JP3221309B2 - Steel for machine structure and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION
m ² or more and excellent in delayed fracture resistance,
The present invention relates to a machine structural steel suitable for use in high tension bolts, PC steel bars, large machines, and the like.

[0002]

2. Description of the Related Art In recent years, as structures have become larger and automobiles, trucks, civil engineering and mining equipment have become lighter, steel for machine structural use having higher strength than ever, especially high-tensile bolts and PCs have been developed.
There is a need for steel materials for steel bars.

The following are low alloy high strength mechanical structural steels generally used so far.

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

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

(C) Tensile strength: 174 kgf / mm class ² :
A steel with the same composition as (a) or (b) above but with a different heat treatment.

In both (a) and (b), quenching and tempering are performed after hot rolling to provide the required strength. The tensile strength of (c) is 174 kgf / m.
In the m ² class, the low-alloy steel is hot-rolled, and then subjected to quenching and tempering under different conditions from the heat treatment performed in (a) or (b) to achieve toughness.

[0008] These mechanical structural steels, however,
Since it may cause delayed fracture during use, it has not been fully used for important parts of automobiles and civil engineering machines, including high-tensile bolts and PC steel bars. “Delayed destruction”
Is a phenomenon in which a steel placed under a static load suddenly breaks brittlely after a certain time elapses, and is considered to be a kind of hydrogen embrittlement caused by hydrogen invading steel from the external environment.

[0009] Since the above-mentioned low alloy steel for mechanical structural use may generally cause delayed fracture, its tensile strength is set to 100%.
It is said that it is desirable to limit the weight to not more than kgf / mm ² .

[0010] As a steel having more excellent delayed fracture resistance than the above low alloy steel, for example, 18% Ni-7.5% Co-5% M
18% represented by o-0.5% Ti-0.1% Al steel
Although there is Ni maraging steel, its use is limited in terms of economy because it is extremely expensive.

Therefore, as a high-strength bolt steel excellent in delayed fracture resistance in consideration of economy, a steel in which Ca and the like are added to control the form of sulfides, and at the same time, a combination of Cr and Mo is proposed. (JP-A-58-84960, JP-A-61-117248 and JP-A-61-1304).
No. 56). However, since these steels do not have sufficient delayed fracture resistance, steels with low Si and Mn have been developed (JP-A-3-243745, JP-A-2-243).
No. 145746). However, these steels with reduced Si and Mn are still insufficient in terms of hydrogen permeability or toughness, and have not yet been generally used as steel materials that do not cause delayed fracture. Here, the hydrogen permeability refers to the degree of permeability of hydrogen that permeates (penetrates) into steel without remaining in the environment as hydrogen gas during a corrosion reaction on the steel surface accompanied by reduction of hydrogen ions. means. Since the smaller the hydrogen permeability, the smaller the amount of hydrogen in the steel that causes embrittlement, delayed fracture is less likely to occur.

[0012] These steels with improved delayed fracture properties are:
After limiting the tensile strength to 100 to 120 kgf / mm ² , the immersion test in a Walpole solution having a pH of 2 (mixed solution of hydrochloric acid and sodium acetate aqueous solution) described later was performed at 2
It is said that cracks should not occur within 00 hours. The environment in which these steels are actually used has a slower action of causing delayed fracture than Walpole's solution,
For a certain period longer than 200 hours, parts (bolts and the like) in which these steel materials are used are replaced every period.

[0013]

SUMMARY OF THE INVENTION
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 in delayed fracture resistance. Specifically, assuming regular replacement instead of permanent use such as high tension bolts for bridges, for example,
A notched tensile test piece was subjected to a constant load of 140 kgf / mm ² in Walpole's solution having a pH of 2 and applied with a constant load of 1 mA / cm 2.
Under a stream of ^second constant current, and to provide an entirely caused no tensile strength 160 kgf / cm ² or more steel to delayed fracture within 750h.

[0014]

SUMMARY OF THE INVENTION Many structural steels that have been proposed to date and have excellent delayed fracture resistance have Mo added thereto. Mo in the steel suppresses the penetration of hydrogen into the steel by increasing the ratio of hydrogen gas remaining in the external environment by lowering the hydrogen overvoltage, that is, by promoting the hydrogen reduction reaction. This effect prevents hydrogen embrittlement, which causes delayed fracture, and improves the delayed fracture resistance of steel. However, in order to provide strength that is higher than the current level and has excellent delayed fracture resistance that is higher than the current level, it is necessary to suppress hydrogen penetration more severely than the current level.

