JPH05255733A - Production of carburized and case hardened steel material having delayed fracture resistance - Google Patents

Abstract

PURPOSE:To easily produce a carburized and case hardened steel material showing superior delayed fracture resistance and having sufficiently satisfactory mechanical properties. CONSTITUTION:A steel stock which has a composition consisting of 0.30-0.45% C, <=1.00% Si, <=2.00% Mn, <=3.50% Ni, <=3.00% Cr, <=3.00% Mo, and the balance Fe with inevitable impurities or further containing one or more kinds among <=0.0050% Nb, <=0.30% V, <=0.30% Pb, <=0.0100% Ca, and 0.0005-0.0050% B is carburized and quenched and is then subjected to tempering treatment at 400-600 deg.C and to induction hardening, by which the hardness of the carburized layer softened by tempering can be restored.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a carburized case-hardened steel material suitable for "carburized parts having a screw portion" in automobiles, construction machines and the like.

[0002]

2. Description of the Related Art Generally, carburized case hardened steel members subjected to carburizing treatment are often used in parts of automobiles, civil engineering construction machines, industrial machines, etc. where wear is likely to occur. This "carburizing treatment" is carried out by, for example, performing heat treatment in a gas atmosphere of CO, N ₂ , H ₂ or the like to infiltrate carbon into the surface layer portion of the steel to increase the carbon concentration (carbon content) of the surface layer portion It is well known that this is a heat treatment for increasing the hardness after quenching, which is intended to improve wear resistance and fatigue characteristics.
On the other hand, there is a fact that when steel is carburized, "delayed fracture" tends to occur.

This "delayed fracture" is a phenomenon in which brittle fracture suddenly occurs during use, and particularly when tensile stress occurs, the tendency becomes more remarkable. Therefore, the occurrence of tensile stress is unavoidable. In the case of parts having a screw portion), delayed fracture caused by the carburizing treatment as described above is a serious problem. Therefore, for carburized parts that have threaded parts (tensile stress is generated during tightening), a carburizing inhibitor should be applied to prevent carburization of the threaded parts, or a cover should be provided on the threaded parts to prevent carburization. It had to be devised.

However, it is very difficult in practice to apply the carburizing inhibitor evenly, and even if a cover is provided, it is not possible to completely prevent the carburizing gas from entering through the gap. However, it was difficult to completely prevent carburization even with the above measures. Moreover,
Since applying these measures leads to a marked decrease in workability, there has been a strong demand for the development of means for more easily improving delayed fracture resistance.

In view of the above, the object of the present invention is to establish means for easily realizing a carburized case-hardened steel material which exhibits excellent delayed fracture resistance and also has sufficient mechanical properties. Was that.

[0006]

The inventors of the present invention have made extensive studies in order to achieve the above objects, and as a result, have obtained the following findings. That is, when a steel material is subjected to a carburizing treatment, hydrogen penetrates into the steel material during the carburizing treatment, which causes delayed fracture of the carburized case-hardened steel. The hydrogen that has penetrated into the steel material is not completely released even if ordinary tempering (heating temperature: 170 to 200 ° C.) is performed. However, in the case of performing "200 ° C tempering", the amount of infiltrated hydrogen is reduced to about 65% by tempering, but beyond that, there is almost no reduction over time, and it remains in a stable state semipermanently.

However, if tempering is performed at a high temperature of 400 ° C. or higher after carburization, most of the hydrogen that has penetrated into the steel material is released, and the hydrogen content level before carburizing can be reduced.

The reason why the hydrogen emission rate differs depending on the tempering temperature is considered as follows. 1) When tempering immediately after carburization at about 200 ° C., χ (kai) carbide is extremely finely precipitated in the carburized layer during the tempering treatment. Then, hydrogen is trapped at the interface between the χ carbide and the base iron, and hydrogen cannot be completely diffused and released during the tempering process. Here, χ carbide is a chemical formula
Fe _{2 ■ 3} C is a special carbide.

2) On the other hand, when tempering is performed at a high temperature of 400 ° C. or more, θ carbide (cementite: Fe ₃ C) does not precipitate but χ carbide precipitates. The θ carbide has good compatibility with the base steel, and hydrogen is easily diffused and released without being trapped at the interface.

3) However, when tempering is carried out at 400 ° C. or higher, the hardness of the carburized layer is lowered, and the wear resistance which is the original purpose of the carburized case-hardening steel is lowered. In addition, the strength of the base material is lowered, and the tensile strength and fatigue strength are lowered.

4) However, even when tempering is performed at a high temperature of 400 ° C. or higher, simple induction hardening is carried out again after tempering, which sufficiently solves the problem of hardness reduction of the carburized layer and is applied. By adjusting the C content of the steel material to a high value, it becomes possible to effectively prevent the strength from decreasing without particularly adversely affecting the desired characteristics.

