JPS59159928A - Carburization hardening treatment of case hardened steel - Google Patents

Abstract

PURPOSE:To obtain a product which has less strain and has no local variance in the strength of the core part in the stage of subjecting a case hardened steel contg. specific ratios of C, Si, Mn, Cr, solAl, and N to carburization hardening by subjecting said steel to a heat treatment under a specific condition. CONSTITUTION:A case hardened steel contg., by weight %, 0.10-0.30% C, 0.03- 0.50 Si, 0.30-1.80% Mn, 0.015-0.600% solAl and 0.005-0.025% N, contg. 0.05- 0.35% Mn if necessary, and contg. further 1 or >=2 kind among 0.04-0.40% S, 0.05-0.40% Pb, 0.005-0.20% Bi, 0.005-0.20% Te, and 0.001-0.0020% Ca according to need and consisting of the balance Fe and unavoidable impurities is once held for 2-5hr in the temp. region of 600-Ac1 transformation point in the midway of heating for the purpose of treatment and is then heated as it is, by which the prescribed treatment is accomplished in the stage of subjecting said steel to a carburization hardening treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a carburizing and quenching process for case hardened steel that can produce a product with little distortion during carburizing and quenching and without local variations in core elasticity in a relatively short time. It is about the method.

In general, case hardened steel is often used for mechanical parts such as gears and crankshafts that require wear resistance on the surface. After leak-machining, it was common to undergo carburizing and quenching to harden the surface.

By the way, in the carburizing and quenching treatment of such machine parts, etc., in order to ensure the toughness of the core part, the core part of the part is not completely martensitic, and from the viewpoint of mass effect, it is not necessary to completely martensite the core part. Since no martensitic formation occurred, the core was only martensitic within a range that satisfied the required hardness.

However, under such conditions, if local differences in austenite grain size occur during heating during carburizing and quenching,
The coarse grain part is easily hardened and forms martensite, while the fine grain part becomes a bainite structure.Due to this difference in structure, the quenching distortion during carburizing and quenching becomes uneven and large. Since the layer had to be partially removed, it caused uneven surface hardness, and if the layer could not be corrected by machining, it would become a defective product, resulting in a 9.9 drop in yield, which was a serious problem. .

On the other hand, if a local bend occurs in a crankshaft or the like, there is a problem in that it requires a great deal of time and expense to correct it.

Furthermore, in recent years, in order to shorten processing time and increase efficiency from the viewpoint of energy and cost savings, high-temperature carburizing (processing temperature = 950 to 1000°C) and vacuum carburizing (processing temperature: 9
High-temperature treatments (50 to 1050°C) are now being carried out, which has a strong tendency to coarsen austenite grains, so expensive grain-refining elements such as Nb are added to uniformize quenching distortion. This has also led to the problem of having to change the format.

However, the present inventors first developed a method of rolling Ae-killed steel and then reheating it to a temperature range of 600°Q=Ac, and a method of rolling Ae killed steel and then reheating it to a temperature range of 600'Q=Ac, and using the retained heat of the material after rolling. -A
AIN is formed in steel by a method of retaining heat in the temperature range of c1 point.
We have proposed a method for manufacturing case-hardened steel in which the austenite crystal grains are precipitated uniformly and finely, thereby preventing the coarsening of austenite grains, and have achieved great results in obtaining products with less carburizing and quenching distortion. In this case, in order to obtain a product with little hardening distortion, it is necessary to use a material that has been specially treated as described above, and the case hardening steel is simply A.
If only killed steel is used, A and 7N will not necessarily precipitate sufficiently, and completely uniform grain refinement cannot be expected, so the thermal history will be mixed, and therefore A7? Precipitation behavior of H: At present, the carburizing and quenching method, which allows a good product to be obtained by applying a uniform carburizing and quenching treatment to materials with greatly different J, has not yet been successful. there were.

From the above-mentioned viewpoint, the present inventors have developed a case hardening method that allows products with less quenching distortion to be obtained in a short time without using materials containing elements such as Nb that prevent coarsening of austenite crystal grains. As a result of research to find a method for carburizing and quenching steel, we found that when carburizing and quenching ordinary Al-killed case-hardened copper, by holding it at a predetermined temperature for a certain period of time during heating, we analyzed the fine precipitation of 9AgN. If the austenitization is performed after the austenite is released, a uniform fine-grained austenite structure can be obtained, and therefore the quenching during carburizing and quenching will be uniform, resulting in ■ small carburizing and quenching distortion υ,
The frequency of product machining and straightening work is reduced, and the occurrence of defective products is extremely reduced; ■ There are fewer local and overall variations in the strength of the product core, and product quality is improved. What can be achieved: ■ If the temperature is below the melting temperature of AlN, high-temperature carburizing or vacuum carburizing can be carried out without adding alloying elements such as Nb, and the carburizing process can be completed in a short time and at low cost. It has come to this.

