JPH0978182A - Mechanical structural steel with excellent delayed fracture resistance - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide a mechanical structural steel having excellent delayed fracture resistance. SOLUTION: C: 0.3-0.6%, Si: 0.5-
2.0%, Cu: 0.1 to 1.0%, Cr: 0.1
1.5%, Al: 0.005-0.010%, Nb:
0.005 to 0.20%, Ni: 0.05 to 0.50
%, V: 0.01 to 0.30%, Mn: less than 0.5%,
N: 0.01% or less, Zr: 0 to 0.15%, Ti: 0
~ 0.10%, B: 0 to 0.0050%, Mo: 0.0
1% or less, P: 0.015% or less, S: 0.01% or less, the balance Fe and the composition of unavoidable impurities, and
93 ≦ Al / {N− (Zr / 6.25) − (Ti / 3.
43)-(B / 0.78)} ≦ 10, which is a steel for machine structural use excellent in delayed fracture resistance. The element symbol in the above inequality means the content of that element. The structure is preferably a quenched and tempered structure.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-tensile bolt and a PC steel rod having a tensile strength of 160 kgf / mm ² or more and excellent in delayed fracture resistance, and a high-tensile steel plate for a large machine. The present invention relates to machine structural steel used as.

[0002]

2. Description of the Related Art In recent years, mechanical structural steels, especially high-tensile bolts and PCs, which have higher strength than ever, have been produced due to the increase in size of structures and the weight reduction of automobiles, trucks, civil engineering and mining machinery, etc.
Development of steel rod is needed.

Conventionally, generally used steel for mechanical structures has a tensile strength of 100 kgf / mm ² level, for example, 0.4% C
JI having a composition of -1.05% Cr-0.23% Mo
In the low alloy steel of SCM440 specified in SG4105 (1989), the tensile strength of 130 kgf / mm ² level, for example,
0.17% C-3% Ni-1.6% Cr-0.5% Mo
It is manufactured by subjecting a hot-rolled material of a low alloy steel of SNCM616 specified in JIS G4103 (1989) having the composition of No. 1 to quench hardening and tempering treatment. Further, those having a tensile strength of 174 kgf / mm ² level are manufactured by subjecting the above hot-rolled material of the low alloy steel to quenching and tempering treatment under different heat treatment conditions. However, when these mechanical structural steels are put into practical use, delayed fracture may occur during use, so when using them as important safety parts for automobiles and civil engineering machinery, including high-tensile bolts and PC steel rods, quality Lack of stability was a problem.

Delayed fracture is a phenomenon in which steel placed under static load suddenly becomes brittle after a certain period of time.
It is considered to be a kind of hydrogen embrittlement due to hydrogen that has penetrated into steel from the external environment.

From the above, the strength level of the above-mentioned steel for machine structural use has so far been 10% in terms of tensile strength.
It is said that it is practically desirable to set it to 0 kgf / mm ² or less.

On the other hand, as a steel having a delayed fracture resistance superior to that of the above-mentioned ordinary low alloy steel, for example, 18% Ni-
7.5% Co-5% Mo-0.5% Ti-0.1% Al
There is an 18% Ni maraging steel having the composition of
Since it is extremely expensive, its use is limited from the economical point of view. Therefore, structural steels and steels for high-strength bolts, which have high strength and excellent delayed fracture resistance in consideration of economic efficiency, are disclosed in, for example, JP-A-58-84960 and 61-117248.
And 61-130456. Furthermore, JP-A-3-243745 and JP-A-2-243745
Japanese Patent No. 145746 and the like propose a steel in which various components are added to improve delayed fracture resistance and a manufacturing method thereof.
However, since the proposals of the above-mentioned respective publications each contain Mo as a component of steel, they are still problematic in terms of economical efficiency, and are not necessarily sufficient in terms of toughness and hydrogen permeability.

Japanese Patent Laid-Open Nos. 58-113317 and 58-1
57921, 58-61219 and 58-11.
No. 7856 proposes a technique that does not contain Mo as a steel component, but it is not always sufficient in terms of toughness and hydrogen permeability.

