JPH09209085A - Steel for machine structural use and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for machine structural use, containing specific weight percentages of specific elements, free from delayed fracture for a fixed period, and having a tensile strength of specific value or above. SOLUTION: This steel for machine structural use has a composition which consists of, by weight, 0.3-0.6% C, 0.5-2% Si, 0.1-1% Cu, 1.5-5% Cr, 0.05-1% Mo, 0.005-0.01% Al, 0.005-0.2% Nb, 0.05-0.5% Ni, 0.01-0.3% V, 0-0.5% Mn, <=0.01% N, 0-0.15% Zr, 0-0.1% Ti, 0-0.005% B, and the balance Fe with inevitable impurities and simultaneously satisfies 0.4(%)<=Cu(%) +Mo(%) and 1.93<=Al(%)/[effective N(%)]=10, where equation [effective N(%)]=N(%)-Zr(%)/6.25-Ti(%)/3.43-B(%)/0.78 is satisfied, and in which the contents of P and S among the inevitable impurities are regulated to <=0.015% and <=0.01%, respectively. Further, this steel for machine structural use, excellent in delayed fracture resistance, can be produced by performing hardening and then tempering at a temp. less than the Ac1 point.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to 160 kgf / m.
Having a tensile strength of m ² or more and excellent delayed fracture resistance,
The present invention relates to a machine structural steel suitable for use in high-tensile bolts, PC steel rods, large machines, and the like.

[0002]

2. Description of the Related Art In recent years, with the increase in size of structures and the weight reduction of automobiles, trucks, civil engineering and mining equipment, etc., mechanical structural steels with higher strength than ever, especially high-tensile bolts and PCs.
A steel material for a steel rod is desired.

The low-alloy, high-strength mechanical structural steels that have been commonly used until now are as follows.

(A) Tensile strength 100 kgf / mm ² grade:
SCM440 defined by JIS G 4105 (1989) represented by 0.4% C-1.05% Cr-0.23% Mo steel.

(B) Tensile strength 130 kgf / mm ² grade:
0.17% C-3% Ni-1.6% Cr-0.5% Mo
SNCM61 stipulated by JIS G 4103 (1989) represented by steel
6.

(C) Tensile strength 174 kgf / mm ² grade:
Steel with the same composition as in (a) or (b) but with different heat treatment.

In both of the above (a) and (b), quenching and tempering treatment is performed after hot rolling to give a required strength. The tensile strength of (C) is 174 kgf / m.
In the m ² class, toughening is achieved by hot rolling the above low alloy steel and then performing quenching and tempering treatment under different conditions from the heat treatment performed in (a) or (b).

These mechanical structural steels, however,
Since delayed fracture may occur during use, it has not been fully used for important parts of automobiles and civil engineering machines, including high-tensile bolts and PC steel rods. “Delayed destruction”
The phenomenon is a phenomenon in which steel placed under a static load suddenly becomes brittle after a certain period of time, and is considered to be a kind of hydrogen embrittlement due to hydrogen that has penetrated into the steel from the external environment.

Since the above low alloy steel for machine structural use generally causes delayed fracture, its tensile strength is 100%.
It is said that it is desirable to limit it to kgf / mm ² or less.

As a steel having a delayed fracture resistance superior to that of the above low alloy steel, for example, 18% Ni-7.5% Co-5% M.
18% typified by o-0.5% Ti-0.1% Al steel
Ni maraging steel is available, but its application is limited from the economical point of view because it is extremely expensive.

Therefore, as a steel for high-strength bolts having excellent delayed fracture resistance in consideration of economic efficiency, a steel is proposed in which Ca or the like is added to control the morphology of sulfides and at the same time, Cr and Mo are added in combination. (JP-A-58-84960, JP-A-61-117248 and JP-A-61-1304)
No. 56). However, even with these steels, the delayed fracture resistance is not sufficient, and therefore steels in which Si and Mn are suppressed to a low level have been developed (JP-A-3-243745 and JP-A-2-).
145746, etc.). However, even those steels having reduced Si and Mn are still insufficient in terms of hydrogen permeability or toughness, and have not been generally used as steel materials that do not cause delayed fracture. Here, hydrogen permeability refers to the degree of easiness of hydrogen permeation (penetration) into the steel as hydrogen gas during the corrosion reaction on the surface of the steel accompanied by reduction of hydrogen ions, without remaining in the environment as hydrogen gas. means. The smaller the hydrogen permeability, the smaller the amount of hydrogen in the steel, which is the cause of embrittlement, and the delayed fracture is less likely to occur.

