JPH02166258A - Machine structural steel with excellent delayed fracture resistance - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention is applicable to high-tensile bolts and PCs that have a tensile strength of 125 kgf/u or more and have excellent delayed fracture resistance.
It relates to steel for machine structures, such as steel bars and high-tensile steel plates for large machines.

[Conventional technology]

In recent years, as structures have become larger and automobiles, trucks, civil engineering machinery, etc. have become lighter, there has been a demand for the development of mechanical structural steels with a tensile strength of 125+tgf/d or higher, especially high-tensile bolts and PC steel bars. It's coming.

Conventionally, generally 1. Strong steel for mechanical structures having a tensile strength of OOkgf/m or more is, for example, 0.35% C1)
．． It has a composition of 0% Cr and 10.2% MO. 1ISG4
Low alloy steel of SCM435 specified in 105 and 0.3
1'%C1,0,8%Cr51.8%N +'% '0
．． 2%M.

SNCM431 low alloy steel specified in JIS G 4103 with a composition of 0.2%C1
It is manufactured by subjecting a hot rolled material such as boron steel having a composition of 0.8% Cr and 0.002% B to quenching and tempering.

However, when these mechanical structural steels are put into practical use, 1
Items with a tensile strength of 25 kgf/- or more may suffer delayed fracture during use, so high-tensile bolts, PC steel rods, and other important safety parts for automobiles and civil engineering machinery are of stable quality. The problem was that it lacked sex.

Note that delayed fracture is a phenomenon in which steel under static load suddenly breaks brittle after a certain period of time, and is considered to be a type of hydrogen embrittlement due to hydrogen penetrating into the steel from the external environment.

For this reason, the strength level of the above-mentioned mechanical structural steel is currently limited to a tensile strength of 125 kgfl- or less. For example, for high-strength bolts, JIS B 1) 86 (1979) "High-strength hexagon bolt, hexagon nut, flat washer set for friction bonding" F8Tl tensile strength = 80 to 100 kgf/
w), FIOT (100-120 kgf/i
), and FIIT (same 1) 0~l 30 bgf/w
Z), and for FLLT,
Please avoid using it as much as possible and take precautions (=1).

Furthermore, even in the case of steel plates for civil engineering and construction machinery that require wear resistance, those with a tensile strength exceeding 125 kgf/m have a problem of delayed fracture during use.

On the other hand, as a steel with superior delayed fracture resistance than the above-mentioned ordinary low alloy steel, for example, 18%Ni57.5%C01
5%Mo10.5%Ti, 0.1%A, 1. There is an 18%N+ maraging steel with a composition of 18%N+, which can be used without fear of delayed fracture up to a tensile strength of about 150kgf/-, but it is extremely expensive and therefore not economical. In this respect, it has only been put to practical use in a few extremely limited applications, and it has not yet come into widespread use for mechanical structures.

On the other hand, as a structural steel that is economical, has high strength, and has excellent delayed fracture resistance, for example, JP-A-58-6121
No. 9, JP-A-513-84960, JP-A-58-1)
No. 3317, JP-A-58-1) No. 7856, JP-A-Sho 58
-157'921, JP-A No. 81-1) 7248, JP-A No. 61-130456, and other publications propose high-strength steels with various components and methods for producing them.

However, even these steels with a tensile strength of over 125 kgf/- cannot completely eliminate the risk of delayed fracture to the extent that they can be used, for example, in high-tensile bolts for bridges; In areas that have been subjected to severe processing with a cross-section reduction rate of 60% or more, especially 70% or more during molding, the austenite grains may become coarse during the next quenching process and may not exhibit sufficient delayed fracture resistance. , their scope of application is neither determinable nor sufficient.

[Problem to be solved by the invention]

This invention was developed in order to meet the above-mentioned demands of the industrial world.
The purpose of this product is to provide mechanical structural steel that has a tensile strength of more than 'kgf/- and has excellent delayed fracture resistance. Or, assuming replacement, for a certain period of time, for example, 5,000
125 kgf with no risk of delayed destruction within hours
The object of the present invention is to provide a mechanical structural steel having a tensile strength of /wr or more. Such uses include high-strength steel for various structures, steel for bolts for automobiles, civil engineering machinery, and industrial machinery, and high-strength steel plates, and by using this invented steel in these, the above-mentioned industrial demands can be met. It is possible to answer.

