JP2001288538A - Steel for high-strength bolts having excellent delayed fracture resistance, bolts, and method of manufacturing the bolts - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high strength steel for a bolt good in delayed fracture resistance and having tensile strength of >=1,300 MPa, to provide a bolt and to provide a method for producing the bolt. SOLUTION: This high strength steel for a bolt excellent in delayed fracture resistance has a composition containing, by mass, 0.50 to 1.00% C, 0.05 to 2.0% Si, 0.2 to 2.0% Mn and 0.005 to 0.1% Al, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high-strength steel for bolts having a tensile strength of 1300 MPa or more and excellent in delayed fracture resistance, a bolt and a method for producing the bolt.

[0002]

2. Description of the Related Art High-strength bolts widely used in machines, automobiles, bridges, and buildings include, for example, JISG 4104 and JIS G410.
C content of 0.20-SCr, SCM, etc. specified in 5
It is manufactured by quenching and tempering using 0.35% medium carbon steel. However, it is well known that the risk of delayed fracture increases when the tensile strength of all types exceeds 1300 MPa.For example, the strength of currently used building bolts is limited to 1150 MPa class. It is the current situation.

[0003] As a conventional finding to improve the delayed fracture characteristics of high strength bolts, for example, Japanese Patent Publication No. 3-243744 proposes that it is effective to make old austenite grains finer and to make the structure bainite. are doing. Certainly, the bainite structure is effective against delayed fracture, but the bainite treatment increases the manufacturing cost. The miniaturization of old austenite grains has also been proposed in Japanese Patent Publication No. 64-4566 and Japanese Patent Publication No. 3-243745. Also,
JP-B-61-64815 proposes to add Ca. However, none of the proposals has significantly improved the delayed fracture resistance in the tests of the present inventors.

As described above, in the prior art, there is a limit in manufacturing a high-strength bolt with drastically improved delayed fracture resistance.

[0005]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and has realized a high-strength bolt steel and a high-strength bolt having good delayed fracture resistance and a tensile strength of 1300 MPa or more. And a method of manufacturing the bolt.

[0006]

Means for Solving the Problems The present inventors first analyzed in detail the delayed fracture behavior using bolt steels of various strength levels manufactured by quenching and tempering. It is already clear that delayed fracture has occurred due to hydrogen in steel. Therefore, the delayed fracture characteristics were evaluated by obtaining the “critical diffusible hydrogen amount” at which delayed fracture did not occur. In this method, various levels of diffusible hydrogen are contained by electrolytic hydrogen charging, and then Cd plating is performed to prevent hydrogen from leaking from the sample to the atmosphere during the delayed fracture test, and then the air is charged in the atmosphere. A predetermined load is applied to evaluate the diffusible hydrogen amount at which delayed fracture does not occur. FIG. 1 shows an example in which the relationship between the amount of diffusible hydrogen and the rupture time until delayed fracture is analyzed. As the amount of diffusible hydrogen contained in the sample decreases, the time until delayed fracture increases, and when the amount of diffusible hydrogen is less than a certain value, delayed fracture does not occur. This amount of hydrogen is defined as “critical diffusible hydrogen amount”. The higher the critical diffusible hydrogen content is, the better the delayed fracture resistance of the steel material is, which is a value inherent to the steel material determined by the composition of the steel material and the manufacturing conditions such as heat treatment. The amount of diffusible hydrogen in a sample can be easily measured by gas chromatography.

In order to increase the critical diffusible hydrogen content of high-strength bolts, that is, to improve delayed fracture resistance,
The austenitic grain size, the effects of quenching and tempering conditions, and the like were repeated. As a result, since delayed fracture is grain boundary cracking along the former austenite grain boundary, it is important to prevent the occurrence of grain boundary cracking in order to achieve a significant improvement in delayed fracture resistance. Reached the conclusion.

[0008] Therefore, as a result of various studies on means for preventing austenite grain boundary cracking, a bolt having a structure in which the phase having the largest area ratio is martensite is obtained.
By performing tempering at a temperature of 0 ° C or higher, preferably 580 ° C or higher, it is possible to prevent austenite grain boundary cracking even in a high-strength region exceeding 1300MPa. It was found that the amount was greatly increased and the delayed fracture resistance was significantly improved.