The steels with improved delayed fracture resistance are
After the tensile strength was limited to 100 to 120 kgf / mm ² , in the dipping experiment in a Walpol solution (mixture of hydrochloric acid and sodium acetate aqueous solution) of pH = 2 described later, 2
It is said that cracks should not occur within 00 hours. The environment in which these steels are actually used has a slower effect of causing delayed fracture than Walpol's liquid, so
A fixed period of time longer than 200 hours is set, and parts (bolts, etc.) using these steel materials are replaced for each period.

[0013]

DISCLOSURE OF THE INVENTION The present invention is 160 kg.
An object of the present invention is to provide a mechanical structural steel having a tensile strength of f / mm ² or more and excellent delayed fracture resistance. Specifically, assuming not to use it permanently like high tension bolts for bridges, etc., but to replace it regularly, for example,
In a Walpol solution of pH = 2, a constant load of 140 kgf / mm ² was applied to a tensile test piece with a notch, and 1 mA / cm 2.
^An object of the present invention is to provide a steel material having a tensile strength of 160 kgf / cm ² or more, which does not cause delayed fracture at all within 750 hours in the state where a constant current of ² is applied.

[0014]

[Means for Solving the Problems] Many of the structural steels which have been proposed so far and have excellent delayed fracture resistance are those to which Mo is added. Mo in the steel suppresses the invasion of hydrogen into the steel by lowering the hydrogen overvoltage, that is, by promoting the hydrogen reduction reaction, thereby increasing the ratio of hydrogen gas remaining in the external environment. This effect prevents hydrogen embrittlement, which is the cause of delayed fracture, and improves the delayed fracture resistance of steel. However, in order to have strength higher than the current level and excellent delayed fracture resistance equal to or higher than the current level, it is necessary to suppress hydrogen penetration more severely than the current level.

Therefore, the present inventors have focused on Cu, which is known to suppress the penetration of hydrogen, like Mo, and
As a result of studying the effect of the combined addition of o and Cu on the delayed fracture resistance, the following items (a) to (c) could be confirmed.

(A) Even if Cu is additionally added to the low alloy steel containing the amount of Mo (0.01 to 0.8% by weight) that has been conventionally proposed, a further hydrogen penetration inhibiting effect is recognized. Absent.

FIG. 1 is a drawing showing the effect of Mo and Cu on the amount of hydrogen permeation through a low alloy steel containing less than 1.5% Cr. According to the figure, even if various low alloy steels with Mo of 0.01 to 0.8% (Cr less than 1.5%) contain Cu 0.3%, the amount of hydrogen permeation is greater than that in the case where Cu is not added. Does not result in a substantial difference, and therefore the amount of hydrogen penetration into the steel does not decrease. The center values of alloying elements other than Cu and Mo of the steel used for obtaining FIG. 1 are C: 0.35%, Si: 0.64.
%, Mn: 0.22%, P: 0.005%, S: 0.0
02%, solAl: 0.005%, Cr: 0.63
%, Ni: 0.25%, Nb: 0.08%, V: 0.0
5% and N: 0.003%. Since the hydrogen permeation amount is measured by ionization, the unit of hydrogen permeation amount (μC / cm) is shown in the figure in a unit including the electric amount (μC: microcoulomb).

(B) Mo steel with Cu containing 1.5% of Cr
When the content is high as above, the effect of suppressing hydrogen invasion can be dramatically enhanced.

FIG. 2 is a drawing showing the effect of Cr and Cu on the hydrogen permeation amount of 0.4% Mo steel. The center values of the alloy elements other than Cu, Mo and Cr of the steel used in the figure are C: 0.35%, Si: 0.28%, Mn: 0.3.
6%, P: 0.005%, S: 0.008%, solA
1: 0.006%, Ni: 0.45%, Nb: 0.17
1%, V: 0.05% and N: 0.003%,
The variability was very small. According to the figure, it can be seen that the hydrogen permeation amount of 0.4% Mo steel containing Cr at 1.5% or more and simultaneously containing 0.3% of Cu is significantly reduced.