In other words, the present invention provides that high tensile strength bolts for bridges do not have the risk of delayed fracture occurring for a predetermined period of time even if they are not resistant to delayed fracture, and therefore are suitable for use in parts etc. that require periodic repair or replacement. It is an object of the present invention to provide a mechanical structural steel having a tensile strength of 125 kgf/- or more that can be suitably used.

[Failure to solve the problem]

In order to achieve the above-mentioned object of the present invention, the inventors of the present invention have conducted extensive experiments and research, and have found that no delayed fracture occurs over a period of 5,000 hours or more, and that 125 kgf/-
It has been found that in order to obtain steel with the above tensile strength, it is effective to satisfy the conditions shown below. That is, (a) reduction and cleaning of grain boundary segregation by lowering P1 and S;
Low Mn is effective in improving delayed fracture resistance.

(b) The reduction in hydrogen diffusion ability due to high Si content and the refinement of steel grains due to Nb addition are effective in improving delayed fracture resistance.

(c) Carbides in steel serve as a place for hydrogen to accumulate. Therefore, if these carbides exhibit a notch defect shape such as needles or rods, or coarsely aggregate, this becomes the starting point and delayed fracture occurs. However, when Zr is contained in steel, carbides are dispersed into fine, spherical shapes, which is effective in significantly improving delayed fracture resistance.

(d) The amount of N is 1.9 for steel that does not contain Ti or B.
3≦Afl/ (N-(Zr/8.52)}≦10, and for steel containing Ti9B, 1.93≦AA/ (N-(Zr/a 52)-(
Ti/a 43) - (B10.78) }≦10 If the formula is restricted to satisfy, even if strong working with an area reduction rate of 70% or more is applied during cold forming, the austenite grains will become coarse during quenching. Excellent delayed fracture resistance can be obtained without causing corrosion.

This invention was made based on the above knowledge, and the gist thereof is that C: 0.20 to 0 in weight%.
．． 45% Si: more than 0.50 ~ zO%Mn: 0.5
Less than 0% P: O, OL 5% or less S: 0.
01% or less Cr: 0.01-5.0%Z
r: 0.01-0.15% Nb: 0.0
05-0.20% AIl: 0.005-0.10%
N: Contains 0.035% or less (% indicates weight%), and 193≦A fl/(N − (Z r/
6.52) }≦10-==---・-(1) is satisfied, or further V: 0.01 to 0.30% Mo: 0.01
~0.80% of one or two types and satisfying the above formula (1), or (1) above, in place of MO, Ti: 0.01 to 0.10% B: 0.0003
Contains ~0.0050% of one or two types and 193≦All/(N − (Z r / 6.52) −
(T i / 3.43) - (B10.78) }≦10
... satisfies the formula (2), or v: o, o 1 to 0.30% Mo: 0.0
1 to 0.80% of one or two types and Ti: 0.01 to 0.10% B: 0.0003 to 0
．． 125 kgf/- or more, which contains one or two of 0.0050% and satisfies the above formula (2), with the remainder consisting essentially of Fe and unavoidable impurities, and a quenched and tempered structure. It is a mechanical structural steel with tensile strength and excellent delayed fracture resistance.

[For production]

Below, the reasons for limiting the composition and structure of the steel in this invention will be described.

C is an effective component for increasing the hardenability and strength of steel, as well as refining the grains, but if its content is less than 0.20%, the hardenability deteriorates and the desired level is not achieved. On the other hand, if the content exceeds 0.045%, the susceptibility to quenching cracking during quenching increases, and the toughness deteriorates in conjunction with other alloy components, so the content is 0.20-
It was set at 0.4'5%.

In addition to deoxidizing steel and increasing its strength, Si is an element that is effective in reducing hydrogen diffusion ability in steel and improving delayed fracture resistance, but when its content is less than 0.50%, it It is difficult to obtain the desired effect on reducing the diffusion ability of hydrogen in steel, which is important for improving delayed fracture properties.On the other hand, if the content exceeds 2.0%, the toughness will deteriorate significantly, so the content should be reduced to 0.0%. Over 50%~z
It was set as 0%.

Mn is an element that is effective in deoxidizing and improving hardenability.
If it is contained in a large amount, grain boundary embrittlement phenomenon will occur, promoting the occurrence of delayed fracture. Furthermore, since Mn combines with S and becomes the starting point for cracking, its content must be reduced as much as possible in order to improve delayed fracture resistance. Therefore, in this invention, which aims to improve delayed fracture resistance, Mn
The content was set to less than 0.5%.

