JPH04297550A - Delayed fracture resistant carburizing steel and its manufacture - Google Patents

Abstract

PURPOSE:To offer a carburizing steel combining sufficient wear resistance and excellent delayed fracture resistance. CONSTITUTION:The chemical componental compsn. of a carburizing steel is constituted of a one contg. 0.10 to 0.30% C, <=0.50% Si, <=2.00% Mn, 0.30 to 2.00% Mo and 0.10 to 0.50% V, furthermore contg. one or more kinds among <=1.0% Cu, <=3.50% Ni, <=5.0% Cr, 0.010 to 0.100% Al, 0.010 to 0.100% Ti, 0.0003 to 0.0050% B, 0.010 to 0.100% Nb and <=0.50% Pb and the balance Fe with inevitable impurities. Or, the steel having the above chemical componental compsn. is subjected to carburizing and quenching and is thereafter subjected to tempering treatment in the temp. range of 400 to 450 deg.C.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carburized case-hardened steel suitable for use in "carburized parts with threaded parts" in automobiles, construction machinery, etc., and a method for manufacturing the same.

0002

[Prior Art and its Problems] In general, carburized case-hardened steel members that have been subjected to a carburizing treatment are useful in areas where wear is likely to occur in automobiles, civil engineering construction machinery, industrial machinery, and the like. This "carburizing treatment" is performed by applying heat treatment in a gas atmosphere such as CO, N2, H2, etc. to infiltrate carbon into the surface layer of the steel, increasing the carbon concentration (carbon content) in the surface layer, and thereby increasing the carbon concentration (carbon content) after quenching. It is well known that this is a heat treatment that increases the hardness of steel and is intended to improve wear resistance and fatigue properties.
On the other hand, there is a fact that carburizing steel materials tends to cause "delayed fracture."

In particular, this tendency becomes even more pronounced when tensile stress is generated, so for bolts (parts with threaded parts) where the generation of tensile stress is unavoidable, the above-mentioned carburizing treatment causes delayed fracture. has become a big problem. Therefore, it has a threaded part (tensile stress occurs when tightening)
In carburized parts, in order to prevent carburization of the threaded parts, measures such as applying a carburizing inhibitor or providing a cover on the threaded parts have been required to prevent carburization. However, since these measures to prevent carburization lead to a significant decrease in workability and an increase in manufacturing costs, there has been a strong desire for improvement measures.

[0004] In view of the above, it is an object of the present invention to provide a method that does not require complicated work that increases costs.
The objective was to find a way to realize carburized case-hardened steel that has sufficient wear resistance (hardness) and also has excellent delayed fracture resistance.

[0005]

[Means for Solving the Problems] In order to achieve the above object, the inventors of the present invention conducted many repeated experiments and researches, and as a result, they were able to obtain the following knowledge. (a) When simultaneously adding Mo and V as constituent components of carburized case-hardening steel and appropriately adjusting the content ratio of other components, desired wear resistance (hardness) and delayed fracture resistance can be achieved. (b) Further, after carburizing and quenching the steel having the above chemical composition, tempering is performed while controlling the tempering temperature to a higher temperature range than before. , sufficiently good wear resistance (
(hardness) and delayed fracture resistance.

The present invention was completed based on the above-mentioned findings, etc., and it is based on the above-mentioned findings.
0% (hereinafter, % representing component ratio is weight percentage),
Si: 0.50% or less, Mn: 2.00% or less, Mo: 0.30 to 2.00%, V: 0.10
~0.50%, Cu: 1.0% or less, Ni: 3.50% or less, C
r: 5.0% or less, Al: 0.010 to 0.100
%, Ti: 0.010 ~ 0.100
%, B: 0.0003-0.0050%,
Nb: 0.010 to 0.100%, Pb: 0.50
% or less, and the balance is Fe and unavoidable impurities.
It is characterized by having excellent delayed fracture resistance, and furthermore, by carburizing and quenching the steel with the above chemical composition, and then tempering it in a temperature range of 400 to 450 degrees Celsius. It is also characterized by the stable production of carburized case-hardened steel with excellent delayed fracture resistance.