(C) In order to enhance the delayed fracture resistance of high strength steel having a tensile strength of 160 kgf / mm ² or more, Al is
In addition to the limitation only by the Al content, the ratio with N, that is Z
It is necessary to limit the ratio to the so-called "effective N", which is not bound to r, Ti and B, to a proper range.

The present invention based on the above matters has the following (1).
The gist is a steel for machine structural use, which is excellent in delayed fracture resistance, and a method for producing the same, as shown in (2).

(1) C: 0.3-0.6% by weight,
Si: 0.5-2%, Cu: 0.1-1%, Cr: 1.
5-5%, Mo: 0.05-1%, Al: 0.005-
0.01%, Nb: 0.005-0.2%, Ni: 0.
05-0.5%, V: 0.01-0.3%, Mn: 0-
0.5%, N: 0.01% or less, Zr: 0 to 0.15
%, Ti: 0 to 0.1% and B: 0 to 0.005%, at the same time satisfying the following formulas and formulas, the balance consisting of Fe and inevitable impurities, and P and inevitable impurities: A steel for machine structural use having excellent delayed fracture resistance, wherein the content of S is P: 0.015% or less and S: 0.01% or less.

0.4 (%) ≤ Cu (%) + Mo (%) ・ ・ ・ 1.93 ≤ Al (%) / effective N (%) ≤10 ... However, effective N (%) = N (%)-{Zr (%) / 6.
25}-{Ti (%) / 3.43}-{B (%) / 0.
78} (2) After quenching, tempering is performed at a temperature of Ac ₁ point or less, The method for producing a steel for machine structural use according to claim 1.

Here, effective N is A contained in steel.
When it is assumed that the nitride-forming elements Zr, Ti and B other than 1, Cr and the like are all combined with N to form a nitride,
Nitrogen other than nitrogen that forms Zr nitride, Ti nitride and B nitride. That is, under the above assumption, solid solution nitrogen ([ N ]) and nitrogen as AlN (N as A
lN) and a nitride with other elements, for example, the sum of nitrogen (N as CrN, etc.) as a nitride of Cr. Zr and Ti nitrides, which are strong nitride-forming elements, are
It has a feature that it is generated at a high temperature of 50 ° C or higher and is likely to coarsen.
Practically all Zr, Ti and B do not bond with N, so the actual solid solution nitrogen, N as AlN and N
The sum, such as as CrN, is greater than the effective N defined above. Further, in the above range, a large amount of Cr nitride is not formed, so in practice, most of the effective N is solid solution nitrogen and N as A.
It consists of lN. However, in the high temperature range of 1150 ° C or higher, Al
Since N dissolves in solid solution, almost all of it becomes solid solution nitrogen in the same temperature range.

"Al / effective N", which is obtained by dividing Al (%) by effective N (%), is used because it is convenient for rearranging the experimental values as will be described later, and its metallic meaning. Has not been completely clarified, but it is considered that it limits the amount of AlN and solid solution Al by limiting Al and N to the above range and displays the easiness of forming fine AlN. To be

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition of steel and the manufacturing method according to the present invention will be described below.

1. Chemical Composition In the present description, “%” represents “% by weight”.

C: C forms a carbide and strengthens the steel by precipitation strengthening, and also forms stable martensite during quenching to strengthen the steel by transformation, so it is essential for increasing the strength. It is an element. Further, it is also effective in improving hardenability and making crystals finer. If it is less than 0.3%, the hardenability is deteriorated, and the amount of precipitated carbide is small and the strength is lowered. 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 the ductility, weldability and workability deteriorate, so 0.3-0.6% To do. In order to stably obtain the desired high strength,
It is desirable to be 0.4 to 0.6%.

Si: Si is effective for deoxidizing steel and increasing strength. If it is less than 0.5%, the effect cannot be obtained. On the other hand, if it exceeds 2%, the cleanliness of the steel may be impaired and toughness may be deteriorated, so the content is made 0.5 to 2%.

