JP3463600B2 - High-strength steel with excellent shore weather resistance - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel suitable for manufacturing high-strength bolts and nuts for friction welding used in various structures in the field of construction and civil engineering, and particularly to corrosion caused by salt water such as structures in coastal areas. The present invention relates to steel suitable for manufacturing high-strength bolts and nuts with excellent coastal weather resistance used in the environment.

[0002]

2. Description of the Related Art Conventionally, when a steel structure is used without being coated, it is known to use weather-resistant steel. Regarding steel plates and shaped steels, JIS G3114 "Weather-resistant hot-rolled steel for welded structure" has already been used. ] And JIS G3125 “High weather resistance rolled steel”.

However, the high-strength bolts and nuts used for joints of those steel structures have not been standardized in Japan at all. High-strength bolts and nuts require extremely high strength for that purpose, unlike steel plates and shaped steels, and are therefore quenched and tempered. Therefore, it is impossible to use the steel of the above standard as it is as a material for high strength bolts and nuts. As a technique for solving such a problem, Japanese Patent Publication Nos. 53-24890 and 56
No. 20350 is disclosed.

All of these are premised on the fact that they are finally subjected to quenching and tempering treatment and have a strength of 1000 MPa or more. JP-B-53-24890 is Cu,
At the same time as securing the weather resistance by adding Cr, Si,
The feature is that delayed fracture peculiar to high strength steel is suppressed by the effect of adding Al. Further, Japanese Patent Publication No. 56-20350 discloses that Si and Si are added after securing the weather resistance by adding Cu and Cr.
It is possible to reduce annealing and omit annealing before bolt molding.

In such weather resistant steel, stable corrosion-resistant rust enriched with effective elements such as Cu and Cr covers the surface of the weather-resistant steel, whereby the progress of corrosion is significantly delayed. Is.

Due to its remarkably good weather resistance, it is known that steel structures utilizing weather resistance often withstand unserviced for decades. However, an environment with a relatively high amount of salt, such as the coastal area (flying salt content of 0.05 mdd
In the above), it is known that the rust of the weather resistant steel is difficult to stabilize, and it is difficult to obtain practical corrosion resistance. As a technique for providing a steel material that can withstand practical use in such an environment, Japanese Patent Laid-Open No. 7-2
No. 07340 and Japanese Patent Laid-Open No. 7-242993 are disclosed.

[0007]

[Patent Document 1] Japanese Unexamined Patent Publication No.
The techniques disclosed in Japanese Patent Application Laid-Open No. 207340 and Japanese Patent Application Laid-Open No. 7-242993 are intended for steel plates, and cannot be used as a material for high-strength bolts and nuts as described above. is there.

As described above, according to the prior art, it was not possible to manufacture a high strength bolt / nut having practical weather resistance in an area where the amount of incoming salt is 0.05 mdd or more. In order to solve such problems, an object of the present invention is to provide a steel suitable for manufacturing high strength bolts and nuts having high weather resistance in an environment where salt, particularly 0.05 mdd or more of salt comes in. To do.

[0009]

In order to solve the above problems and achieve the object, the present invention uses the following means.

(1) The steel of the present invention has a C: 0.
15-0.3%, Si: 0.15-0.5%, M
n: 0.6 to 1%, P: 0.01% or less, S: 0.
01% or less, Ni: 1-3%, Mo: 0.25-
0.8%, Al: 0.01 to 0.05%, N: 0.
002 to 0.01% , the balance being steel consisting of Fe and inevitable impurities, and having a hardenability index: DI shown by the following formula (1) of 2 or more,
It is a high strength steel with excellent weather resistance.

DI = 0.31√C% (1 + 0.64Si
%) (1 + 4.1 Mn%) (1 + 2.83 P%) (1-
0.62S%) (1 + 2.33Cr%) (1 + 0.52
Ni%) (1 + 3.14Mo%) (1 + 0.27Cu
%) {1 + 1.5 (0.9-C%)} ... (1) However, each element symbol represents the mass% . And the last section:
{1 + 1.5 (0.9-C%)} is multiplied only when B is added.

[0012]

[0013] (2) the steel of the present invention, as the steel components, mass
% , Further Cu: 0.1-0.5%, Cr: 0.8-
1.5%, Ti: 0.01 to 0.05%, and B: 0.
The high-strength steel excellent in coastal weather resistance according to (1) above , which contains one or more selected from the group of 0003 to 0.005%.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In order to achieve the above object, the present inventors have made earnest studies on the relationship between the chemical composition and the weather resistance of steel materials on the premise that a tensile strength of 980 MPa or more is ensured. As a result, we have obtained the following findings.

In a high strength bolt, the DI value is set to 2 or more in order to sufficiently quench the bolt to the center thereof. In such a case, if Ni and Mo are added in combination,
It was found that weather resistance with no practical problems can be obtained in an environment with a flying salt content of 0.05 mdd or more, and that addition of Cr and Cu is effective for improving weather resistance. That is, in the case of a martensite-based hard structure obtained with a DI value of 2 or more, it was found that the combined addition of Ni and Mo improves the weather resistance in the marine atmosphere containing Cl ions. Similar knowledge was obtained regarding the hardenability and weather resistance of the nut.