Based on the above examination results, it was concluded that a high-strength bolt excellent in delayed fracture resistance can be realized by optimally selecting the steel material composition, microstructure, and heat treatment conditions.
The present invention has been made. The present invention has been made based on the above findings, and the gist thereof is as follows.
It is as follows. (1) In mass%, C: 0.50 to 1.00%, Si: 0.05 to 2.0
%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.1%, the balance being Fe and unavoidable impurities, characterized by having a tensile strength of not less than 1300 MPa and excellent delayed fracture resistance. For steel. (2) In mass%, C: 0.50 to 1.00%, Si: 0.05 to 2.0
%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.20%, B: 0.0003 to 0.00
50%, Cr: 0.05-2.0%, Mo: 0.05-1.0%, N
i: 0.05-5.0%, Cu: 0.05-1.0%, V: 0.05-2.
0%, Nb: 0.005 to 0.2%, Ta: 0.005 to 0.5% or W: 0.05 to 0.5%, characterized in that it has one or more of the following: excellent in delayed fracture characteristics with a tensile strength of 1300 MPa or more. (3) A bolt comprising the component described in (1) or (2) above, wherein the phase having the largest area ratio is a tempered martensite structure, and the average size of the spherical carbides at the prior austenite grain boundary is
A high-strength bolt excellent in delayed fracture resistance, having a thickness of 0.05 μm or more and a tensile strength of 130 kgf / mm 2 or more. (4) A steel having a structure composed of the component described in the above (1) or (2) and having a phase having a maximum area ratio of martensite is used.
A method for producing a high-strength bolt excellent in delayed fracture resistance having a tensile strength of 1300 MPa or more, characterized by tempering at 0 ° C or more and 650 ° C or less.

[0010]

Next, an embodiment of the present invention will be described. Steel composition: The reasons for limiting the composition of the steel targeted by the present invention will be described. C: C is an essential element for securing the strength of the bolt, but if it is less than 0.50%, the required strength cannot be obtained in a predetermined tempering temperature range, and if it exceeds 1.00%, the toughness is deteriorated. It was limited to the range of 0.50 to 1.00%, preferably 0.70 to 1.00%.

Si: Si has an effect of increasing strength by a solid solution hardening effect. If the content is less than 0.05%, the above effect cannot be exerted. On the other hand, if it exceeds 2.0%, an effect commensurate with the added amount cannot be expected, so the content is limited to the range of 0.05 to 2.0%. Mn: Mn is an element not only necessary for deoxidation and desulfurization but also effective for enhancing hardenability for obtaining a martensitic structure, but if less than 0.2%, the above effects cannot be obtained. On the other hand, if it exceeds 2.0%, it is segregated at the grain boundaries during heating in the austenite region, embrittles the grain boundaries and deteriorates the delayed fracture resistance, so that it is limited to the range of 0.2 to 2.0%.

Al: Al has the effect of preventing austenite grains from being coarsened by forming AlN during deoxidation and heat treatment, and also has the effect of fixing N. However, if it is less than 0.005%, these effects are exhibited. Not 0.1
%, The effect is saturated, so that the range is limited to the range of 0.005 to 0.1%. The above are the basic components of the steel targeted by the present invention. In the present invention, the steel further contains Ti: 0.005 to 0.20.
%, B: 0.0003 to 0.0050%, Cr: 0.05 to 2.0%, M
o: 0.05-1.0%, Ni: 0.05-5.0%, Cu: 0.05-
1.0%, V: 0.05 to 2.0%, Nb: 0.005 to 0.2%, T
One or more of a: 0.005 to 0.5% or W: 0.05 to 0.5% can be contained.

Ti: Like Al, Ti has the effect of forming TiN during deoxidation and heat treatment and has the effect of preventing the austenite grains from becoming coarser, and also has the effect of fixing N. Effect is not exhibited, and even if it exceeds 0.20%, the effect saturates.
Limited to the range of 0.20%. B: B has the effect of suppressing grain boundary fracture and improving delayed fracture characteristics. Further, B segregates at austenite grain boundaries to significantly enhance hardenability. However, if the content is less than 0.0003%, the above effect is not exerted. .