Cu: Cu suppresses hydrogen invasion into the steel from the outside, and by adding Nb and Cr in combination, the tempering softening resistance of the steel is remarkably increased and the tempering temperature can be increased, so that delayed fracture resistance Greatly improve. Further, as shown in FIG. 2, 1.5% or more of Cr
Also, by coexisting with a fixed amount of Mo, the hydrogen permeation amount is remarkably reduced, and it is effective in improving delayed fracture resistance. If less than 0.1%, the effect is not sufficient, while 1
%, The weldability, hot workability and toughness are deteriorated, so the content is made 0.1 to 1%.

Cr: Cr improves the hardenability of the steel and enhances the temper softening resistance of the steel. In particular, the addition of Nb and Cu in combination significantly improves the temper softening resistance. 1.
If it is less than 5%, the effect of suppressing hydrogen intrusion by the combined addition of Cu and Mo cannot be obtained, and the delayed fracture resistance cannot be dramatically improved. On the other hand, if it exceeds 5%, weldability, hot workability and toughness deteriorate. Therefore, Cr is 1.5
To 5%. In order to stably obtain a very excellent delayed fracture resistance, the content is preferably 1.7 to 4%.

Mo: Mo has a different mechanism but exhibits a hydrogen invasion suppressing effect by itself like Cu. It also improves the hardenability of steel and at the same time increases the resistance to temper softening. In particular, C
By adding u, Nb, and Cr in combination, the temper softening resistance is remarkably increased, a high tempering temperature can be adopted, and the delayed fracture resistance is greatly improved. It is also effective in suppressing the amount of hydrogen permeation due to the coexistence of Cu and 1.5% or more of Cr. If less than 0.05%, these effects cannot be obtained, while
Even if it exceeds 1%, the effect is saturated and only the cost is increased, so the content is made 0.05 to 1%.

Al: Al is effective for deoxidizing and refining steel. If it is less than 0.005%, the desired effect cannot be obtained, while if it exceeds 0.01%, inclusions (alumina, etc.) are present at the high strength level targeted by the present invention.
Is in the range that deteriorates the delayed fracture resistance, so 0.
005-0.01%. Further, as described later, Al needs to be in a proper range also in the ratio with effective N.

Nb: Nb causes grain refinement of steel and further improves delayed fracture resistance. If it is less than 0.005%, the desired effect cannot be obtained, while if it exceeds 0.2%, the strength and ductility are impaired, so the content is made 0.005 to 0.2%.

Ni: Ni enhances the toughness of the steel, and at the same time prevents hot pebbles on the surface due to molten Cu concentrated on the surface during hot rolling. If it is less than 0.05%, the desired effect cannot be obtained. On the other hand, if it exceeds 0.5%, the effect is saturated, and since Ni is expensive as an alloying element, it is economically considered to be 0.05. ~ 0.5%.

V: V has a grain refining effect on steel similarly to Nb, and further improves delayed fracture resistance by reducing grain boundary segregation. Further, V increases the resistance to temper softening of steel, so
Allows the use of high tempering temperatures to improve delayed fracture resistance. If it is less than 0.01%, the effect is not sufficient, while if it exceeds 0.3%, the toughness is deteriorated.
It is set to 01 to 0.3%.

Mn: Mn is an element effective in deoxidizing and improving hardenability, but if contained in a large amount, a grain boundary embrittlement phenomenon occurs and promotes the occurrence of delayed fracture. Further, Mn easily forms a dense segregation portion during solidification, and coarse MnS is formed there. This expands by rolling and becomes a starting point of delayed fracture. Therefore, the amount thereof should be minimized to improve delayed fracture resistance. Must be lowered. 0.5 to ensure hardenability
%, But Mn may be substantially 0 if the hardenability can be supplemented with other elements.

N: N is effective for making the steel finer and improving the delayed fracture resistance. However, if it exceeds 0.01%, coarse nitrides are generated and the delayed fracture resistance is deteriorated, so the content is made 0.01% or less. On the other hand, 0.0
If it is less than 02%, the grain refining effect is not sufficient, but if N is low, it becomes difficult to form nitrides linked to austenite grain boundaries and the like, and the delayed fracture resistance is improved accordingly. Therefore, 0.002% or more is desirable, but
0.002% or more is not essential. Note that N
Is an inequality condition of 1.93 ≦ Al (%) / effective N together with Al, Zr, Ti and B, as will be described later.
It is necessary to satisfy (%) ≦ 10.