[0008]

SUMMARY OF THE INVENTION The present invention provides a mechanical structural steel having a tensile strength of 160 kgf / mm ² or more and excellent delayed fracture resistance in order to meet the above-mentioned demands of the industrial world. That is the purpose. More specifically, for example,
Rather than using it permanently like high tension bolts for bridges, it is assumed that it will be repaired or replaced regularly.
An object of the present invention is to provide at low cost a mechanical structural steel having a tensile strength of 160 kgf / mm ² or more, which does not cause delayed fracture within a certain period. Such applications include high-strength steels for various structures, automobiles, civil engineering machines, steels for bolts and high-strength steel sheets for industrial machines, and by using the steel of the present invention for these, the above industrial requirements Can meet.

That is, the present invention provides a mechanical structural steel having a tensile strength of 160 kgf / mm ² or more, which has no risk of delayed fracture for a predetermined period and therefore can be safely used on the premise of periodic replacement. It is intended to be provided at a low price.

[0010]

[Means for Solving the Problems] Many structural steels which have been proposed so far and have excellent delayed fracture resistance have Mo added thereto.

Mo in steel has an action of suppressing the invasion of hydrogen by lowering the hydrogen overvoltage. This effect prevents hydrogen embrittlement, which causes delayed fracture, and improves delayed fracture resistance of steel. However, since Mo is an expensive element, the steel containing Mo is often limited in application from the viewpoint of economy.

Therefore, the inventor of the present invention is cheaper than Mo and
C, which is known to suppress the invasion of hydrogen as with o
As a result of studying the effect of Cu-containing steel on the delayed fracture resistance, focusing on u, the following findings (a) to (d) have been obtained.

(A) Cu has been proposed for a low alloy steel having a Mo content range (about 0.01 to 0.8% by weight) that has been proposed so far.
Even if additional is added, no further corrosion inhibition effect and hydrogen penetration inhibition effect are recognized. That is, as shown in FIG.
Even if 0.3% by weight of Cu is added to various low alloy steels with an o content of 0.01 to 0.8% by weight, the amount of hydrogen permeation and the corrosion rate are higher than those in the case of not adding Cu There is no substantial difference. Note that FIG. 2 shows the results of investigations using various low alloy steels each having a Cr content of 2% by weight or less.

(B) If the alloy composition other than Mo is left as it is and only Cu is added to the low alloy steel containing no Mo, the desired corrosion inhibiting effect cannot be obtained.

(C) If the Cr content is lowered together with the addition of Cu, the effect of suppressing hydrogen invasion by adding Cu to the low alloy steel to which Mo is not added (b) is dramatically improved. Can be increased. Moreover, the effect of suppressing hydrogen invasion in this case is larger than that in the case of containing Mo, as shown in FIG. Note that FIG. 1 shows that 0.5 wt% Mo-added steel containing 0.3 wt% Cu and non-Mo-added steel (Mo content is 0.003 wt%, which is an impurity level), and Cr content is hydrogen. It is the result of investigating the influence on the permeation amount. The chemical composition of the steel used in the figure other than Cu, Mo and Cr is% by weight, C: 0.31%, Si: 0.26
%, Mn: 0.35%, P: 0.005%, S: 0.0
08%, Al: 0.006%, Ni: 0.46%, N
b: 0.173%, V: 0.05% and N: 0.003
%.

(D) In order to enhance the delayed fracture resistance of high strength steel having a tensile strength of 160 kgf / mm ² or more, Al content and N content, which are also not combined with Zr, Ti and B It is important to optimize the ratio with the so-called "effective N amount (EN amount)".

The gist of the present invention based on the above findings is the steel for machine structural use which is excellent in delayed fracture resistance as shown in the following (1) and (2).

(1) C: 0.3 to 0.6% by weight,
Si: 0.5-2.0%, Cu: 0.1-1.0%, C
r: 0.1 to 1.5%, Al: 0.005 to 0.010
%, Nb: 0.005 to 0.20%, Ni: 0.05 to
0.50%, V: 0.01 to 0.30%, Mn: 0.5
%, N: 0.01% or less, Zr: 0 to 0.15%,
Ti: 0 to 0.10%, B: 0 to 0.0050%, M
o: 0.01% or less, P: 0.015% or less, S: 0.
A mechanical structural steel having a composition of 01% or less, the balance being Fe and unavoidable impurities, and excellent in delayed fracture resistance satisfying 1.93 ≦ fn1 ≦ 10.

Here, fn1 = Al / {N- (Zr /
6.25)-(Ti / 3.43)-(B / 0.7
8)}. The element symbol in the above fn1 means the content of the element in% by weight.

(2) A steel for machine structural use, which has a quenched and tempered structure and is excellent in delayed fracture resistance as described in (1) above.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition and structure of steel in the present invention as described above will be described below. In addition, "%" means "weight%."