This invention was made based on the above knowledge, υ, C: 0.10 to 0.30 inches (hereinafter referred to as weight percentage)
, St: 0.03-0.50%, Mn: 0.30
~1.80%, Cr: 0.30~1.80%, soe, A, g: 0.015~0.600%, N
:0.006 to 0.025%, and if necessary, Mo20. S: 0.04-0.40 chi, Pb: 0.05-0
．． 40%, Bi: 0.005 to 0.20%, Te: 0.005 to 0.20%, Ca: 0.001 to 0.020%, and also contains one or two of the following: When carrying out carburizing and quenching treatment on case hardened steel having a composition consisting of Fe and unavoidable impurities: residue, during heating for the treatment, the case hardened steel is once heated to a temperature range of 600°0-A as transformation point. It is characterized by the ability to produce surface-hardened products in a short time with little distortion and no variation in the strength of the core by holding the product for 2 to 5 hours, then raising the temperature and subjecting it to the specified treatment. It is something.

It should be noted that the above-mentioned "predetermined treatment" refers to a normal carburizing and quenching treatment performed after the case hardening steel is heated to an austenite region, and does not refer to any special treatment.

Figure 1 shows the conventional carburizing and quenching method with the JIS standard symbol S.
This is an example of a heating and cooling curve when applied to case hardened steel represented by 0M420. Figure 2 shows a heating and cooling curve when the carburizing and quenching method of the present invention is applied to 80M420. As is clear, this invention
In the middle of the heating process during carburizing and quenching, 600°C-
It is characterized in that after maintaining the temperature in the AC, transformation point temperature range for 2 to 5 hours, the same treatment as in the conventional method is continued.

Next, the reason why the composition of the case hardened steel is limited as described above in the method of the present invention will be explained.

■ C The C component has the effect of ensuring the required strength of the steel material, but if its content is less than 0.104, it is difficult to ensure the desired strength where the hardness of the steel material core is HR020 or more. On the other hand, if the content exceeds 0.30%, the hardness of the uncarburized core exceeds HRC4o, making it impossible to secure the desired toughness. .30%.

■ 5i St acid component An essential element as a deoxidizing agent for molten steel,
If the content is less than 0.034, the desired deoxidizing effect cannot be obtained, while if the content exceeds 0.50%, inclusions of Sin and other compounds will increase and the toughness will deteriorate. Therefore, the content was determined to be 0.03 to 0.50.

■ Mn The Mn component has a deoxidizing effect and a desulfurizing effect on molten steel) and is essential for improving the toughness of steel materials.
．． If the content is less than 30%, the desired effects such as those described above cannot be obtained, while if the content exceeds 1.80%, the stiffness will decrease, so the content should be adjusted to 0.30 to 1.8%.
It was set as 0%.

■ Cr The Cr component has the effect of improving the hardenability, strength, toughness, and carburizability of steel materials, but if its content is less than 0.30%, the desired effects cannot be obtained; .80
Since the performance of the carburized product deteriorates if the content exceeds 0.30 to 1.80%.

■ The soe, A, e sol, and Al components have the effect of suppressing the growth of austenite crystal grains, but if their content is less than 0.015 grains, the desired effect cannot be obtained; If it is contained in excess of
．． 06%.

■N The N component has the effect of combining with the Al component in steel to precipitate AeN compounds and suppressing the growth of stenite grains, but if its content is less than 0.006%, this effect will not be achieved. The desired effect cannot be obtained, and even if the content exceeds 0.02.5%, no further improvement effect can be expected, and it is not preferable from an economic standpoint, so the content should be increased from 0.006 to 0.02.5%. 02
It was set at 5%.

■　M.

The Mo component has the effect of improving the hardenability and toughness of steel materials, so it is added as necessary, but if the content is less than 0.05%, the desired effect cannot be obtained. On the other hand, if the content exceeds 0.35%, machinability deteriorates and the effect of improving hardenability and toughness is saturated, so the content is set at 0.05 to 0.35%. Ta.

■ S, Pb, Bi + Te + and Ca These components have the effect of improving the stiffness of steel, and when it is necessary to improve the stiffness of steel, one or two of them are used. Although it is added, S: 0.04
%, Pb: less than 0.05%, Bi: less than 0.005%, 'pe: O, QO less than 5%, and Ca: O, 00
If it is less than 14, the desired effect cannot be obtained, and on the other hand, S: 0.40%, pb: 0.40%, Bi
: 0.20%, Te: 0.20%, and Ca: 0.02
Even if the content exceeds 0%, the effect is saturated and there is no point in increasing the content, and it is economically disadvantageous. Therefore, S: 0.04 to 0.40%, Pb: 0.
05-0.40%, Bi: 0.005-0.20%,
Te: 0.005-0.20%, ca: 0.0
The respective contents were determined to be 01 to 0.020%. It goes without saying that adding a combination of three or more of these elements will also bring about the effect of improving machinability, but considering the balance with economic efficiency, adding one or two of these elements as described above will improve machinability. We decided to add .