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

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

FIG. 1 is a drawing showing the effect of Mo and Cu on the amount of hydrogen permeation of a low alloy steel having a Cr content of less than 1.5%. According to the figure, even when various low alloy steels with Mo of 0.01 to 0.8% (Cr less than 1.5%) contain 0.3% of Cu, the amount of hydrogen permeation is higher than in the case of not adding Cu. Does not substantially differ, and therefore the amount of hydrogen penetrating into the steel does not decrease. The central values of alloying elements other than Cu and Mo used in obtaining the steel in 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 the hydrogen permeation amount (μC / cm) is shown in the figure in a unit including the electric amount (μC: microcoulomb).

(B) 1.5% Cr in Mo steel combined with Cu
By increasing the content as described above, the effect of suppressing hydrogen intrusion can be dramatically increased.

FIG. 2 is a drawing showing the effect of Cr and Cu on the amount of hydrogen permeation of 0.4% Mo steel. The central values of alloy elements other than Cu, Mo and Cr in 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 amount of hydrogen permeation of a 0.4% Mo steel containing 1.5% or more of Cr and 0.3% of Cu at the same time is remarkably low.

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

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

(1) C: 0.4 to 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 to 0.2%, Ni: 0.
05 to 0.5%, V: 0.01 to 0.3%, Mn: 0 to 0%
0.5%, N: 0.01% or less, Zr: 0 to 0.15
%, Ti: 0 to 0.1% and B: 0 to 0.005%, and at the same time, satisfying the following formulas and formulas, with the balance being Fe and unavoidable impurities, and P and unavoidable impurities. the content of S, P: 0.015% or less and S: 0.01% or less der is, tensile strength 160k
gf / mm ² or more, a steel for machine structural use having excellent delayed fracture resistance. 0.4 (%) ≦ Cu (%) + Mo (%) 1.93 ≦ Al (%) / Effective N (%) ≦ 10 However, Effective N (%) = N ( %)-{Zr (%) / 6.25}-{Ti (%) / 3.43}-{B (%) / 0.78} (2) Mechanical structure of chemical composition according to (1) above after a use steel quenched <br/>, method of manufacturing Oko of the Hare machine <br/>械structural steel tempered at temperatures below ₁ point Ac.

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 steel for machine structural use according to claim 1, wherein after quenching, tempering is performed at a temperature of ₁ point or less of Ac.

Here, effective N means A contained in steel.
When it is assumed that all of the nitride-forming elements Zr, Ti and B other than l, Cr, etc. are combined with N to form a nitride,
It means nitrogen other than nitrogen that forms Zr nitride, Ti nitride and B nitride. That is, based on the above assumption, the solid solution nitrogen ([ N ]) and nitrogen as AlN (N as A
1N) and nitrides with other elements, for example, the sum of nitrogen (such as N as CrN) as a nitride of Cr. The nitrides of Zr and Ti, which are strong nitride-forming elements,
It is formed at a high temperature of 50 ° C. or higher and tends to be coarse.
In practice, not all Zr, Ti and B will combine with N, so that the actual dissolved nitrogen, Nas AlN and N
The sum, such as as CrN, is greater than the effective N defined above. In addition, since a large amount of Cr nitride is not formed in the above range, most of the effective N is actually dissolved nitrogen and N as A
consists of lN. However, in the high temperature range of 1150 ° C or higher, Al
Since N forms a solid solution, almost all of it becomes solid solution nitrogen in the same temperature range.

"Al / effective N" obtained by dividing Al (%) by effective N (%) is used for convenience in organizing experimental values as described later, and its meaning in terms of gold. Although not completely elucidated, it is believed that the amount of AlN and solid solution Al is restricted by limiting Al and N to the above ranges, and then the easiness of formation of fine AlN is indicated. Can be

[0026]

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

1. Chemical composition In this description, "%" represents "% by weight".

C: C forms carbides and strengthens the steel by precipitation strengthening, and also forms stable martensite at the time of quenching and strengthens the steel by transformation. Therefore, C is indispensable for increasing the strength. Element. Further, it is effective for improving hardenability and refining crystals. If it is less than 0.4 %, the hardenability deteriorates,
In addition, the amount of carbide precipitation is small and the strength is reduced. on the other hand,
It exceeds 0.6%, an increase in quenching crack susceptibility baked Nyutoki, because the steel is hardened considerably ductility, weldability and workability is lowered in addition to the 0.4 to 0.6%.