Zr: Zr may not be added. When added, the carbide is dispersed in the steel in a spherical fine form to further improve the delayed fracture resistance. Therefore, it may be contained for the purpose of imparting high delayed fracture resistance to particularly high strength steel. In order to surely obtain the above effect, it is desirable to set it to 0.01% or more. However, if it exceeds 0.15%, the toughness deteriorates, so even if it is added, the content is made 0.15% or less.

Ti: Ti may not be added. If added, it has the effect of increasing the strength and improving the surface properties of the continuously cast slab. Therefore, they may be added when the surface properties are a problem. In order to surely obtain this effect, it is desirable that Ti is 0.01% or more. However, if the amount exceeds 0.1%, the toughness of the steel deteriorates, so even if it is added, it is made 0.1% or less.

B: B may not be added. If added, the hardenability of the steel is further improved, the strength is increased, and the grain boundary is strengthened, whereby the delayed fracture resistance is further improved. Therefore, it may be added for the purpose of ensuring high strength particularly when the product size is large. In order to surely obtain this effect, it is desirable to be 0.0003% or more. However, if it exceeds 0.005%, the toughness of the steel deteriorates, so the content is made 0.005% or less.

Cu + Mo: It is very important in the present invention to add Cu and Mo together after the content of Cr is 1.5% or more. By combining these elements having different hydrogen invasion suppressing mechanisms, it is possible to obtain a higher hydrogen invasion suppressing effect than the combined effect obtained by adding each element alone.

FIG. 3 shows the effect of C on the amount of hydrogen permeation of low alloy steel.
It is a figure showing the influence of u + Mo (content which combined Cu and Mo). As shown in the figure, Cu + Mo is 0.4%
If it is less than the desired value, the desired effect cannot be obtained. Therefore, in order to provide excellent delayed fracture resistance, Cu + Mo is added to 0.
Must be 4% or more. The center values of the alloying elements of the steel used to obtain FIG. 3 are C: 0.52%, Si:
1.31%, Cr: 4.0%, Al: 0.01%, N
b: 0.09%, Ni: 0.09%, V: 0.02%,
Mn: 0.15%, P: 0.013%, S: 0.01%
And Cu and Mo are 0.04 to 0.53, respectively.
%, And within the range of 0.04 to 0.55%, Cu
+ Mo was adjusted.

Al / Effective N: The present inventors set Al and N as alloying elements within the scope of the present invention as described above.
A large effect of Al / effective N was confirmed by investigating the delayed fracture resistance of steels with different 1 / effective N. That is, by controlling Al / effective N to be 1.93 to 10, it is possible to obtain desired excellent delayed fracture resistance even with steel having a tensile strength of 160 kg / mm ² or more. Next, FIG. 4 to FIG. 7 show the results of experiments conducted to complete the present invention. In these figures, it can be seen that "Al / effective N" is a suitable index for organizing the experimental results.

FIG. 4 is a drawing showing the effect of Al / effective N on the delayed fracture resistance of steel not containing Zr, Ti and B. The symbol "EN" in the figure represents an effective N.

FIG. 5 is a drawing showing the effect of Al / effective N on the delayed fracture resistance of steel containing neither Ti nor B.

FIG. 6 is a drawing showing the influence of Al / effective N on the delayed fracture resistance of Zr-free steel.

FIG. 7 is a drawing showing the effect of Al / effective N on the delayed fracture resistance of the steel containing all of Zr, Ti and B.

From these FIGS. 4 to 7, it is clear that cracking occurs both when the Al / effective N is less than 1.93 and when it exceeds 10. Furthermore, it was found that all cracks originated from coarse nitride. Based on this result, Al / effective N is set to 1.93 or more and 10 or less.

P: The grain boundary segregation of P cannot be completely eliminated by any heat treatment, the grain boundary strength is lowered and the delayed fracture resistance is deteriorated. However, lowering P raises the steelmaking cost, so it is made 0.015% or less, which is an allowable range.

S: S, as described above, serves as a base point for cracking by combining with Mn, and even alone, segregates at the grain boundaries in a solid solution state to promote hydrogen embrittlement, which causes delayed fracture. It is necessary to limit it as low as possible. However, although lowering the S does not increase the steelmaking cost as compared with the P, the cost certainly increases. Therefore, the amount is set to 0.01% or less, which is an allowable range for performance.