(A) Chemical Composition of Steel C: C forms a carbide and strengthens the steel by precipitation strengthening,
It also forms a stable martensite structure during quenching and strengthens the steel by transformation strengthening, so it is an essential element for increasing the strength. Further, it is an effective component for increasing hardenability and making crystals finer.

However, if the content is less than 0.3%, the hardenability is deteriorated, and the amount of precipitated carbide is small and the strength is impaired. On the other hand, if it exceeds 0.6%, the susceptibility to quench cracking during quenching increases and, in addition, the steel remarkably hardens and ductility and workability deteriorate. Therefore, the content of C is 0.3 to 0.
6%. In addition, in order to stably obtain a desired high strength, the C content is preferably 0.4 to 0.6%.

Si: Si is an element effective for deoxidizing steel and increasing strength. Particularly, expensive Mo such as the steel of the present invention
In order to enhance the delayed fracture resistance of the steel, the strengthening action of Si is effective for high strength steel having a low Cr content. However, if the content is less than 0.5%, the desired effect cannot be obtained on the other hand, while the content is 2.0%.
If it exceeds, the cleanliness of the steel may be impaired and the toughness may deteriorate. Therefore, the Si content is set to 0.5 to 2.0%.

Cu: Cu suppresses the penetration of hydrogen into the steel from the external environment, and by adding Nb and Cr in combination, the tempering softening resistance of the steel can be remarkably increased, so that a high tempering temperature can be taken. Together, they have the effect of improving delayed fracture resistance. However, if its content is less than 0.1%, its effect is small, while if it exceeds 1.0%, weldability, hot workability and toughness deteriorate. Therefore, the content of Cu is set to 0.1 to 1.0%.

Cr: Cr has the effects of improving the hardenability of steel and imparting temper softening resistance to the steel. Especially Nb
Addition of Cu and Cu gives a remarkable temper softening resistance to steel, but if its content is less than 0.1%, the effect of addition is poor. On the other hand, if it exceeds 1.5%, C in the steel without Mo addition
This impairs the effect of u to suppress hydrogen intrusion and deteriorates delayed fracture resistance. Therefore, the Cr content is 0.1 to 1.5%.
And In addition, in order to reliably exhibit the effect of suppressing hydrogen intrusion of Cu in the Mo-free steel, the upper limit of the Cr content is preferably set to about 0.5%.

Al: Al is effective for stabilizing the deoxidation of steel, homogenizing it, and making it finer, but if it is less than 0.005%, the desired effect cannot be obtained. On the other hand, 0.010
If it is contained in excess of%, at the high strength level targeted by the present invention, the amount of inclusions falls within the range that deteriorates the delayed fracture resistance. Therefore, in the present invention, the Al content is set to 0.005 to 0.010%.

By adding Nb: Nb, grain refinement in steel is promoted, grain boundary segregation is reduced, and delayed fracture resistance is further improved. However, if it is less than 0.005%, the desired effect cannot be obtained, while if it exceeds 0.20%, toughness, ductility and the like are impaired. Therefore, the content of Nb is set to 0.005 to 0.20%.

Ni: Ni is effective in increasing the toughness of steel and is effective in preventing the deterioration of hot workability due to Cu checking. However, if the content is less than 0.05%, the desired effect cannot be obtained. On the other hand, if it exceeds 0.50%, the effect is saturated, and since Ni is an expensive alloying element, its content is 0.0 in the present invention in consideration of economical efficiency.
It was set to 5 to 0.50%.

V: Addition of V has the effect of promoting grain refinement of the steel as in the case of Nb, and the delayed fracture resistance is further improved by reducing grain boundary segregation. Further, since V has the effect of imparting temper softening resistance to steel, delayed fracture resistance is also improved by making it possible to adopt a high tempering temperature.
However, if the content is less than 0.01%, the effect of addition is poor, and if it exceeds 0.30%, the toughness deteriorates. Therefore, the V content is set to 0.01 to 0.30%.

Mn: Mn is an element effective for deoxidizing and improving the hardenability, but if it is contained in a large amount, a grain boundary embrittlement phenomenon occurs and promotes the occurrence of delayed fracture. Further, since Mn is combined with S and this becomes the starting point of cracking, its content must be reduced as much as possible in order to improve delayed fracture resistance. Therefore, the Mn content is set to less than 0.5% in the present invention for improving delayed fracture resistance. The Mn content may be substantially zero.