[0007] Here, the mechanism by which carburized case-hardened steel with excellent delayed fracture resistance is realized by the present invention will be explained in more detail while following the research process of the present inventors.

[Operation] First, the inventors of the present invention repeatedly conducted basic studies on the relationship between "carburizing treatment" and "delayed fracture," and the following facts were clarified. That is, when carburized case hardening steel is subjected to carburizing treatment, hydrogen penetrates into the steel during the carburizing treatment. The delayed fracture of carburized case-hardened steel is caused by this "hydrogen that has entered during the carburizing process," and the hydrogen that has entered is not completely dissipated even if normal tempering (200° C.) is performed.

However, when tempering is carried out at 200° C., the amount of hydrogen penetrating is reduced to about 65% by the tempering. However, after that, it hardly decreases over time and remains in a stable state semi-permanently. However, the inventors of the present invention found that when tempering is performed at a high temperature of 400°C or higher, which is far from conventional wisdom after carburizing, most of the penetrating hydrogen is dissipated and the state returns to the state before carburizing. It was done.

[0009] The reason why the hydrogen dissipation rate differs depending on the tempering temperature was considered to be as follows. (1) When tempering is performed at around 200°C immediately after carburizing, χ carbides (chi) are formed in the carburized layer during the tempering process.
A special carbide with the chemical formula Fe2■3C) precipitates extremely finely. Hydrogen is then trapped at the interface between the chi carbide and the steel base, and hydrogen cannot be completely diffused and released during the tempering process. (2) On the other hand, when tempering is performed at a temperature of 400°C or higher, χ carbide does not precipitate and θ carbide (cementite Fe
3C) is precipitated. Since this θ carbide has good compatibility with the base iron, hydrogen is not trapped at the interface and is easily diffused and released by heat treatment.

However, when tempering is carried out at a temperature of 400°C or higher, the hardness of the carburized layer decreases to 800 or less on the Vickers hardness, and the wear resistance, which is the main purpose of carburized case-hardened steel, is impaired. The problem arose of deterioration. Therefore, 400℃
We further investigated the composition system of carburized case-hardened steel, which does not easily reduce its hardness even after tempering, and found that "Mo and V
When Mo and V were added simultaneously, the tempering softening resistance was significantly improved to an extent that could not be expected when Mo and V were added alone, and even when tempered at 400°C or higher, there was a significant decrease in hardness (decreased wear resistance). They found that it is possible to obtain a steel that has significantly improved delayed fracture resistance without causing any damage.

As mentioned above, the present invention has investigated in detail the relationship between carbide precipitation and hydrogen capture during tempering treatment, and based on this, has developed a carburized case-hardened steel with a new component system that has high resistance to temper softening. This invention is designed to prevent delayed fracture of carburized parts due to hydrogen without impairing required performance such as wear resistance. The reason for the limitation will be explained in more detail.

A) Chemical composition of steel C Although C has the effect of imparting a certain static strength to steel, it is also an element that deteriorates toughness. In particular, case hardening steel that undergoes carburizing requires a balance between static strength and toughness, but if the C content is less than 0.10%, the desired static strength cannot be secured; If C is contained in excess of this amount, toughness will be reduced. Therefore, the C content is between 0.10 and 0.
．． It was set at 30%.

Si Si is an element necessary for deoxidizing steel, and also has the function of imparting a predetermined static strength to steel. However, 0.5
If the content exceeds 0%, the carburizability deteriorates, which is disadvantageous for case hardening steel that requires carburization treatment, and the formation of an abnormal carburized layer becomes significant, leading to a softening of the hardness of the outermost surface. Therefore, the Si content was set to 0.50% or less.

Mn Like Si, Mn is an element necessary for deoxidizing steel, but at the same time it is an element necessary for imparting hardenability. However, if the content exceeds 2.00%, the high temperature softening resistance decreases, leading to a decrease in static strength. Therefore, the Mn content was determined to be 2.00% or less.