2. Manufacturing method Usually, the steel of the present invention is used by applying a heat treatment of quenching and tempering. The structure formed by quenching and tempering is not particularly limited. However, even in the case of steel having the above chemical composition, in order to have tensile strength of 160 kgf / mm ² or more and good delayed fracture resistance, it is necessary to use a tempered structure of martensite or a mixed structure of martensite and bainite. Is desirable. The heat treatment for the usual hot rolling (heating temperature: 1000 to 1250 ° C.) performs, after rolling, it is Ar ₃ point or more temperature without cooling to a temperature lower than the Ar ₃ point (preferably 850 to
Quenching with water or oil from 1020 ° C), so-called direct quenching is performed. Alternatively, after being rolled and then once cooled, it is 850 to 1050 ° C. (preferably 920 to 1020).
Reheat to ℃) and quench with water or oil. It is desirable that the martensite or the mixed structure of martensite and bainite obtained by these direct quenching or reheating quenching is then tempered at a temperature of Ac ₁ point or less.

However, quenching and tempering are not necessarily required. 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 largely depend on the structure inside the steel.
For example, even with a structure such as hot rolling or as-quenched (AsQ), as shown in Examples described later, 160 kgf /
It exhibits a 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 it is used as a steel for machine structural use, there is a problem that stress relaxation increases during use. Therefore, in order to obtain a predetermined strength and delayed fracture resistance, it is desirable to apply quenching and tempering treatment to obtain mainly tempered martensite or a tempered martensite + bainite mixed structure.

[0055]

EXAMPLES The present invention will now be described by way of examples in comparison with comparative steels.

Tables 1 and 2 are a list of chemical compositions and production methods of the present invention steels and comparative steels (including conventional steels).

[0057]

[Table 1]

[0058]

[Table 2]

Steels (steels 1 to 49) having the chemical compositions shown in Tables 1 and 2 were melted in a 50 kg vacuum melting furnace by a usual method. Steels 1 to 37 are steels of the present invention, and steel 38
~ 49 is a steel having a component marked with * in Table 2,
It is outside the scope of the present invention. In addition, steel 50 shown in Table 2
52 are conventional steels, steel 50 is SCM440 of JIS G 4105 (1989), steel 51 is SNCM of JIS G 4103 (1989)
Steel corresponding to 616, and steel 52 is disclosed in
This is the steel proposed in Japanese Patent Publication No. 84960.

Steel 1 to 4, steel 8 to 15, steel 19 to 24, steel 28 to 32, steel 36 to 39, and steel 43 to 49 are 1100.
Hot forging and hot rolling at ~ 1200 ° C to a thickness of 15
After making it into a plate material of mm, it was reheated to 950 ° C., held for 45 minutes and water-quenched, and then tempered and air-cooled. In the tempering, the tempering temperature was adjusted so that the tensile strength was 160 kgf / mm ² or more. Further, similar quenching and tempering treatment was performed on the conventional steels of the steels 50 to 52. That is,
870, 900, 950 for steels 50-52, respectively
After being reheated to ℃ and held for 45 minutes, it was water-quenched and then tempered. Further, hot-rolled materials other than the above (steels 5, 16, 25, 33, 40), as-quenched materials (steels 6, 17, 26, 34, 41), hot-rolled and accelerated cooling (steel) 7, 18, 27, 35, 42), the test material was prepared. In accelerated cooling, water cooling was performed so that the cooling rate from Ar ₃ point or higher to 400 to 500 ° C was 10 to 15 ° C / sec.

The delayed fracture resistance of these test materials was investigated by the constant load test method.

FIG. 8 is a drawing showing the shapes of the test pieces used for the evaluation of delayed fracture resistance and the notches provided in the test pieces. FIG. 8A shows the shape of the test piece,
FIG. 8B shows an enlarged view of the notch portion of the test piece.