Cr: Cr is an element effective for improving hardenability and increasing softening resistance during tempering, but if it is less than 0.05%, its effect cannot be sufficiently exhibited.
If it exceeds 2.0%, the toughness and the cold workability deteriorate, so the content is limited to 0.05 to 2.0%. Mo: Mo is an element that has a strong tempering softening resistance like Cr and is effective for increasing the tensile strength after heat treatment.
If it is less than 0.05%, the effect is small.On the other hand, if it exceeds 1.0%, the effect is saturated and the cost is increased.
Limited to 0%.

Ni: Ni is added in order to improve the ductility, which deteriorates with the increase in strength, and also to improve the hardenability at the time of heat treatment and to increase the tensile strength. On the other hand, if the content exceeds 5.0%, the effect corresponding to the added amount cannot be exhibited, so the content is limited to the range of 0.05 to 5.0%. Cu: Cu is an effective element for increasing the tempering softening resistance. However, if it is less than 0.05%, the effect cannot be exhibited, and if it exceeds 1.0%, the hot workability deteriorates, so Cu is limited to 0.05 to 1.0%.

V: V has the effect of reducing the size of austenite grains by forming carbonitride during quenching, but if it is less than 0.05%, the above effect cannot be obtained. Because the effect is saturated, 0.05-2.
Limited to 0%. Nb: Like Nb, Nb is also an effective element for refining austenite grains by forming carbonitrides. However, if it is less than 0.005%, the above effect is insufficient.
If it exceeds 0.2%, this effect is saturated, so 0.005 to 0.2
%.

Ta: Ta also has an effect of refining austenite grains like Nb. However, if it is less than 0.005%, the above effect is not exhibited, and even if it exceeds 0.5%, the effect is saturated. , 0.005 to 0.5%. W: W is an element effective for improving the delayed fracture characteristics of high-strength bolts. However, if it is less than 0.05%, the above-mentioned effect is not exerted. On the other hand, if it exceeds 0.5%, the effect is saturated. Therefore, it was limited to the range of 0.05 to 0.5%.

Although P and S, which are impurity elements, are not particularly limited, from the viewpoint of improving delayed fracture characteristics,
Each is preferably 0.015% or less. As for N, 0.002 to 0.1% is a desirable range because the formation of nitrides of Al, V, Nb, and Ti has the effect of refining old austenite grains and increasing the yield strength.

Manufacturing conditions: The method of manufacturing a high-strength bolt according to the present invention is as follows. The steel having the above-mentioned components is heated to a temperature range of 3 or more AC and then quenched to convert the phase having the largest area ratio to martensite. Or less, preferably 580 ° C or more and 650
Tempering in a temperature range of not more than ℃. The heating temperature is
If it is too high, it promotes the coarsening of the prior austenite grains.

The present invention reduces the stress concentration at the tip of a microcrack at a grain boundary by tempering a steel material at a high temperature of 550 ° C. or more, thereby coarsening and spheroidizing the grain boundary carbide to 0.05 μm or more. It suppresses the boundary cracking.
If the tempering temperature is lower than 550 ° C, a structure having the above-mentioned form of carbide cannot be obtained.
0 ° C. or less. More desirable conditions are 580 ° C or more and 650
It is the range below ° C.

The effect of the present invention can be obtained without any particular upper limit of the average short axis length of the above-mentioned spheroidized carbide. However, since excessively coarse carbide becomes a starting point of crack generation, it is preferably 1 μm or less. In the present invention, the area ratio of martensite or tempered martensite is determined by using a C-section t / 4 part of a steel rod or a C-section t / 4 of a bolt at a magnification of 200 to 1000 times with an optical microscope.
This is the average value when performing visual field observation. Other structures can include retained austenite, bainite, ferrite, and pearlite.

The average short axis length of the spherical carbides at the prior austenite grain boundaries was 5000 to 50000 times by SEM after speed etching in the above sample, and the lower limit of measurement was 0.005 μm.
The shortest length existing in the prior austenite grain boundary when observed as above is defined as the short axis length, and is defined as the average value when observed in 10 visual fields. The effect of the present invention can be obtained without particularly setting the upper limit of the tensile strength of the steel for bolts and the bolt of the steel of the present invention.However, in order not to deteriorate the toughness, 1700 MPa
a The following is desirable.