Mo Mo is an element necessary to give steel a certain hardenability and improve static strength and toughness, but at the same time, together with V, it also exerts an important effect of improving high temperature softening resistance. In order to ensure the desired hardness of the carburized layer (Hv800 or higher) even after tempering at 400° C. or higher, it is necessary to contain 0.30% or more of Mo along with a predetermined amount of V. However, even if Mo is contained in an amount exceeding 2.00%, the above effect will be saturated and economical efficiency will be impaired, so the upper limit of the Mo content is set at 2.00%.

VV, together with Mo, has the effect of improving the high temperature softening resistance of carburized case hardened steel. In order to ensure the desired hardness of the carburized layer (Hv 800 or higher) even after tempering at 400°C or higher, V must be added to 0.30% or higher along with 0.30% or higher Mo.
It is necessary to contain 10% or more. However, 0.50%
The upper limit of the V content is 0.50% because even if V is contained in an amount exceeding
It was determined that

[00016] Cu, Ni, Cr, Al, Nb, Ti,
B and Pb Each of these components has the effect of further improving the desired properties of carburized case hardening steel, so it is preferable to add one or more of them in a predetermined amount. The reason for limiting the amount added will be explained.

a) Cu Cu is an effective element for increasing the hardenability and static strength of steel. Cu content can be added as appropriate to achieve its effect, but if it is added in excess of 1%, it will deteriorate the hot workability of the steel and conversely reduce the static strength. The upper limit was set at 1.0%.

b) Ni Like Cu, Ni is effective in imparting a certain hardenability to steel and increasing its static strength, and also has the effect of improving the toughness of steel. It can be added arbitrarily to ensure predetermined hardenability and toughness. but,
The upper limit of Ni content was set at 3.5% because the effect saturates and economic efficiency is impaired even if Ni is contained in excess of 3.5%.

c) Cr Cr is an effective additive element for imparting hardenability to steel. Additionally, since it improves carburizing properties, it is often added to general case hardening steel. However, if it is contained in an amount exceeding 5.0%, Cr oxides will be generated and the carburizability will be reduced. Therefore, the upper limit of the Cr content was set at 5.0%.

d) Al Al has the effect of improving toughness by refining high-temperature crystal grains, but if its content is less than 0.010%, the desired effect of the above effect cannot be obtained. Can not. On the other hand, if Al is contained in an amount exceeding 0.100%, the cleanliness of the steel will deteriorate and the machinability will be impaired, and the crystal grains of the steel will become coarser, resulting in a decrease in toughness. Therefore, the Al content was determined to be 0.010 to 0.100%.

e) Nb Nb, like Al, has the effect of refining the crystal grains of steel and improving toughness, but when its content is 0.01
If it is less than 0%, the desired effect cannot be obtained due to the above action. On the other hand, if Nb is contained in an amount exceeding 0.100%, it not only impairs the machinability when machining steel parts, but also coarsens the crystal grains of the steel, which even deteriorates the toughness. Therefore, the Nb content is 0.0
It was set at 10% to 0.100%.

f) Ti Ti, like Al or Nb, has the effect of refining the grains of steel and improving the toughness of the steel, but if its content is less than 0.010%, the above-mentioned The desired effect cannot be obtained by the action. On the other hand, if Ti is contained in an amount exceeding 0.100%, the cleanliness of the steel will decrease, resulting in poor machinability, and will also cause the crystal grains of the steel to become coarser, resulting in a decrease in toughness. Therefore, the Ti content is 0.0
It was set at 10% to 0.100%.

g) B B has the effect of improving the hardenability of steel and increasing the static strength, but if its content is less than 0.0003%, the desired effect of the above effect is not achieved. On the other hand, if the B content exceeds 0.0050%, the crystal grains of the steel will become coarser and the toughness will decrease.
．． It was set at 0003% to 0.0050%.

h) Pb Pb has the effect of improving the machinability of steel, but 0.5
If added in excess of 0%, both static strength and toughness will decrease. Therefore, the upper limit of the Pb content was set at 0.50%.