FIG. 9 is a drawing showing a constant load test apparatus for evaluating delayed fracture resistance. A test piece having a shape and dimensions as shown in FIG. 8 is set in the constant load tester shown in FIG.
= 2 under Walpol liquid (mixture of hydrochloric acid and sodium acetate solution) static load (tensile stress: 140 kgf / mm
While cathodic charging the specimen hydrogen flowing over ²⁾ constant current (1 mA / cm ^2), it was observed the occurrence of fracture.
PH = 2 as the test environment corresponds to the most severe environment that can be realized in the actual use environment. During the test, the test temperature was kept at 25 ° C., which is the standard temperature for the delayed fracture test, by the temperature controller.

Tables 3 and 4 are a list of the breaking times in the above-mentioned constant load tests, in addition to the measurement results of the tensile strength, impact absorption energy and high temperature compressive stress of each steel. In the same table, those that did not break within 750 h were indicated by ◯, and those that broke were indicated by ×, and the achievement of the task was clearly indicated.

[0065]

[Table 3]

[0066]

[Table 4]

From Tables 3 and 4, the steels of the present invention show that the constant load rupture time does not break even after 750 hours, which is much longer than the conventional standard of 200 hours, and exceeds 2000 hours in all, indicating delayed fracture resistance. It is clear that it is excellent. Further, in terms of toughness, the absorbed energy value of the Charpy test is high, and in terms of ductility, the required deformation stress of the high temperature compression test is small, and thus it can be understood that they have been improved. In other words, it is a material that is not only delayed fracture resistant but also easier to use in practice than conventional steels in manufacturing and use.

[0068]

INDUSTRIAL APPLICABILITY The steel of the present invention has a tensile strength of 160 kgf / mm ² or more, and there is no risk of delayed fracture even if it is replaced for a certain period of time longer than in the past. It can be used with high reliability in high-tensile bolts, PC steel rods, and mechanical parts that require high strength, and it has a great effect on related industries.

[Brief description of drawings]

FIG. 1 is a drawing showing the effect of Mo and Cu on the hydrogen permeation amount of a low alloy steel with Cr less than 1.5%.

FIG. 2 is a drawing showing the influence of Cr and Cu on the hydrogen permeation amount of 0.4% Mo steel.

FIG. 3 is a diagram showing Cu + which affects the hydrogen permeation amount of a low alloy steel.
It is a figure showing the influence of Mo.

FIG. 4 is a drawing showing the effect of Al / effective N on the delayed fracture resistance of steels not containing Zr, Ti and B.

FIG. 5 is a drawing showing the effect of Al / effective N on the delayed fracture resistance of steels not containing Ti and B.

FIG. 6 is a drawing showing the effect of Al / effective N on the delayed fracture resistance of Zr-free steel.

FIG. 7 is a drawing showing the influence of Al / effective N on the delayed fracture resistance of steel containing all of Zr, Ti and B.

FIG. 8 is a drawing showing shapes and dimensions of a test piece used for evaluation of delayed fracture resistance and a notch of the test piece, FIG. 8 (a) showing the test piece, and FIG. Shows an enlarged view of the notch portion of the test piece.

FIG. 9 is a drawing showing an outline of a constant load tester used for evaluation of delayed fracture resistance.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Test piece (tensile test piece for delayed fracture resistance evaluation) 2 ... Walpole liquid 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%, Cu: 0.1-1%, Cr: 1.5-5
%, Mo: 0.05 to 1%, Al: 0.005 to 0.0
1%, Nb: 0.005 to 0.2%, Ni: 0.05 to
0.5%, V: 0.01 to 0.3%, Mn: 0 to 0.5
%, N: 0.01% or less, Zr: 0 to 0.15%, T
i: 0 to 0.1% and B: 0 to 0.005%, and at the same time satisfy the following formula and formula, with the balance being Fe
And unavoidable impurities, and the content of P and S in the unavoidable impurities is P: 0.015% or less and S: 0.01% or less, which is excellent in delayed fracture resistance. Steel for machine structure. 0.4 (%) ≤ Cu (%) + Mo (%) ・ ・ ・ ・ ・ 1.93 ≤ Al (%) / effective N (%) ≤10 ... However, effective N (%) = N ( %)-{Zr (%) / 6.
25}-{Ti (%) / 3.43}-{B (%) / 0.
78} 2. The method for producing a steel for machine structural use according to claim 1, wherein after quenching, tempering is performed at a temperature of Ac ₁ point or less.