N: N is an element effective for reducing the grain size of steel and enhancing delayed fracture resistance. However, in low alloy steels, it is usually difficult to contain more than 0.01%, so the content was made 0.01% or less. It should be noted that the N content is the above-mentioned inequality regarding fn1, 1.93 ≦ fn1
Since it is necessary to satisfy ≦ 10, the upper and lower limits are determined in relation to the contents of Al, Zr, Ti and B, respectively.

Zr: Zr may not be added. If added, it has the effect of dispersing carbides in the steel in a spherical and fine manner to further enhance delayed fracture resistance. Therefore, it may be contained especially for the purpose of ensuring high delayed fracture resistance in the case of high strength steel. In order to reliably obtain the above effect, it is preferable that the Zr content be 0.01% or more. However, if the content exceeds 0.15%, the toughness deteriorates.
Therefore, the Zr content is set to 0 to 0.15%.

Ti: Ti may not be added. If added, it has the effect of further enhancing the hardenability of the steel to increase its strength and strengthening the grain boundaries to further improve the delayed fracture resistance. Therefore, it may be added for the purpose of ensuring high strength particularly when the product size is large. In order to reliably obtain the above effect, it is desirable that the content of Ti be 0.01% or more. However, if its content exceeds 0.10%,
It deteriorates the toughness of steel. Therefore, the content of Ti is set to 0 to 0.10%.

B: B may not be added. If added, it has the effect of further enhancing the hardenability of the steel to increase its strength and strengthening the grain boundaries to further improve the delayed fracture resistance. Therefore, it may be added for the purpose of ensuring high strength, especially when the product size is large. In order to surely obtain the above effects, the content of B is preferably 0.0003% or more. However, if its content exceeds 0.0050%,
It deteriorates the toughness of steel. Therefore, the B content is 0
Was set to 0.0050%.

Mo: Mo reduces the hydrogen invasion suppressing effect of Cu in the steel containing the above-mentioned amount of Cr.
This is because Cu has an effect of suppressing hydrogen invasion by reducing the corrosion rate, while Mo has an action of increasing the corrosion rate. Therefore, in the present invention, the content of Mo should be 0.01 or less so as not to exceed the range of impurities in steel.
% Or less.

P: P cannot completely eliminate the grain boundary segregation by any heat treatment, and lowers the grain boundary strength and deteriorates the delayed fracture resistance. Therefore, P as an impurity in the present invention. Was set to 0.015%.

S: S is combined with Mn as a starting point of cracking as described above, and further promotes hydrogen embrittlement which causes segregation at grain boundaries and causes delayed fracture, so its content should be as low as possible. It is necessary to limit it. Therefore, in the present invention, the S content as an impurity is set to 0.01% or less.

Fn1: As shown in FIGS. 3 to 6, by controlling the above-mentioned value of fn1 to be 1.93 ≦ fn1 ≦ 10, the desired excellent resistance to the steel of the present invention which is directed to extremely high strength can be obtained. It became clear that delayed fracture can be imparted.

That is, the present inventor has found that C, Si, Mn,
P, S, Cu, Cr, Mo, Al, Nb, Ni, V, Z
Steels in which the contents of r, Ti and B are within the range of the steel of the present invention described above, the N content is 0.01% or less, and fn1 has various values are produced in a laboratory. . Then, hot rolling was performed by a usual method, and then quenching and tempering treatment was performed to adjust the tensile strength to 160 kgf / mm ² or more. Next, the delayed fracture characteristics of these steel materials were investigated by the test method described in detail in the later examples. 3 to 6 show the results. 3 includes Al, but Zr, T
FIG. 4 shows the results for steels not containing i and B, FIG. 4 shows the results for steels containing Al and Zr but not Ti and B, and FIG. 5 shows the results for steels containing Al, Ti and B but not Zr. The result is shown.
Further, FIG. 6 shows the results in the steel containing all of Al, Zr, Ti and B.

In FIGS. 3 to 6 above, the mark ◯ indicates that cracking (delayed fracture) did not occur, and the mark ● indicates that cracking occurred. From these figures, the value of fn1 is 1.
It is clear that cracks occur in both cases of less than 93 and more than 10. Furthermore, it was found that all cracks originated from nitride. Therefore, in the present invention, the definition of 1.93 ≦ fn1 ≦ 10 is provided.