B) Tempering Temperature In order to completely dissipate the hydrogen that entered during carburizing (usually the hydrogen concentration before carburizing is 0.3 ppm or less) and improve delayed fracture resistance, the steel after carburizing should be It is necessary to heat at 400° C. or higher, and hydrogen cannot be sufficiently diffused below this temperature. However, when tempering is performed at a temperature exceeding 450° C., the hardness of the carburized layer decreases, making it difficult to ensure desired wear resistance. Therefore, the tempering temperature was set at 400 to 450°C.

Next, the effects of the present invention will be explained in detail with reference to examples.

[Example] First, steel having the chemical composition shown in Tables 1 and 2 was melted in a 150 kg vacuum melting furnace, and the obtained steel ingot was heated to 1250°C for 1 hour and forged to 30 mmφ. . Then, a test material was further subjected to normalizing treatment at 925° C. for 1 hr.

[Table 1]

[Table 2]

[00027] Next, using the above test material,
"Impact test", "Measurement of austenite grain size" and "
In addition to measuring static bending strength, we also carried out hydrogen content investigation, delayed fracture testing, and hardness measurement.

[00028] Hydrogen content investigation was conducted using the sample material with a thickness of 6 mm.
After processing to φ×12 mm, carburizing and quenching was performed under the carburizing conditions shown in FIG. 1, and immediately after tempering under the tempering conditions shown in FIG. The results are shown in Tables 3 and 4.

[Table 3]

[Table 4]

[00029] In the delayed fracture test, the specimen material was processed into the test piece shown in Fig. 3, then carburized and quenched under the carburizing conditions shown in Fig. 1, further tempered under the tempering conditions shown in Fig. 2, and then fixed. The test was carried out using a load tensile tester with a load of 630 kg (in the atmosphere). The results are shown in Tables 5 and 6.

[Table 5]

[Table 6]

[00030] For hardness measurement, the sample material was
After processing to 10 mm, carburizing and quenching was performed under the carburizing conditions shown in FIG. 1, and further tempering was performed under the tempering conditions shown in FIG. 2, after which the hardness distribution was measured using a Vickers hardness tester. The hardness of the carburized layer was measured at a position 0.1 mm inside from the surface. The results are shown in Tables 7 and 8.

[Table 7]

[Table 8]

[00031] In the abrasion test, the above sample material was
After processing to 100 mm, carburizing and quenching was performed under the carburizing conditions shown in FIG. 1, and further tempering was performed at 400° C. for 1 hr, followed by testing using a gouging wear tester. The test conditions were: surface pressure: 28 kg/cm2, wear rate: 33 m/min, wear distance: constant 500 m, abrasive material: SiC. The results are shown in Tables 9 and 10.

[Table 9]

[Table 10]

[00032] In the Charpy impact test, the above sample material was
After cutting to 0 mmφ, quenching at 950°C x 1 hr, and further tempering at 180°C x 1 hr, processed into JIS No. 3 (U notch) test piece and subjected to Charpy impact tester.
It was conducted at room temperature. The results are also shown in Tables 9 and 10 above.

Regarding the static bending strength, after processing into the test piece shown in FIG. 4, carburizing and quenching was performed under the carburizing conditions shown in FIG.
The measurements were performed by performing a static bending test at a strain rate of s. In addition,
Strength was evaluated by breaking load. The results are also shown in Tables 9 and 10 above.

Tables 9 and 10 above show the "carburized layer hardness", "delayed fracture test results" and "hydrogen content" when tempered at 400°C after carburizing and quenching under the conditions shown in FIG. ” was also included.

[00035] Now, from the hydrogen content survey results shown in Tables 3 and 4, tempering at 400°C or higher is necessary to dissipate the hydrogen that entered during carburizing to the level before carburizing (≦0.3 ppm). It turns out that it is. In addition, from the delayed fracture test results shown in Tables 5 and 6, it is shown that the delayed fracture resistance is significantly improved (target fracture time ≧ 1000 hr).
It can be confirmed that it is also necessary to perform tempering at a temperature of 400°C or higher. That is, the results shown in Tables 3 to 6 show that ``In order to improve the delayed fracture resistance of steel, 400
℃ or higher to control the hydrogen content to 0.3 ppm or less.''