Si: Si is effective for deoxidizing steel and increasing strength. If the content 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 the toughness may be deteriorated.

Cu: Cu inhibits the intrusion of hydrogen into the steel from the outside and, when combined with Nb and Cr, significantly increases the tempering softening resistance of the steel and increases the tempering temperature. Greatly improve. Further, as shown in FIG.
By coexisting with a certain amount of Mo, the amount of hydrogen permeation is remarkably reduced, which is effective for improving delayed fracture resistance. If it is less than 0.1%, the effect is not sufficient.
%, The weldability, the hot workability and the toughness are deteriorated.

Cr: Cr improves the hardenability of the steel and increases the tempering softening resistance of the steel. In particular, the tempering softening resistance is significantly improved by the complex addition with Nb or Cu. 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 drastically improved. On the other hand, if it exceeds 5%, the weldability, hot workability and toughness are deteriorated. Therefore, Cr is 1.5
To 5%. In order to stably obtain extremely excellent delayed fracture resistance, the content is desirably 1.7 to 4%.

Mo: Mo has a different mechanism, but exhibits an effect of suppressing hydrogen invasion by itself, similarly to Cu. In addition, it improves the hardenability of steel, and at the same time, increases the tempering softening resistance. In particular, C
The addition of u, Nb, and Cr significantly increases the tempering softening resistance, enables the use of a high tempering temperature, and greatly improves delayed fracture resistance. It is also effective in suppressing the amount of hydrogen permeation due to the coexistence of Cu and 1.5% or more of Cr. Below 0.05%, these effects cannot be obtained, while
Even if it exceeds 1%, the effect is saturated and only the cost is increased.

Al: Al is effective for deoxidizing and refining steel. If it is less than 0.005%, the desired effect cannot be obtained. On the other hand, if it exceeds 0.01%, inclusions (such as alumina) may not be obtained at the high strength level of the present invention.
Is in the range where the delayed fracture resistance is deteriorated.
005 to 0.01%. Further, Al needs to be in an appropriate range in terms of ratio with effective N, as described later.

Nb: Nb brings about 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 cracking of 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. Also, since Ni is expensive as an alloy element, Ni is considered to be economical. To 0.5%.

V: V, like Nb, has the effect of grain refinement of steel, and further improves delayed fracture resistance by reducing grain boundary segregation. Also, V increases the tempering softening resistance of steel,
High tempering temperature can be adopted to improve delayed fracture resistance. If it is less than 0.01%, the effect is not sufficient, while if it exceeds 0.3%, toughness is deteriorated.
01 to 0.3%.

Mn: Mn is an element effective for improving deoxidation and hardenability, but when contained in a large amount, grain boundary embrittlement occurs and accelerates the occurrence of delayed fracture. Further, Mn easily forms a thick segregated portion during solidification, and forms coarse MnS there, which is expanded by rolling and becomes a starting point of delayed fracture. Therefore, in order to improve delayed fracture resistance, the amount of Mn should be as small as possible. Must be lowered. Although it is set to less than 0.5% in consideration of securing hardenability, Mn may be substantially zero as long as hardenability can be supplemented by another element. That is, do not add and mix as impurities
It may be the amount that comes.

N: N is effective for reducing the grain size of steel and improving delayed fracture resistance. However, if the content exceeds 0.01%, coarse nitrides are formed, and the delayed fracture resistance is rather deteriorated. On the other hand, 0.0
If it is less than 02%, the effect of grain refining is not sufficient, but since N is low, it is difficult to form nitrides connected to austenite grain boundaries and the like, and accordingly, delayed fracture resistance is improved. Therefore, it is desirable that the content is 0.002% or more,
0.002% or more is not essential. Note that N
Is the condition inequality together with Al, Zr, Ti and B, as described below, 1.93 ≦ Al (%) / effective N
(%) ≦ 10 must be satisfied.

Zr: Zr may not be added. If added, carbides are finely dispersed in a spherical shape in steel to further improve delayed fracture resistance. Therefore, it may be contained particularly for the purpose of imparting high delayed fracture resistance to high-strength steel. In order to reliably obtain the above-mentioned effects, it is desirable that the content be 0.01% or more. However, if it exceeds 0.15%, the toughness is deteriorated. Therefore, even when it is added, the content is made 0.15% or less.

Ti: Ti need not be added. When added, it has the effect of increasing the strength and improving the surface properties of the continuously cast slab. For this reason, when surface properties become a problem, they may be added. In order to reliably obtain this effect, it is desirable that the content of Ti be 0.01% or more. However, if the amount exceeds 0.1%, the toughness of the steel deteriorates. Therefore, even when it is added, the content is set to 0.1% or less.