P cannot completely eliminate its grain boundary segregation no matter what heat treatment is applied, and it lowers the grain boundary strength and deteriorates delayed fracture resistance, so its second limit was set at 0.045%.

As mentioned above, S combines with Mn and becomes the starting point of cracking.
Furthermore, even if it is used alone, it segregates at grain boundaries and promotes embrittlement, so it is necessary to limit its content as low as possible. Therefore,
In this invention, S is set to 0.01% or less.

Cr has the effect of improving the hardenability of steel and imparting temper softening resistance to the steel, but if its content is less than 0.01%, the desired effect cannot be obtained; on the other hand, Cr Since it is an expensive alloying element, its content was determined to be 0.01 to 5.0% in consideration of economic efficiency.

Zr is an important element in this invention, and has the effect of dispersing carbides into fine spherical particles in the steel to significantly improve delayed fracture resistance, but if it is less than 0.1%, the effect is small; If the content exceeds 0.01% to 0.15%, the toughness deteriorates, so the content was set at 0.01 to 0.15%.

Nb has the effect of improving the strength and toughness of steel and making the grains finer. In particular, it prevents grains from becoming coarser in high-Sim steels such as the steel of this invention, and improves delayed fracture resistance due to grain coarsening. It is extremely effective in preventing deterioration.

However, in order to ensure the effect, 0.005
It is necessary to add more than %. On the other hand, if it is added in excess of 0.020%, the effect will be saturated and the cost will be high, so the content was set at 0.005 to 0.020%.

Al is effective in stabilizing the deoxidation of steel, homogenizing it, and refining the grains, but if it is less than 0.005%, the desired effect cannot be obtained; on the other hand, if it exceeds o,io% Even if it is contained, its effect will be saturated, and the increase in inclusions will cause cracks and deteriorate toughness, so the content should be reduced to 0.
0.005 to 0.10%.

N is also an important element in this invention. In other words, N is an effective element for making steel grains finer and improving delayed fracture resistance, but it is difficult to add more than 0.035% to low alloy steel, so it is necessary to reduce the content to 0. .035%
The following was made.

On the other hand, regarding the amount of N, further the above formula (1) or (2)
The reason why the formula was determined to be summing was based on the following test results.

That is, the inventors have C1, SilMn, P, and S.

Crs Zr, Nbs All, VN MO% Ti
and B content is within the range of this invention, and the N amount is 0.
035% or less, A, 1. / (N-(Z r/8.
52) ) or 12/ (N-(Zr/6.52)-
Steels having various values of (Ti/3.43) - (B10.78) ) are melted in the laboratory, then hot rolled, and further reduced in area by 80% after spheroidizing annealing. The delayed fracture characteristics were tested after cold working and quenching and tempering to give a tensile strength of 130 to 14 (ltgf/-). Figure 3 (a) shows the delayed fracture characteristics of the constant load type used. This is an explanatory diagram schematically showing the configuration of the test device, in which a rod-shaped test piece (1) with an outer diameter of 63.φ and a notched portion is placed in constant temperature water (2
) was circulated and immersed in Walbor's buffer solution (3) of pH 2 maintained at 25°C, and a constant load of 1600 kgf was applied to examine the appearance of cracks after 200 hours had elapsed. The notch part of test piece 0) has an angle of 6 as shown in Figure 3(b).
It has a shape of 0°, a depth of 1 inch, and a groove bottom radius R0, 1 ml-, and is formed all around the test piece (1). Five test pieces (1) having such a notch were cut out from each of the above-mentioned quenched and tempered materials and used for testing.

The results of the above test are shown in FIGS. 1 and 2. Figure 1 is T
In the case of steel that does not contain i or B, FIG. 2 shows the case of steel that contains Ti or B, the horizontal axis shows the Afl content, and the vertical axis shows (N-Zr/8. , .52), and the Nioite in Figure 2 is (N - (Zr/a52) -=
Ti/3.43)-(B10.78)).
In addition, in both figures, the ○ mark indicates that no cracking was observed in any of the five test pieces used in the above test.
An X mark indicates that cracking was observed in at least one of the five test pieces. From Fig. 1, it can be seen that AIl(N − (Z
When the value of r/8.52>) is less than 1.93 or more than 10, cracking occurs, and as shown in Figure 2, l! / (N(
Zr/8.52) -(Ti/3.43) -(B10
．． 78) It can be seen that cracks occur when the value of ) is less than 1.93 or more than 10.