(B) Structure of Steel Even if the steel has the above-mentioned chemical composition, it is desirable to have a quenching and tempering structure in order to have tensile strength of 160 kgf / mm ² or more and good delayed fracture resistance. . As an example of heat treatment for that purpose, normal hot rolling (heating temperature: 1000 to
1250 ° C.) and immediately after rolling, quenching with water or oil from a temperature of Ar ₃ or higher (preferably 850 to 1020 ° C.), or 850 to 1050 ° C., preferably 920 to 920 ° C.
Reheated to 1020 ℃ and then quenched with water or oil to obtain low-temperature transformation products (martensite and bainite).
There is a treatment for tempering this at a temperature of Ac ₁ point or less. However, the structure of the steel of the present invention does not necessarily have to be quenched and tempered (QT).
It does not have to be an organization. This is because the steel of the present invention reduces the amount of hydrogen penetration during use and improves delayed fracture resistance, and therefore does not depend so much on the internal structure. For example, as hot rolled or as quenched (As
Even in organizations such as Q), as shown in the examples below, 160 kgf
It exhibits tensile strength of / mm ² or more and excellent delayed fracture resistance.

Although the as-quenched steel has a high tensile strength, it has a low yield point, and when used as a mechanical structural steel, there is a problem that stress relaxation increases during use.

Therefore, in order to impart predetermined strength and delayed fracture resistance to the steel, it is desirable to carry out tempering treatment after quenching so that the structure of the steel becomes a quenched and tempered structure (mainly tempered martensite structure).

[0045]

EXAMPLES The present invention will now be described by way of one example in comparison with a comparative steel. It should be noted that these examples are examples showing the effects of the present invention, and do not limit the technical scope of the present invention at all.

First, the following Tables 1 to 5 are prepared by a usual method.
Steel (No. 1 to 46) having the composition shown in Table 1 was melted in a 50 kg vacuum melting furnace. Steels 1 to 37 have the composition of the present invention, and steels 38 to 46 are steels having compositions outside the scope of the present invention in that they are marked with * in Table 5. Steels 47 to 49 are conventional steels, 47 is JIS G 4105 (1989) SCM440 steel and 48 is JIS
G 4103 (1989) SNCM616 steel, 47 is disclosed in JP-A-58-849.
This is the steel proposed in Japanese Patent Laid-Open No. 60.

Steel 1 to 4, 8 to 15, 19 to 24, 28 to 32, 36 to
39, 43 to 46 are hot forged and hot rolled at 1100 to 1200 ° C. to be a plate material having a thickness of 15 mm, reheated to 950 ° C., held for 45 minutes, water-quenched, tempered and air-cooled,
Its structure is a quenched and tempered structure and its tensile strength is 160 kgf.
The delayed fracture property was investigated by adjusting so as to be not less than / mm ² .
Further, the same quenching and tempering treatment was performed on the conventional steels of 47 to 49. 87 for 47, 48 and 49 respectively
After reheating to 0, 900, 950 ° C and holding for 45 minutes,
It was water-quenched and then tempered. Furthermore, as-hot-rolled materials other than the above (No. 5, 16, 25, 33, 40), as-quenched materials (No. 6, 17, 26, 34, 41), and subjected to accelerated cooling after hot-rolling Steel (No. 7, 18, 27, 35, 42) was also investigated.
Accelerated cooling conditions are 10 to 15 ° C / 400 to 500 ° C.
Water cooling was performed so that the cooling rate was s.

The investigation of the delayed fracture property was based on the constant load test method. That is, a test piece 1 having a shape and dimensions as shown in FIG. 7 is set in a constant load tester 8 as shown in FIG. 8 and a pH = 2 Walpol solution (mixed solution of hydrochloric acid and sodium acetate aqueous solution) 2 is set. 750 in an environment circulated by pump 3
A static load (tensile stress: 140 kgf / mm ² ) is applied by the weight 4 for a period of time, and a constant current (1 mA) is applied between the test piece 1 as a cathode and the counter electrode 6.
/ cm ² ) and flowing hydrogen into the test piece 1 while observing the occurrence of breakage. The test temperature is the temperature controller 7
And kept at 25 ° C. The results of this test are shown in Tables 6 to 10, in which those that did not break were marked with ◯, and those that did break were marked with x and the strength level of each steel. In FIG. 7, the numeral is m
Indicates the length in units of m.

PH = 2 as the test environment corresponds to the most severe environment that can be realized in the actual use environment. Therefore, this result is considered to evaluate delayed fracture resistance in the most severe environment of actual use. 25 ° C. as a test temperature is one standard temperature for performing a delayed fracture test.