B: B may not be added. If added, the hardenability of the steel is further improved and the strength is increased, and the delayed fracture resistance is further improved by strengthening the grain boundaries. For this reason, especially when the product size is large, it may be added for the purpose of securing high strength. In order to surely obtain this effect, the content is preferably set to 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 in combination after making Cr 1.5% or more. By combining these elements having different hydrogen intrusion suppressing mechanisms, it is possible to obtain a higher hydrogen intrusion suppressing effect than that obtained by combining the effects obtained by adding each element alone.

FIG. 3 shows the effect of C on the hydrogen permeation rate of low alloy steel.
It is a figure showing the influence of u + Mo (content which combined Cu and Mo). As shown in FIG.
If it is less than 30, the desired effect cannot be obtained. Therefore, in order to provide excellent delayed fracture resistance, Cu + Mo is added in an amount of 0.1%.
Must be at least 4%. The central values of the alloy 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 respectively 0.04 to 0.53.
% And within the range of 0.04 to 0.55%,
+ Mo was adjusted.

Al / effective N: The present inventors set the alloying elements including Al and N within the scope of the present invention as described above, and
The effect of Al / effective N was confirmed by investigating the delayed fracture resistance of the steel in which 1 / effective N was changed. That is, by controlling the ratio of Al / effective N to 1.93 to 10, even with steel having a tensile strength of 160 kg / mm ² or more, a desired excellent delayed fracture resistance can be obtained. 4 to 7 show the results of experiments performed 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 containing no Zr, Ti and B. The symbol “EN” in the figure represents effective N.

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

FIG. 6 is a drawing showing the effect of Al / effective N on the delayed fracture resistance of steel containing no Zr.

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

It is apparent from FIGS. 4 to 7 that cracks occur when the ratio of Al / effective N is less than 1.93 or more than 10. Further, it was found that all cracks originated from coarse nitrides. Based on this result, Al / effective N is set to 1.93 or more and 10 or less.

P: No heat treatment can completely eliminate grain boundary segregation of P, lowering the grain boundary strength and deteriorating delayed fracture resistance. However, lowering P increases steelmaking costs, so it is set to an acceptable range of 0.015% or less.

S: As described above, S combines with Mn to serve as a starting point for cracking. Further, S alone segregates at the grain boundary in a solid solution state to promote hydrogen embrittlement which causes delayed fracture. It is necessary to limit it as low as possible. However, lowering S certainly increases the steelmaking cost, although the increase in steelmaking cost is smaller than that of P. Therefore, the content is set to 0.01% or less, which is an allowable range in 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 steel having the above chemical composition, in order to provide a tensile strength of 160 kgf / mm ² or more and good delayed fracture resistance, it is necessary to use a structure obtained by tempering martensite or a mixed structure of martensite and bainite. Desirably. 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. Alternatively, after rolling and once cooling, 850 to 1050 ° C. (preferably 920 to 1020 ° C.)
℃) and quenched with water or oil. It is desirable that the martensite or the mixed structure of martensite and bainite obtained by the direct quenching or the reheating quenching be thereafter tempered at a temperature of ₁ point or less of Ac.

However, it is not always necessary to perform quenching and tempering. This is because the steel of the present invention reduces the amount of hydrogen infiltration during use and improves delayed fracture resistance, and thus does not largely depend on the internal structure of the steel.
For example, even in a structure such as hot rolled or quenched (AsQ), as shown in Examples described later, 160 kgf /
It exhibits a tensile strength of at least ² mm ² and excellent delayed fracture resistance.

However, as-quenched steel has a high tensile strength, but has a low yield point and has a problem that stress relaxation increases during use when used as a machine structural steel. Therefore, in order to obtain a predetermined strength and delayed fracture resistance, it is desirable to perform quenching and tempering to obtain a tempered martensite or a mixed structure of tempered martensite and bainite.

[0055]

EXAMPLES Next, the present invention will be described by way of examples with reference to comparative steels.

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

[0057]

[Table 1]

[0058]

[Table 2]

In a conventional manner, 50 kg of steel (steel 2 to 49) having the chemical composition shown in Tables 1 and 2 was melted in a vacuum melting furnace. Steels 2 to 36 are steels of the present invention,
-49 are steels having the components 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), and steel 51 is SNCM of JIS G 4103 (1989).
616, and steel 52 is disclosed in
No. 84960.