Therefore, the amount of N was determined to satisfy the above formula (1) when Ti and B are not contained, and to satisfy the above (2:J) when Ti and B are contained.

(2) has the effect of making the steel grain finer and further improving the strength of the steel by precipitation hardening, so it is added as necessary when higher strength is required, but if it is less than 0.01%, the above effect will not be achieved. On the other hand, if the content exceeds 0.3%, the addition effect will be saturated and further strength improvement effect will not be obtained, so the content was set at 0.01 to 0.30%. .

Since Mo1Ti and B have the effect of further improving the hardenability of steel, it is recommended to include them in order to ensure high strength especially when the steel product size is large.
Less than 0.01%, Ti less than 0.01%, BO, 0003%
If it is less than M, the desired effect cannot be obtained;
Even if O is contained in an amount exceeding 0.8%, the effect will be saturated and the cost will only increase, Ti is 0.10%, and B is 0.8%. If the content exceeds 50% of OO, the toughness and delayed fracture resistance of the steel will deteriorate, and in the case of Ti, the machinability will also deteriorate. Therefore, for Mo1Ti and B, the contents are MoO, 0.01~080%, Ti0.01~01%, respectively.
0% and 80.0003 to 0.0050%.

Even if the steel has the above-mentioned composition, 125kgf
In order to have a tensile strength of /- or more and good delayed fracture resistance, it is necessary to carry out normal hot rolling and furnace immediately after rolling, or to reheat and then quench, or to perform cold rolling. It is necessary to heat the product after molding at a warm temperature and then quench it to obtain a low-temperature transformation product (martensis F or bainite), which is then tempered to form a so-called quenching furnace returned structure.

Namely, furnace conditioned material, normalized and tempered material, as-rolled material,
A structure consisting mainly of high-temperature bainite, ferrite, and pearlite, which are transformation products at high temperatures, such as that of tempered rolled material, has a stable tensile strength of 125 kgf/w.
It is difficult to obtain a high strength of 125 kgf/- or more in terms of delayed fracture resistance and tensile strength. On the other hand, as-quenched steel has high tensile strength but low yield point, and when used as mechanical structural steel, stress relaxation increases during use. There is a problem that the characteristics are not good.

Therefore, in order to impart a predetermined strength and delayed fracture resistance f'N+ to the steel, it is necessary to perform a quenching and tempering treatment to transform the structure of the steel into a quenched and tempered structure.

〔Example〕

Next, the present invention will be explained using examples and comparing with comparative examples.

Example 1 First, fi4 of the component composition shown in Table 1 was prepared by a conventional method.
(Symbol A-3) was melted. Steel A-L has a composition within the scope of this invention, and Steel M-3 has a composition outside the scope of this invention in terms of the points marked with this mark in Table 1. be.

(Left below) These molten steels are made into steel slabs by continuous casting or ingot-forming, heated to 1200-1250°C, and then hot-rolled into 15mm thick steel plates using normal methods. , then convert this to 920
From a temperature of ~1020°C, directly quenching is performed, which is quenching immediately after hot rolling, or after reheating and quenching, which is quenching after being reheated to the above temperature range, it is 200~680°C.
Tempered at a temperature of , the structure is a quenched and tempered structure,
The tensile strength was adjusted to be 125 kgf/i or more, and the vibration fracture characteristics were investigated.

The results are shown in Table 2.

The presence or absence of delayed fracture was confirmed by the wedge insertion type delayed fracture test method shown in Fig. 4.

In other words, a wedge as shown in Fig. 4(C) is inserted into the notch part (shown in Fig. 4(b)) of a monthly test specimen having the shape shown in Fig. 4(a), and a static load is applied. Multiply this by 55°
The specimen was placed in hot water maintained at a temperature of 100° C., and the time required for cracking to occur was determined.

In addition, in the figure, the numbers indicate the length of the unit of ff1)1.

In the above test, one criterion for delayed fracture resistance was 5000 hours, but this was because we assumed that 3 months was the period for regular maintenance or inspection of the equipment, and estimated an error of about twice that. . The test environment in warm water at 55°C corresponds to the most severe environment in actual use.

Therefore, the obtained delayed fracture time is considered to correspond to the delayed fracture occurrence time in the harshest environment in actual use.