Based on the above findings, the inventors of the present invention added a certain amount of Ni and Mo in combination to improve the weather resistance, and further specified the C content to secure the strength.
The I value is controlled to be a certain value or more, and 0.05 md
The inventors have found a high-strength steel having high weather resistance in an environment in which a salt content of d or more comes in, and completed the present invention.

That is, according to the present invention, by specifying the steel composition and the quenching index: DI value, salinity, particularly 0.05
It is possible to provide a high-strength steel having high weather resistance in an environment where a salt content of mdd or more comes in.

The reasons for adding the components of the present invention, the reasons for limiting the components, and the reasons for limiting the hardenability index: DI value will be described below.

(1) Component composition range and hardenability index range C: 0.15 to 0.3% C is an element necessary for ensuring tensile strength by quenching and tempering. If it is less than 0.15%, the required tensile strength cannot be obtained, and if it exceeds 0.3%, the deformation resistance during bolt forming increases and the workability decreases, so the content is 0.15%. It is 0.3%.

P: 0.01% or less P is effective for obtaining excellent weather resistance, but 0.01%
When the content exceeds 0.01, the toughness deteriorates, so the content is 0.01.
% Or less.

Ni: 1 to 3% Ni is an important element in the present invention, and when added together with Mo, it has the effect of significantly improving the weather resistance in an environment where a salt content of 0.05 mdd or more comes in. If it is less than 1%, the effect cannot be obtained, and if it exceeds 3%, the effect is saturated, so that the content is 1 to 3%.

Mo: 0.25 to 0.8% Mo is an important element in the present invention, and when added together with Ni, it has the effect of significantly improving the weather resistance in an environment where a salt content of 0.05 mdd or more comes in. Have. If it is less than 0.25%, the effect cannot be obtained, and if it exceeds 0.8%, the effect is saturated, so the content is 0.25 to 0.8%.

DI value (defined by the following formula (1)): 2 or more DI = 0.31√C% (1 + 0.64Si%) (1 + 4.1Mn%) (1 + 2.83P%) (1-0.62S% ) (1 + 2.33Cr%) (1 + 0.52Ni%) (1 + 3.14Mo%) (1 + 0.27Cu%) {1 + 1.5 (0.9-C%)} ... (1) However, each element symbol is the weight. Represents%. And the last section:
{1 + 1.5 (0.9-C%)} is multiplied only when B is added.

In order to secure the hardenability required for manufacturing high strength bolts, the DI value defined by the above formula (1) is 2 or more. In the present invention, among the elements used for calculating the DI value, C,
P, Ni, Mo are indispensable, other elements are DI value 2
Appropriate selection may be made to satisfy the above. Although the material diameter of the nut is slightly larger than the bolt,
Since it is usually forged and quenched, the quenching temperature is much higher than that of bolts, and since there is a hole in the center of the material, sufficient hardenability can be secured if it has the same composition as the bolt. The other components and the addition amount thereof which are preferable in actual production are as follows.

Si: 0.15 to 0.5% Si is required as a deoxidizer, so 0.15% or more is added. However, if it exceeds 0.5%, the deformation resistance increases, so the content is 0. 0.1 to 0.5%.

Mn: 0.6 to 1% Mn is an element that enhances the hardenability and is effective in obtaining the required tensile strength, but if it is less than 0.6%, that effect cannot be obtained and 1%. If it exceeds, the weather resistance will be adversely affected, so
Its content is 0.6 to 1%.

S: 0.01% or less S exists as MnS in steel, but if it exceeds 0.01%, the amount of MnS increases and the toughness deteriorates. Therefore, the content is 0.01% or less. is there.

Al: 0.01 to 0.05% Al is not only added as a deoxidizing agent, but also precipitates as AlN to refine the structure and improve toughness. % Or more. But,
If it exceeds 0.05%, the amount of oxide-based inclusions increases and the toughness deteriorates. Therefore, the content is 0.01 to 0.
It is 05%.

N: 0.002 to 0.01% N precipitates as AlN to refine the structure and improve toughness, so the addition amount is 0.002% or more. However, if it exceeds 0.01%, the solid solution N deteriorates the toughness, so the content is 0.002-0.01.
%.

(Selective component) In the present invention, by defining the DI value to be 2 or more and adding Ni and Mo in combination, it is possible to obtain coastal weather resistance without any practical problems, but in order to further improve the characteristics. , Cu, and Cr, and in order to improve the strength, Ti and B are added in the following ranges.

Cu: 0.1 to 0.5% Like Ni and Mo, Cu is an element effective for improving weather resistance, but if the content is less than 0.1%, it has no effect. If it exceeds 0.5%, the workability is deteriorated, so the addition amount is 0.1 to 0.5%.

