JP3458762B2 - 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, there is no standardization in Japan for high-strength bolts used for joints of those steel structures.

High-strength bolts require extremely high strength, unlike steel plates and shaped steels, for the purpose thereof, and are therefore subjected to quenching and tempering treatment. Therefore, it is impossible to use the steel of the above standard as it is as a material for high strength bolts. As a technology for solving such problems, Japanese Patent Publication No.
JP-A-3-24890 and JP-B-56-20350 are 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 omit annealing before bolt cold forming by reducing the above.

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, steel structures utilizing weather resistance are often known to survive decades of service unpainted. However, it is known that in an environment where the salt content is relatively high (flying salt content of 0.05 mdd or more) such as in the coastal area, 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 be practically used in such an environment, JP-A-7-207340 and JP-A-7-242 are known.
Japanese Patent No. 993 is disclosed.

[0006]

However, the techniques disclosed in Japanese Patent Application Laid-Open Nos. 7-207340 and 7-242993 are all directed to steel plates and have high strength as described above. It cannot be used as a material for bolts and nuts.

As described above, according to the conventional technique, steel having practical weather resistance in a region having a flying salt content of 0.05 mdd or more can be omitted in the production of high strength bolts before annealing for cold forging. In addition, it was not possible to manufacture the above-mentioned weather resistant nut.

In order to solve such a problem, an object of the present invention is to have high weather resistance in an environment where salt content, particularly salt content of 0.05 mdd or more comes in, and the hardness after hot rolling is HRB85. The purpose of the present invention is to provide a steel suitable for manufacturing a high-strength bolt and nut that does not need annealing and has the following tensile strength after quenching and tempering: 980 MPa or more.

[0009]

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

[0010] (1) the steel of the present invention, in mass%, C: 0.
15-0.3%, Si: 0.15% or less, Mn:
0.3-0.6%, P: 0.01% or less, S: 0.
01% or less, Al: 0.01 to 0.05%, N:
0.002-0.01%, Ni: 1-2%, Mo:
0.1 to 0.3%, Ti: 0.01 to 0.05%,
B: 0.0003 to 0.005%, balance Fe and improper
It is a high-strength steel with excellent weather resistance that is composed of evasive impurities .

(2) The steel of the present invention has a mass of
% , Further Cu: 0.1 to 0.5%, Cr: 0.3 to
The high-strength steel containing 1% or 2% of 1% and excellent in coastal weather resistance according to the above (1). (3) The bolt of the present invention has the steel component as described in (1) or (2) above. ( 4 ) The nut of the present invention has the steel component described in (1) or (2) above.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In order to achieve the above object, the present inventors assume that a tensile strength of 980 MPa or more is secured, and the chemical composition and weather resistance of the steel material and the hardness after hot rolling are As a result of diligent examination of the relationship, the following findings were obtained.

(1) In order to obtain excellent weather resistance in an environment with a flying salt content of 0.05 mdd or more, the combined addition of Ni and Mo is effective.

(2) P is effective for obtaining excellent weather resistance in an environment with a flying salt content of 0.05 mdd or more, but in high strength bolts, the adverse effect of promoting delayed fracture is remarkable, so the addition amount thereof is Need to be suppressed. However, the decrease in corrosion resistance caused by reducing P can be sufficiently compensated by increasing the amount of Ni.

(3) While adding Ni and Mo, Si
By reducing the amount of addition of Mn and Mn, the HRB hardness after hot rolling can be suppressed to 85 or less.

Based on the above findings, the present inventors have found that
In an environment where a salt content of 05 mdd or more comes in, a fixed amount of Ni and Mo are added in combination to improve weather resistance, the amount of P is reduced for delayed fracture resistance, and in the production of high strength bolts, annealing before cold forging can be omitted. Therefore, the inventors have found a high-strength steel in which the amounts of Si and Mn added are reduced, and completed the present invention.

That is, according to the present invention, by limiting the steel composition to the following range, it has high weather resistance in an environment in which salt, particularly 0.05 mdd or more of salt comes in, and in the production of high strength bolts Hardness after rolling is HRB: 85 or less, annealing before cold forging can be omitted, and subsequent tensile strength after quenching and tempering: suitable for manufacturing high strength bolts of 980 MPa or more. It is possible to provide a high-strength steel suitable for manufacturing a nut having properties.

The reasons for adding the components of the present invention and the reasons for limiting the components will be described below.

(1) Component composition range C: 0.15 to 0.3% C is an element necessary for securing 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 at the time of bolt forming increases and the workability decreases, so the content is 0.15 to 0. It is 0.3%.

Si: 0.15% or less Si is necessary as a deoxidizing agent, but if it exceeds 0.15%, the hardness after hot rolling increases, so its content is 0.
It is 15% or less.

Mn: 0.3 to 0.6% Mn is added to fix S, but if it is less than 0.3%, solid solution reprecipitation of MnS occurs during hot rolling, and ductility decreases. Further, if it exceeds 0.6%, the hardenability becomes high, and the hardness after hot rolling increases, so the content is 0.3-.
It is 0.6%. 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.

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

Al: 0.01-0.05% Al is not only added as a deoxidizing agent, but also precipitates as AlN to refine the structure and improve toughness, so the addition amount is 0.01. % Or more. However, 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: 0.002-0.01% 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.
%.