From Table 2, the steel of this invention has a long delayed fracture occurrence time;
It is clear that it has excellent delayed fracture resistance.

That is, according to the present invention, it is possible to obtain a mechanical structural steel that has a tensile strength of 125 kgf/- or more and does not cause delayed fracture for a period of 5,000 hours or more, and can be periodically repaired or replaced as described above. Based on this premise, the A-machine structural steel according to the present invention can be widely used for steel materials for applications where it is clear how much delayed fracture resistance is required.

(Left below) Example 2 m< of the component composition shown in Table 1 above by a qualified method.
A steel (code A-1-) and a steel (code TY) having the composition shown in Table 3 were melted. As mentioned above, @A-L is responsible for the composition within the scope of this invention, while m
TY has a composition that is outside the scope of the present invention, as indicated by the * mark (t) in Table 3.

(Left below) These steels were hot-rolled into a round bar with a diameter of 17mm, then subjected to spheroidizing annealing, and then cold-processed into a round bar with a diameter of 7I1m (section reduction rate of 83%). After quenching at 940'C, the specimens were tempered at various temperatures to obtain a quenched and tempered structure with a tensile strength of 125 kgf/- or more, and their delayed fracture characteristics were investigated.

The results are shown in Table 4.

In addition, the delayed fracture property was confirmed by the above-mentioned constant load type delayed fracture test.

That is, the test piece (1) shown in FIG. 3(b) was attached using the apparatus shown in FIG. , 160
A constant load of 0 kgf was applied and the occurrence of cracks was examined after 200 hours had elapsed.

From the results shown in Table 4, it is clear that the steel of the present invention has excellent delayed fracture resistance, with no cracking observed in any of the five test specimens.

(The following is a blank space) Table 4 [Effects of the Invention] As described above, the present invention can provide a mechanical structural steel having a tensile strength of 125 kgf/- or more and excellent delayed fracture resistance. In particular, the mechanical structural steel of this invention has no risk of delayed fracture within a certain period of time, so it can be used as high-strength steel for various structures, automobiles, civil engineering machinery, industrial machinery, etc. on the premise of periodic repair or replacement. It is an inexpensive, low-alloy, high-strength steel that can be suitably used as steel for machine bolts, etc., and has extremely high practical value.

[Brief explanation of the drawing]

Figure 1 shows the effects of chemical composition and steel structure on the delayed fracture resistance of steel within the scope of this invention.
5)) Figure 2 shows the influence of the same <Aj2/(N
-(Zr/a52)-(Ti/3.43)-CB10.
78)), and Figure 3 is an explanatory diagram of the constant load type delayed fracture test device used in the examples. Fig. 4 is an explanatory drawing showing the shape and dimensions of the test piece and wedge used in the delayed fracture test in other examples; Fig. 4(b) shows the details of the notch part of the test piece, and Fig. 4(C) shows the wedge to be inserted into the notch part of the test piece to apply a load. , the number indicates the length of one unit. 1...Test piece 2...Water 3...Walbol buffer solution (zi army 9 (do!;'9/'Z) N (Z1 toughness (gl da seconds) ~ to V Lee-c゛Vgu8~

Claims (3)

[Claims] (1) C: 0.20 to 0.45% Si: More than 0.50 to 2.0% by weight
Mn: less than 0.50% P: 0.015% or less S: 0.0
1% or less Cr: 0.01-5.0% Zr: 0.01-0
．． 15%Nb: 0.005-0.20%Al: 0.00
5 to 0.10% N: Contains 0.035% or less and satisfies the formula 1.93≦Al/{N-(Zr/6.52)}≦10, and the remainder is substantially Fe and A mechanical structural steel having a tensile strength of 125 kgf/mm^2 or more and excellent delayed fracture resistance, characterized by comprising a component consisting of unavoidable impurities and a quenched and tempered structure. (2) Further as component elements: V: 0.01-0.30% Mo: 0.01-0.80%
2. The mechanical structural steel having excellent delayed fracture resistance according to item 1, which contains one or two of the following. (3) As component elements, Ti: 0.01 to 0.10% B: 0.0003 to 0.0
050% of one or two types, and the formula to be satisfied is 1.93≦Al/{N-(Zr/6.52)-(Ti/
3.43)-(B/0.78)}≦10 The mechanical structural steel having excellent delayed fracture resistance according to item 1 or 2.