Table 7 showing hardness after carburizing and quenching and tempering
From Table 8, the following can be confirmed. In other words, in order to ensure wear resistance, which is the original purpose of carburized case-hardened steel, it is necessary to ensure the hardness of the carburized layer: Hv 800 or more, but it is possible to ensure a hardness of Hv 800 or more. The tempering temperature was set for steels with Mo and V within the range specified by the present invention among the present invention steel and comparative steel (however, Cr: 5 of comparative steel 21).
．． (excluding 10% material) from 0 to 450°C. In other words, it can be seen that the tempering temperature needs to be 450°C or lower. On the other hand, among comparative steels, Hv
The tempering temperature range that can ensure a hardness of 800 or higher is 0.
-300°C" and "0-350°C." That is, 35
Tempering must be carried out at a temperature below 0°C, and in this case, delayed fracture resistance decreases.

As mentioned above, Tables 9 and 10 show various properties, Charpy absorbed energy, austenite grain size, and static bending strength when tempered at 400°C after carburizing and quenching. However, from Tables 9 and 10, it is clear that ``the steel of the present invention is heated to 400℃ for the purpose of improving delayed fracture resistance (dissipating hydrogen to the level before carburizing).
Even after high-temperature tempering, the hardness of the carburized layer is Hv 800.
It can be confirmed that the above requirements are met, and all other characteristic values are also good.

On the other hand, among the comparative steels, those whose Mo and V are outside the lower limit of the range specified by the present invention satisfy the target value for delayed fracture resistance, but the hardness of the carburized layer is low ( Hv less than 800), the wear resistance is poor. Also,
If the C content is outside the lower limit of the range specified by the present invention, the static bending strength will be below the target value, and conversely, if the C content is outside the upper limit of the range specified by the present invention, the impact properties will be poor. (Charpy absorbed energy) has decreased significantly. When the Si content and the Mn content are outside the upper limits of the range defined by the present invention, the abnormal carburized layer develops significantly. When the Ti content and Al content are outside the range specified by the present invention, the austenite crystal grains become coarse and the impact properties are significantly inferior. Note that when the Cr content is outside the range specified by the present invention, the carburizing properties are poor due to the formation of Cr oxides, resulting in a significantly low carburized layer hardness.

[00039]

[Summary of Effects] As explained above, according to the present invention, a carburized case-hardened steel that has sufficient properties necessary for a carburized case-hardened steel and has extremely excellent delayed fracture resistance can be produced with ease of workability. This brings about extremely useful effects industrially, such as making it possible to provide products in a good and stable manner.

[Brief explanation of the drawing]

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

FIG. 2 is a diagram showing a tempering pattern.

FIG. 3 is a schematic diagram illustrating the shape of a delayed fracture test piece.

FIG. 4 is a schematic diagram illustrating the shape of a static bending test piece.

Claims (2)

[Claims] [Claim 1] C: 0.10 to 0.3 in weight ratio
0%, Si: 0.50% or less, M
n: 2.00% or less, Mo: 0.30 to 2.00%,
Contains V: 0.10 to 0.50%, and C
u: 1.0% or less, Ni: 3.50% or less, Cr: 5.0% or less, Al: 0.0
10 to 0.100%, Ti: 0.01
0 ~ 0.100%, B: 0.0003 ~ 0.005
0%, Nb: 0.010 ~ 0.100
%, Pb: 0.50% or less, and the remainder consists of Fe and inevitable impurities, and is characterized by having excellent delayed fracture resistance. [Claim 2] C: 0.10 to 0.3 in weight ratio
0%, Si: 0.50% or less, M
n: 2.00% or less, Mo: 0.30 to 2.00%,
Contains V: 0.10 to 0.50%, and C
u: 1.0% or less, Ni: 3.50% or less, Cr: 5.0% or less, Al: 0.0
10 to 0.100%, Ti: 0.01
0 ~ 0.100%, B: 0.0003 ~ 0.005
0%, Nb: 0.010 ~ 0.100
%, Pb: 0.50% or less of steel, the remainder of which is Fe and inevitable impurities, is carburized and quenched, and then tempered in a temperature range of 400 to 450°C. , a method for manufacturing carburized case-hardened steel with excellent delayed fracture resistance.