pb: 0.03-0.15% under the regulation of 20% or less
, Bi: 0.03-0.15% and Ca: 0.0003
A case hardening steel for warm forging with excellent grain coarsening resistance, characterized in that it contains one or more of the following: ~0.0050%, with the remainder consisting of Fe and unavoidable impurities.