The present invention has been completed based on the above-mentioned findings and the like. "C: 0.30 to 0.45% (hereinafter,% representing a component ratio is a weight ratio), Si: 1.00% or less, Mn : 2.00% or less, Ni: 3.50
% Or less, Cr: 3.00% or less, Mo: 3.00% or less, or Nb: 0.050% or less, V: 0.30% or less, Pb: 0.
30% or less, Ca: 0.0100% or less, B: 0.0005 to 0.0050%, including at least one kind, the balance of which is steel and unavoidable impurities, and after carburizing and quenching, steel is tempered at 400 to 600 ° C. Next, induction hardening is performed and the hardness of the carburized layer softened by tempering is restored to enable easy and stable production of a carburized case-hardened steel material with excellent delayed fracture resistance. is doing.

As described above, in the present invention, the relationship between "carbide precipitation" and "capture of hydrogen which causes delayed fracture" in the tempering process was investigated, and a carburized case hardened steel material excellent in delayed fracture resistance was obtained. Although it provides a simple method,
Next, the reason why the chemical composition of the steel material and the treatment conditions are limited as described above in the method of the present invention will be explained together with its action.

A) Chemical Composition C C is an element necessary for imparting desired static strength to steel. In particular, in the present invention, tempering is performed at a high temperature of 400 ° C. or higher in order to improve delayed fracture resistance, but in order to maintain the required static strength even after such high temperature tempering, C of 0.30% or more is required. It is necessary to secure the content.
However, when C is contained in excess of 0.45%, the toughness becomes conspicuous. Therefore, the C content is 0.30 to 0.45.
Defined as%.

Si Si is an element necessary for deoxidizing steel, and has an effect of imparting a predetermined static strength to steel. However, if the content exceeds 1.00%, intergranular embrittlement occurs and the toughness decreases, so the Si content was defined as 1.00% or less.

Mn Mn is an element necessary for deoxidizing steel, similar to Si, but at the same time can be said to be an element necessary for imparting hardenability. However, if the content exceeds 2.00%, grain boundary embrittlement occurs and the toughness is reduced, as in the case of Si. Therefore, Mn
The content was determined to be 2.00% or less.

Ni Ni is an element effective for improving the hardenability of steel and improving the static strength and toughness, so it is a component added for improving these properties, but 3.50% The Ni content was set to 3.50% or less because the effect is saturated and the economical efficiency is impaired even if it is contained in excess.

Cr Cr also has the effect of imparting hardenability to steel, and is also an element often added to case-hardening steel to improve carburization. However, when the Cr content exceeds 3.00%, Cr
On the contrary, an oxide is formed to lower the carburizing property.
Therefore, the Cr content is set to 3.00% or less.

Mo Mo, like Ni, is also an element effective in improving the hardenability of steel and improving the static strength and toughness. But 3.00
Even if added in excess of%, the effect will be saturated and the economic efficiency will be impaired, so the Mo content was defined as 3.00% or less.

Nb, V, Pb, Ca and B each have an effect of further improving the desired characteristics of the carburized case-hardening steel, and therefore it is preferable to add one or more of them. The reason why the addition amount of each component is limited will be described below.

A) Nb Since Nb has an action of suppressing grain boundary embrittlement of steel, it is a preferable component to be added in order to improve toughness and delayed fracture resistance, but more than 0.050% Content of Nb causes coarsening of the crystal grains and a decrease in toughness.
It was defined as 0.050% or less.

B) V V has the effect of forming carbonitrides in the steel to increase the high temperature strength of the steel. However, even if the content of V exceeds 0.30%, the effect is saturated and the economy is impaired. Therefore, the V content is set to 0.30% or less.

C) Pb Pb has the action of improving the machinability of steel, so it is a preferable component to be added if necessary. However, if it is contained in excess of 0.30%, the static strength and fatigue strength are improved. The Pb content was determined to be 0.30% or less, because it markedly decreases.

D) Ca Since Ca also has an action of improving the machinability of steel, it is a preferable component to be added if necessary, but if it is contained in excess of 0.0100%, the toughness is remarkably reduced. Ca
The content was set to 0.0100% or less.

E) BB B is an element effective in improving the hardenability of steel and increasing the static strength, but if its content is less than 0.0005%, no sufficient effect appears, while 0.0050 If it is contained in excess of%, the crystal grains of the steel become coarse and the toughness is lowered. Therefore, the B content is set to 0.0005 to 0.0050%.

B) Tempering temperature after carburizing and quenching When the tempering temperature (heating temperature) after carburizing and quenching is less than 400 ° C., hydrogen that has penetrated into the steel during carburizing is not completely released and desired delayed fracture resistance is obtained. Cannot be secured.
On the other hand, if the tempering temperature exceeds 600 ° C., the hardness of the base material (inner layer of steel) decreases, and the tensile strength and fatigue strength decrease significantly. Therefore, the tempering temperature range after carburizing and quenching is 4
It was set at 00 to 600 ° C.

C) Induction hardening after tempering In order to restore the hardness of the carburized layer softened by high temperature tempering and to secure a desired surface hardness, it is necessary to carry out induction hardening after tempering. The induction hardening conditions are not particularly restricted, and ordinary induction hardening conditions are sufficient, but the heating temperature and the heating time must be careful so that the carburized layer is sufficiently hardened.