From Tables 6 to 10, it is apparent that the steels of the present invention have excellent constant fracture resistance since their constant load fracture time exceeds 750 hours. Further, it can be seen that the toughness is improved because the shelf energy value of the Charpy test is high, and the ductility is that the deformation required stress of the high temperature compression test is small, and thus the improvement is achieved.

That is, according to the present invention, it is possible to obtain a steel for machine structural use having a tensile strength of 160 kgf / mm ² or more, and as described above, on the premise of regular repair or replacement, the required degree of delayed fracture resistance. The steel for machine structural use according to the present invention can be widely used for the steels having clear applications.

[0052]

[Table 1]

[0053]

[Table 2]

[0054]

[Table 3]

[0055]

[Table 4]

[0056]

[Table 5]

[0057]

[Table 6]

[0058]

[Table 7]

[0059]

[Table 8]

[0060]

[Table 9]

[0061]

[Table 10]

[0062]

As described above, the mechanical structural steel excellent in delayed fracture resistance according to the present invention has a tensile strength of 160 kgf / mm ²
Above, it is also excellent in delayed fracture resistance. Therefore, there is no risk of delayed fracture occurring within a certain period of time on the premise of regular repairs or replacements.
Further, it can be used as a steel for machine structure used for a high-strength steel plate for large machines.

[Brief description of drawings]

FIG. 1 0.5 wt% Mo with 0.3 wt% Cu
It is a figure which shows the influence which Cr content has on hydrogen permeation | transmission amount in addition steel and Mo non-addition steel (Mo content is 0.003 weight% which is an impurity level).

FIG. 2 shows a case where 0.3 wt% of Cu is added to various low alloy steels having Mo contents of 0.01 to 0.8 wt% and Cu.
It is a figure which compares and shows the hydrogen permeation | transmission amount in the case of no addition, and a corrosion rate.

FIG. 3 is a diagram showing the influence of the contents of Al and N in the steel containing Al but not Zr, Ti and B and having a tensile strength of 160 kgf / mm ² or more on the occurrence of delayed fracture.

FIG. 4 Al content and EN = N in steel having a tensile strength of 160 kgf / mm ² or more containing Al and Zr but not Ti and B.
It is a figure which shows the influence which the quantity of- (Zr / 6.25) has on delayed fracture occurrence.

FIG. 5: Al content and EN = in a steel containing Al, Ti and B but not Zr and having a tensile strength of 160 kgf / mm ² or more.
It is a figure which shows the influence which the quantity of N- (Ti / 3.43)-(B / 0.78) gives to delayed fracture generation.

FIG. 6 Tensile strength including all of Al, Zr, Ti and B
Al content in steel of 160 kgf / mm ² or more and EN = N-
(Zr / 6.25)-(Ti / 3.43)-(B / 0.
It is a figure which shows the influence which the quantity of 78) has on delayed fracture occurrence.

7A and 7B are diagrams showing shapes and dimensions of a test piece and a notch used in a constant load test in Examples, where (A) shows the test piece and (B) shows details of the notch portion of the test piece.

FIG. 8 is a diagram showing an outline of a constant load test method.

[Explanation of symbols]

1: Test piece, 2: Walpole solution, 3: Pump, 4: Weight, 5: Potentiostat, 6: Counter electrode, 7: Temperature control device, 8: Constant load tester

Claims (2)

[Claims] 1. C: 0.3-0.6% by weight, Si:
0.5-2.0%, Cu: 0.1-1.0%, Cr:
0.1-1.5%, Al: 0.005-0.010%,
Nb: 0.005 to 0.20%, Ni: 0.05 to 0.
50%, V: 0.01 to 0.30%, Mn: less than 0.5%, N: 0.01% or less, Zr: 0 to 0.15%, T
i: 0 to 0.10%, B: 0 to 0.0050%, Mo:
0.01% or less, P: 0.015% or less, S: 0.01
% Or less, the balance Fe and the composition of unavoidable impurities, and
A steel for machine structural use which is excellent in delayed fracture resistance satisfying 1.93 ≦ fn1 ≦ 10. However, fn1 = Al / {N- (Zr /
6.25)-(Ti / 3.43)-(B / 0.78)} 2. The steel for machine structural use according to claim 1, which has a quenched and tempered structure and is excellent in delayed fracture resistance.