Cr: 0.8 to 1.5% Cr, like Ni and Mo, is an element effective for improving weather resistance, but if the content is less than 0.8%, it has no effect. If it exceeds 0.5%, the workability deteriorates, so the addition amount is 0.8 to 1.5%.

Ti: 0.01 to 0.05% By fixing N, Ti has the effect of suppressing precipitation of B as a nitride and effectively acting on the hardenability. When the content is 0.01% or more, the effect is obtained, but when the content is more than 0.05%, the effect is saturated.
It is 0.05%.

B: 0.0003 to 0.005% B has an effect of enhancing hardenability, but if it is less than 0.0003%, the effect cannot be obtained, and if it exceeds 0.005%, the effect is saturated. Its content is 0.0003 to 0.0
It is 05%.

The high-strength bolt is produced by melting the steel having the above-mentioned chemical composition, hot rolling it into a round bar, and then forming the bolt.
It is manufactured by thread rolling and quenching and tempering.
The nut is manufactured by forming a round bar by hot rolling steel having the same chemical composition as the bolt, forming it by hot forging, forging and quenching as it is, and subsequently tempering it. When installing bolts, it is desirable to use washers made of gold together with bolts and nuts. Examples of the present invention will be given below to prove the effects of the present invention.

[0036]

EXAMPLES Steels having the chemical composition shown in Table 1 (No. 1 to No. 1)
10: Steel of the present invention, No. 11 to 15: Comparative steel) was melted and hot rolled into a 22 mmφ round bar. After being formed into a bolt, it was heated to a temperature of 850 ° C. to 1000 ° C. for quenching, and then tempered to a tensile strength of 1030 to 1130 MPa class. Comparative steel No. Since No. 11 had a small amount of C and a DI value of less than 2.0, the center of the bolt was not sufficiently quenched, and a predetermined tensile strength could not be obtained after tempering. For nuts,
Steel with the same chemical composition (Table 1) as the bolt was hot-rolled into a 28 mm diameter round bar, then hot-forged into a nut and quenched to obtain a hardness of HRC 25-30. Was tempered.

The bolts and nuts (other than comparative steel No. 11) thus obtained were subjected to an exposure corrosion test. Tightening was performed with an axial force of 85% of the bolt proof stress. At the time of construction, all bolts, nuts and washers were made of gold. The exposure corrosion test site is Choshi, the test period is one year, and the amount of salt coming in is about 0.25 mdd. After one year of exposure, the maximum perforated corrosion depth of each bolt and nut made of each sample steel was measured.

Table 1 shows the measurement results of the maximum pitting corrosion depth (the maximum pitting corrosion depth of each of the bolt and the nut and the larger pitting corrosion depth). It can be seen that each of the steels of the present invention has a maximum pitting corrosion depth of less than 0.4 mm and exhibits good weather resistance even under severe exposure conditions.

On the other hand, comparative steel No. Since No. 12 has a large amount of Mn, the weather resistance is deteriorated. Comparative steel No. Since No. 13 has a small amount of both Ni and Mo, its weather resistance is significantly deteriorated. Comparative steel No. 14 and 15 are Ni and M, respectively
Due to the small amount of o, the weather resistance is deteriorated.

[0040]

[Table 1]

[0041]

As described above, according to the present invention,
By specifying the steel composition and hardenability index, 0.0
It is possible to provide steel suitable for manufacturing high-strength bolts and nuts, which has high weather resistance in an environment in which a salt content of 5 mdd or more comes in.

Therefore, the high-strength bolts and nuts made of the steel of the present invention can be used even in coastal areas where the amount of flying salt is high.
It can be used as high-strength bolts and nuts for friction joining without painting, and is extremely useful industrially.

Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00

Claims (4)

(57) [Claims] 1. C: 0.15 to 0.3% in mass%,
Si: 0.15 to 0.5%, Mn: 0.6 to 1%,
P: 0.01% or less, S: 0.01% or less, Ni:
1 to 3%, Mo: 0.25 to 0.8%, Al: 0.
01-0.05% and N: 0.002-0.01% , the balance is steel consisting of Fe and unavoidable impurities, and the hardenability index: DI shown by the following formula (1): Two
High strength steel with excellent coastal weather resistance, characterized by the above. DI = 0.31√C% (1 + 0.64Si%) (1+
4.1 Mn%) (1 + 2.83 P%) (1-0.62S
%) (1 + 2.33Cr%) (1 + 0.52Ni%)
(1 + 3.14Mo%) (1 + 0.27Cu%) {1+
1.5 (0.9-C%)} ... (1) However, each element symbol represents the mass% . And the last section:
{1 + 1.5 (0.9-C%)} is multiplied only when B is added. 2. As a steel component, in addition by mass% Cu:
0.1-0.5%, Cr: 0.8-1.5%, Ti:
0.01-0.05%, and B: 0.0003-0.0
Contains one or more selected from the group of 05%
A high-strength steel excellent in coastal weather resistance according to claim 1 . 3. The method according to claim 1 or 2,
Bolts with the listed steel components. 4. The method according to any one of claims 1 and 2.
Nuts with the listed steel components.