Next, the effects of the present invention will be specifically described by way of examples.

EXAMPLES First, a steel having a chemical composition as shown in Table 1 was melted in a 150 kg vacuum melting furnace, and the obtained steel ingot was
It was heated to 250 ° C. for 1 hour and forged to 30 mmφ. Then, a normalizing treatment of 925 ° C. × 1 hr was further performed to obtain a test material.

[0029]

[Table 1]

Next, the hydrogen content in the steel material was measured using the above test material, and the "delayed fracture test", "tensile test",
"Charpy impact test" and "hardness measurement of carburized layer" were carried out. In addition, "measurement of hydrogen content" is a diameter of 6mmφ,
A test piece having a length of 12 mm was prepared, carburized and quenched by the heat pattern shown in FIG. 1, and then tempered at 200 to 700 ° C. for 1 hour.

In the "delayed fracture test", after the test material is processed into the test piece shown in FIG. 2, carburizing and tempering are performed under the conditions shown in FIG. 1 as in the case of measuring the hydrogen content. Induction hardening was performed under the conditions shown below, and the load stress σ / σ _{N of} this treated test piece was measured using a constant load tensile tester.
= 0.8 (σ: load stress, σ _N : notch tensile stress). The delayed fracture test time was up to 1000 hours, and the test was stopped at that time even if no fracture occurred. [Induction hardening conditions] Frequency: 200 kHz, heating method: moving hardening, cooling method: water cooling (spray type), heating temperature: 950 ° C, moving speed: 10 mm / s.

In the "tensile test", a test piece having the same shape as the delayed fracture test piece was used, and this was subjected to carburizing quenching, tempering and induction hardening treatment under the same conditions as in the delayed fracture test.

In the "Charpy impact test", after the test material was processed into a JIS No. 3 test piece, carburizing and quenching under the conditions shown in FIG. 1 and a tempering treatment at 700 ° C. × 1 hr were performed, and the delayed fracture test was performed. After induction hardening under the same conditions, the treated test piece was subjected to a Charpy impact tester to investigate impact properties. Furthermore, "hardness measurement of carburized layer" was carried out using this residual material. The results are shown in Tables 2 to 5.

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

[Table 4]

[0037]

[Table 5]

As is clear from the results shown in Tables 2 to 5, all of the carburized case-hardened steel products manufactured under the conditions specified in the present invention have a sufficiently low hydrogen content and excellent lag resistance. It can be seen that, while exhibiting destructiveness, the tensile strength, impact properties, and carburized layer hardness all satisfy sufficient required levels.

From the results shown in Table 2, it can be confirmed that "the tempering temperature must be 400 ° C. or higher in order to sufficiently reduce the hydrogen that has entered during carburization". In addition, from the results shown in Table 3, "tempering 40
If it is carried out at 0 ° C or higher, delayed fracture characteristics will also be significantly improved.
You can confirm that.

However, from the results shown in Table 4, "C
When using a steel whose content deviates from the lower limit of the present invention, 4
If tempered at a temperature of 00 ° C or higher, the strength will be insufficient and the original characteristics of the carburized case-hardened steel will not be satisfied. ” The results shown in Table 5 indicate that the impact value is inferior when using steel in which C, Si, and Mn deviate from the upper limits of the present invention, and "Cr deviates from the upper limits of the present invention." However, the hardness of the carburized layer does not meet the required level when the steel is used.

[0041]

[Summary of Effects] As described above, according to the present invention, it is possible to easily obtain a carburized case hardened steel material that sufficiently secures various characteristics required for the carburized case hardened steel material and that has extremely excellent delayed fracture resistance. It is possible to provide a very useful effect in the industry, such as being provided to

[Brief description of drawings]

FIG. 1 is a diagram showing a carburizing heat pattern and a tempering pattern.

FIG. 2 is a schematic diagram illustrating the shape of a delayed fracture test piece.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C22C 38/60

Claims (2)

[Claims] 1. By weight ratio, C: 0.30 to 0.45%, Si: 1.00% or less, Mn: 2.
00% or less, Ni: 3.50% or less, Cr: 3.00% or less, Mo: 3.
A steel material containing 00% or less and the balance consisting of Fe and unavoidable impurities is carburized and quenched, then tempered at 400 to 600 ° C, and then induction hardened to restore the hardness of the carburized layer softened by tempering. A method for producing a carburized case-hardened steel material having excellent delayed fracture resistance, which is characterized by the following. 2. By weight ratio, C: 0.30 to 0.45%, Si: 1.00% or less, Mn: 2.
00% or less, Ni: 3.50% or less, Cr: 3.00% or less, Mo: 3.
In addition to containing less than 00%, Nb: 0.050% or less, V: 0.30% or less, Pb: 0.
30% or less, Ca: 0.0100% or less, B: 0.0005 to 0.0050%, including at least one type, the balance of which is Fe and unavoidable impurities, and is then carburized and quenched, and then tempered at 400 to 600 ° C. Then, induction hardening is performed to restore the hardness of the carburized layer that has been softened by tempering, and a method for producing a carburized case-hardened steel material having excellent delayed fracture resistance.