[0023]

EXAMPLES Hereinafter, the effects of the present invention will be described more specifically with reference to examples. Bolt steel having the chemical composition shown in Table 1 was worked into a bolt, heated to 880 ° C. to 950 ° C., then quenched, and heat-treated under various conditions shown in Table 2 to obtain a tempered martensite structure.

Table 2 shows the results of evaluation of the mechanical properties, microstructure, and delayed fracture characteristics using the above samples.
The delayed fracture characteristics were evaluated based on the critical diffusible hydrogen content described above, and the load stress was evaluated under the condition of 90% of the tensile strength. Test Nos. 1 to 13 in Table 1 are examples of the present invention, and others are comparative examples. As can be seen from Table 2, the examples of the present invention all have a tempering temperature of 550 ° C. or higher and a bolt tensile strength of 1300 MPa or higher. In these, the delayed fracture type is intragranular cracking, the critical diffusible hydrogen content is higher than that of the conventional bolt, and a bolt excellent in the delayed fracture characteristic is realized.

On the other hand, Nos. 14 to 2 which are comparative examples
In the case of 0, since the C content is low, the tempering temperature for obtaining a strength of 1300 MPa or more is as low as 500 ° C. or less, so the delayed fracture mode is grain boundary cracking, the critical diffusible hydrogen content is low, and the delayed fracture characteristics are poor. It is an example. No. 21 which is a comparative example is an example in which the C content was too high, and No. 22 was an example in which the Mn content was too high, so that the delayed fracture characteristics were all poor.

FIG. 2 is a diagram showing the relationship between the tempering temperature and the critical diffusible hydrogen amount. Tempering temperature is 550 ℃ ~ 650 ℃
Among them, the critical diffusible hydrogen amount is high, and the delayed fracture resistance is improved. FIG. 3 shows the relationship between the average short axis length of the spheroidized carbide at the grain boundary and the critical diffusible hydrogen amount. When the average short axis length is 0.05 μm or more, the critical diffusible hydrogen amount is high, and the delayed fracture resistance is improved.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

As is clear from the above examples, the present invention makes the delayed fracture mode of the bolt from the intergranular crack to the intragranular crack by growing the grain boundary carbide into a coarse sphere, thereby increasing the tensile strength. It is possible to significantly improve the delayed fracture characteristics of high-strength bolts of 1300MPa or more, and to optimally select the chemical composition of steel, heat treatment conditions, and the like, to improve the steel for bolts, the bolt and the method of manufacturing the bolt. It is established.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of the relationship between the amount of diffusible hydrogen and delayed fracture time.

FIG. 2 is a graph showing a relationship between a tempering temperature and a critical diffusible hydrogen amount.

FIG. 3 is a graph showing the relationship between the average short axis length of grain boundary spherical carbides and the amount of critical diffusible hydrogen.

Claims (4)

[Claims] (1) In terms of mass%, C: 0.50 to 1.00%, Si: 0.
05-2.0%, Mn: 0.2-2.0%, Al: 0.005-0.1
%, With the balance being Fe and unavoidable impurities, characterized by having a tensile strength of 1300 MPa or more and having excellent delayed fracture resistance. 2. In mass%, C: 0.50 to 1.00%, Si: 0.
05-2.0%, Mn: 0.2-2.0%, Al: 0.005-0.1
%, And further, Ti: 0.005 to 0.20%, and B: 0.00
03-0.0050%, Cr: 0.05-2.0%, Mo: 0.05-1.0
%, Ni: 0.05-5.0%, Cu: 0.05-1.0%, V: 0.
05-2.0%, Nb: 0.005-0.2%, Ta: 0.005--0.
High-strength bolt steel with excellent delayed fracture resistance having a tensile strength of 1300 MPa or more, characterized by containing one or more of 5% or W: 0.05 to 0.5%. 3. A bolt comprising the component according to claim 1 or 2, wherein the phase having the largest area ratio is tempered martensite, and the average short axis length of the spherical carbide at the prior austenite grain boundary is
A high-strength bolt excellent in delayed fracture resistance, characterized by having a tensile strength of not less than 0.05 μm and a tensile strength of not less than 1300 MPa. 4. A steel comprising a component according to claim 1 or 2, wherein the phase having the largest area ratio is martensite, at a temperature of 550.degree.
Tensile strength characterized by tempering below 0 ° C: 1300MPa
A method of manufacturing a high-strength bolt excellent in delayed fracture resistance having the above.