Steels 2 to 4, steels 8 to 14 , steels 19 to 24, steels 28 to 32, steels 36 to 39, and steels 43 to 49 are 1100
Hot forging and hot rolling at ~ 1200 ° C to a thickness of 15
mm, reheated to 950 ° C., held for 45 minutes, 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. The same quenching and tempering treatment was performed on the conventional steels 50 to 52. That is, 870, 900, 9 for steels 50 to 52, respectively.
After reheating to 50 ° C and holding for 45 minutes, water quenching
Thereafter, tempering was performed. Furthermore, other than the above-mentioned hot-rolled material (steel 5 , 25, 33, 40) and quenched material (steel 6 ,
26, 34, 41) and those subjected to accelerated cooling after hot rolling (steel 7 , 27 , 35 , 42) were also prepared as test materials. In the accelerated cooling, 400 to 500 Ar from ₃ or more Ar points
Water cooling was performed so that the cooling rate to 10 ° C was 10 to 15 ° C / sec.

Examination of the delayed fracture resistance of these test materials was conducted by a constant load test method.

FIG. 8 is a drawing showing a test piece used for evaluating delayed fracture resistance and the shape of a notch provided in the test piece. FIG. 8A shows the shape of the test piece,
FIG. 8B is an enlarged view of a 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 the shape and dimensions shown in FIG. 8 was set on the constant load tester shown in FIG.
= 2 static load (tensile stress: 140 kgf / mm) under the environment of Walpole liquid (mixed solution of hydrochloric acid and sodium acetate aqueous solution)
² ), a constant current (1 mA / cm ² ) was passed, and hydrogen was negatively charged on the test piece, and the occurrence of breakage was observed.
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 performing the delayed fracture test by the temperature controller.

Tables 3 and 4 are tables summarizing the results of measurement of the tensile strength, impact absorption energy and high temperature compressive stress of each steel, as well as the rupture time in the above constant load test. In the same table, those that did not break within 750 hours were indicated by ○, and those that did break were indicated by x, indicating whether the task could be achieved.

[0065]

[Table 3]

[0066]

[Table 4]

As can be seen from Tables 3 and 4, the steels of the present invention did not break even at constant load rupture time, which greatly exceeded the conventional standard of 200 h, even at 750 h. It is clear that it is excellent. In addition, it can be seen that the absorption energy value in the Charpy test is high in terms of toughness, and the required stress for deformation in the high-temperature compression test is small in terms of ductility. In other words, it is a material that is actually easier to use than conventional steel not only in delayed fracture resistance but also in production and use.

[0068]

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 longer than before, so that it has a structure close to civil life. It can be used for high-tensile bolts and PC steel bars, which are frequently used in products, and for mechanical parts requiring high strength, with higher reliability than before, and has a great effect on related industries.

[Brief description of the drawings]

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

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

FIG. 3 shows the effect of Cu + on the hydrogen permeation rate of low alloy steel.
It is a drawing showing the influence of Mo.

FIG. 4 is a graph showing the effect of Al / effective N on delayed fracture resistance of steel containing no Zr, Ti and B.

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

FIG. 6 is a drawing showing the effect of Al / effective N on delayed fracture resistance of steel containing no Zr.

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

FIG. 8 is a drawing showing a test piece used for evaluation of delayed fracture resistance and the shape and dimensions of a notch of the test piece. FIG. 8 (a) shows 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 evaluating 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 controller 8 ... Constant load tester

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 6/00 C21D 9/00

Claims (2)

(57) [Claims] C .: 0.4 to 0.6% by weight, Si:
0.5-2%, Cu: 0.1-1%, Cr: 1.5-5
%, Mo: 0.05-1%, Al: 0.005-0.0
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%, simultaneously satisfying the following formulas and formulas, with the balance being Fe
And consists inevitable impurities, the content of P and S among unavoidable impurities, P: 0.015% or less and S: 0.01% or less der is, a tensile strength of 160kgf
/ Mm ² or more, a steel for machine structural use having excellent delayed fracture resistance. 0.4 (%) ≦ Cu (%) + Mo (%) 1.93 ≦ Al (%) / Effective N (%) ≦ 10 However, Effective N (%) = N ( %)-{Zr (%) / 6.25}-{Ti (%) / 3.43}-{B (%) / 0.78} 2. A steel for machine structural use having the chemical composition according to claim 1.
The After quenched, method of manufacturing to that machinery structural steel and performing tempering at a temperature of ₁ point Ac.