Ni: 1 to 2% 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 2%, the hardenability becomes high and the hardness after hot rolling increases, so the content is 1
~ 2%.

Mo: 0.1 to 0.3% 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 0.1%, the effect cannot be obtained, and if it exceeds 0.3%, the hardenability becomes high and the hardness after hot rolling increases, so the content is 0.1 to 0.3%. Is.

Ti: 0.01-0.05% Ti fixes N to prevent precipitation of B as a nitride and has an effect of effectively acting on 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 the 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, so the content thereof is Is 0.0003 to 0.0
It is 05%.

In the present invention, in addition to the above chemical components, Cu,
One or two kinds of Cr can be added appropriately.

The reasons for limiting these chemical components will be described.

Cu: 0.1 to 0.5% Like Ni and Mo, Cu is an element effective for improving the 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: 0.3 to 1% Cr, like Ni and Mo, is an element effective for improving weather resistance, but if the content of Cr is less than 0.3%, there is no effect and 1% is added. If it exceeds, the hardness after hot rolling increases, so the addition amount is 0.3 to 1%.

The high-strength bolt is manufactured by melting the steel having the above-mentioned chemical composition, making it into a round bar by hot rolling, cold-forming the bolt, thread-rolling it, and quenching and tempering it. It 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 that are the same as the bolts and nuts. Examples of the present invention will be given below to prove the effects of the present invention.

[0033]

EXAMPLES Examples of the present invention will be described below. Steel having the chemical composition shown in Table 1 (No. 1 to 9: Inventive steel, No. 10 to 15: Comparative steel) was melted and hot rolled into a 22 mmφ round bar. HRB of the cross section of this round bar
The hardness is measured at 5 points at 1 / 4d position, and the average value is shown in Table 1.

Inventive Steel No. In all of Examples 1 to 9, the HRB hardness is less than 85, and the annealing before cold forging is unnecessary, while the comparative steel No. Since Nos. 10, 11, 13, and 15 each contained a large amount of C, Mn, Ni, and Mo, the hardness after hot rolling increased, and cold forging could not be performed without annealing. Comparative steel No. For 12 and 14, the HRB hardness was less than 85.

Bolts were formed using the above round bar as a raw material. Comparative steel No. For 10, 11, 13, and 15, cold forging was performed after softening annealing, and for others, direct cold forging (without annealing) was performed. After the bolt is formed, it is heated to a temperature of 850 ° C to 1000 ° C for quenching, then 1
The tempering treatment was performed so that the tensile strength was in the range of 030 to 1130 MPa. For the nut, steel with the same chemical composition (Table 1) as the bolt was hot rolled to 28 mm
After forming a round bar having a diameter, it was formed into a nut by hot forging, quenched as it was, and tempered so that a hardness of HRC 25 to 30 was obtained.

The bolts and nuts obtained as described above were subjected to an exposure corrosion test. Tighten bolt strength of 85
% Axial force was used. All bolts, nuts and washers were made of gold during the construction. 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 also shows the measurement results of the maximum pitting corrosion depth (larger pitting corrosion depth at the maximum pitting corrosion depth of each of the bolt and the nut). Invention Steel No. 1
It can be seen that all of Nos. 9 to 9 have a maximum pitting corrosion depth of less than 0.4 mm and exhibit good weather resistance even under severe exposure conditions.

On the other hand, comparative steel No. Nos. 12 and 14, respectively, had a small amount of Ni and Mo, respectively, and thus had poor weather resistance. Comparative steel No. Nos. 10, 11, 13, and 15 showed weather resistance equivalent to that of the steels of the present invention.

[0039]

[Table 1]

[0040]

As described above, according to the present invention,
By specifying the steel composition, it has high weather resistance in the environment where salt content of 0.05 mdd or more comes in, and in the production of high strength bolts, the hardness after hot rolling is HRB: 85.
It is the following, and it is possible to provide high-strength steel that does not require annealing before cold forging and that can also manufacture the above-described nut having weather resistance.

The high-strength bolt made of the steel of the present invention can be used as a high-strength bolt for friction welding without coating even in a coastal area where the amount of incoming salt is large, and the heat treatment cost is low. It is extremely useful in industry.

Continuation of the front page (56) Reference JP 2000-119815 (JP, A) JP 2000-63993 (JP, A) JP 2000-54066 (JP, A) JP 11-172370 (JP, A) JP-A-6-220576 (JP, A) JP-A-6-212346 (JP, A) JP-A-62-86149 (JP, A) JP-A-10-36940 (JP, A) JP-A-5-51668 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00

Claims (4)

(57) [Claims] 1. C: 0.15 to 0.3 % by mass %
And Si: 0.15% or less, Mn: 0.3 to 0.6%
, P: 0.01% or less, S: 0.01% or less, A
1: 0.01-0.05%, N: 0.002-0.0
1%, Ni: 1-2%, Mo: 0.1-0.3%
And Ti: 0.01 to 0.05%, B: 0.0003
~ 0.005% and the balance Fe and unavoidable impurities
Becomes, coastal weather resistance excellent high strength steel. 2. As a steel component, in addition by mass% Cu:
The high-strength steel excellent in coastal weather resistance according to claim 1, containing one or two of 0.1 to 0.5% and Cr: 0.3 to 1%. 3. A bolt having the steel component according to claim 1 or 2. 4. A nut having the steel component according to claim 1 or 2.