Furthermore, during heating during carburizing and quenching, the
The reason for holding the temperature in the temperature range of A c, transformation point for 2 to 5 hours will be described below.

During the carburizing and quenching process, maintaining the temperature in the above temperature range for a predetermined period of time is necessary to sufficiently precipitate keN in the steel material and to make it into a uniform and fine state. This is to prevent grain coarsening, but if the holding temperature is less than 600°C or the holding time is less than 2 hours, fine AeN
On the other hand, if the holding temperature exceeds the A+4 point, austenite transformation will occur before sufficient precipitation of IN, and the effect of suppressing coarsening of austenite crystal grains will be ensured. Furthermore, if the holding time exceeds 5 hours, agglomeration of iN will occur, resulting in coarsening of polycrystalline grains, and it is also uneconomical, so the holding temperature range is set to 600°C to Ac1 transformation. Hold time 2-5 to
It was set as time.

Next, the present invention will be specifically explained by comparing examples and comparative examples.

Example First, steel having the respective compositions shown in Table 1 was melted by ordinary atmospheric melting, and then bloomed into a billet of 125 mm, and then hot-rolled into a bar of 30 mm diameter. Finished. At this time, the heating temperature is 1150°C or higher, and the finishing temperature f′i, 1
000°C or higher.

Next, the obtained steel bar was used as a sample and subjected to carburizing and quenching treatment under the conditions shown in FIG.

In addition, at this time, once the holding temperature T+ ('O
) and retention time t1 (hr) are also shown in Table 1.

In order to evaluate the degree of quenching distortion for each sample that has been carburized and quenched, we investigated the degree of coarsening of the austenite crystal grains that are enclosed in a proportional relationship with this, and the results are also shown in the first table. Also listed in the table.

As a method for determining the degree of coarsening of austenite crystal grains, the cross section of each sample that has been water-quenched after carburizing simulation as shown in Figure 1 above is corroded with 5-thinitric alcohol to coarsen the grains. We adopted a method to determine the degree of In this case, the degree of coarse graining of the structure is determined by Jl
ll) Particle size is divided into five grades from 1 to 5, and the carburizing temperature T shown in FIG.

(“-C) is 900-1100°C and this is 50°C
It was evaluated by changing the pitch and having a temperature at which the coarse graining judgment was grade 3 (coarse grain part: 20 inches or more and less than 30 inches).

Therefore, the lower the temperature, the more likely the austenite crystal grains become coarse, and the greater the distortion during carburizing and quenching.

From the results shown in Table 1, it is clear that the method of the present invention, which includes a process of precipitating iN uniformly and finely in the temperature range of 600°Q=Ac, the transformation point, when the temperature is raised during carburizing and quenching treatment, Compared to the conventional method that does not include step 1, the coarsening temperature is about 10
It is clear that the temperature has increased by 0°C.

In addition, here, test numbers 5, 41. The reason why the coarsening temperature of No. 47 and No. 47 was as low as 950'C is considered to be because the N content was relatively low and the absolute amount of IN, which is a growth inhibitory factor for austenite crystal grains, was insufficient.

As described above, the present invention makes it possible to produce high-quality surface-hardened products with very little distortion during carburizing and quenching, and without local variations in core strength, with good yield, low cost, and high cost. This brings about industrially useful effects such as increased efficiency.

[Brief explanation of the drawing]

Figure 1 shows the heating and cooling curves in the conventional carburizing and quenching process, and Figure 2 shows the heating and cooling curves in the carburizing and quenching process of the present invention.
An example of a cooling curve, FIG. 3, is a heating/cooling curve in an embodiment of the method of the present invention. Applicant: Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] C: 0.10 to 0.30%, Si: 0.03 to 0.50%, Mn: 0.30 to 1.80%, Cr: 0.30 to 1.80%, sog, A/! : 0.015~0.600%, N
: 0.006 to 0.025%, and if necessary, Mo: 0.05 to 0.35%, and if necessary, S: 0.04 to 0.40%. , Pb: 0.05-0.40%, Bi: o,o5-0.20%, Te: 0.005-0.20%, Ca: 0.001-0.020%, When carrying out carburizing and quenching treatment on case-hardened steel containing one or two of the above, and having a composition (weight ratio) consisting of Fe and unavoidable impurities: residue, during heating for the treatment, The case hardened steel was once heated in a temperature range of 00°C to the AcI transformation point.
A method for carburizing and quenching case-hardened steel, which comprises holding the steel for ~5 hours, then raising the temperature and subjecting it to a predetermined treatment.