JPH06271975A - High strength steel excellent in hydrogen embrittlement resistance and its production - Google Patents

Abstract

PURPOSE:To obtain a high strength steel having superior hydrogen embrittlement resistance even if heat treatment is omitted by treating a steel, containing specific amounts of C, Si, Mn, Al, and Fe, under prescribed conditions and forming the structure into martensite and/or bainite structure. CONSTITUTION:A steel, having a composition consisting of, by weight, 0.01-0.3% C, 0.01-2.5% Si, 0.01-3% Mn, 0.005-0.1% Al, and the balance Fe, is refined. After hot rolling or subsequent hot forging or heat treatment is applied to this steel, continuous cooling or isothermal transformation is performed, by which the structure can mainly be composed of martensite and/or bainite, thus the objective high strength steel excellent in hydrogen embrittlement resistance can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having high strength and excellent hydrogen embrittlement resistance even if it is not hardened and tempered, and its manufacturing method. The steel material is exposed to a corrosive environment such as sea breeze. It is useful as a high-strength bolt steel for bridges.

[0002]

2. Description of the Related Art There are two types of delayed fracture that occur in a corrosive environment and a non-corrosive environment, which occur after a certain period of time has elapsed since a metal material was stressed, and their causes are complicatedly entangled. Since it is working, it is difficult to identify the cause of the destruction. However, it is said that the hydrogen embrittlement phenomenon acts in some way on the delayed fracture of most steel materials.

On the other hand, as control factors for suppressing hydrogen embrittlement, tempering temperature, metal structure, material hardness, grain size,
Although the types of alloying elements and their contents are tentatively recognized, their degree of influence is not quantified.In reality, hydrogen embrittlement occurs on a trial and error basis depending on the type of steel. The reality is that measures are being taken to prevent this.

As a typical example of delayed fracture mainly caused by hydrogen embrittlement, there is high strength bolt steel used for bridges and the like, and the tensile strength including these high strength bolt steel is 120 kgf. Delayed fracture is often a major problem in tempered steels with a hardness of about / mm ² or more. Therefore, for example, Japanese Patent Laid-Open No. 58-113317 and Japanese Patent Laid-Open No. 1-1917
Japanese Patent Laid-Open No. 62-62, etc. proposes a technique in which the composition of the steel material, the heat treatment method, and the like are devised on the premise that tempering treatment is performed. However, even with these methods, the risk of delayed fracture due to hydrogen embrittlement is completely eliminated. It is not wiped out, and their application is limited to a narrow range. Moreover, all of these methods are premised on heat treatment for quenching and tempering after processing, and in order to reduce the cost of various processed products, they are excellent in hydrogen embrittlement resistance even if the heat treatment for quenching and tempering is omitted. Development of high-strength steel materials that exhibit chemical properties is required.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to obtain excellent hydrogen resistance without heat treatment such as quenching and tempering as in the conventional example. An object of the present invention is to provide a high-strength steel capable of exhibiting embrittlement characteristics and a manufacturing method thereof.

[0006]

The structure of the high-strength steel according to the present invention, which was able to solve the above-mentioned problems, has a composition of C: 0.01.
To 0.3% (hereinafter, unless otherwise specified, means% by weight), Si: 0.01 to 2.5%, Mn: 0.01 to
3.0% and Al: 0.005-0.1%,
The balance is composed of iron and unavoidable impurities, and the main point is that the structure is martensite, bainite, or a mixed structure thereof.

As the above steel materials, Cu: 0.1 to 3.0% and Ni: 0.1 to 3.0 are added as other elements.
%, Cr: 0.1 to 3.0%, Mo: 0.05 to 1.0
%, Nb: 0.01 to 0.2%, V: 0.01 to 0.5
%, Ti: 0.01 to 0.2% and B: 0.0003
A material containing at least one element selected from the group consisting of .about.0.005% or having P and S each limited to 0.02% or less exhibits further excellent hydrogen embrittlement resistance.
Further, a steel material having a lath-like structure and containing retained austenite in a volume ratio of 1 to 30% between the laths more reliably exhibits excellent hydrogen embrittlement resistance and is very strong. It will be excellent.

The method for producing high-strength steel according to the present invention is
Using a steel material satisfying the requirements of the above component composition, after hot rolling or hot forging or heat treatment after that, continuous cooling, or by isothermal transformation, the main constituent of the structure martensite or bainite or those It has a gist in the place where it is a mixed tissue of.

[0009]

The function of the composition of the steel material according to the present invention will be described first. C: 0.01 to 0.3% C is a component essential as a strengthening element, and 1
0.01 to secure strength above 20kgf / mm ² level
% Must be contained. However, if it is too much, very brittle lenticular martensite is formed and the toughness deteriorates, so it must be suppressed to 0.3% or less. The more preferable content of C is in the range of 0.03 to 0.25%.

Si: 0.01 to 2.5% Si acts as a deoxidizing agent and is also an effective element as a strengthening element. To effectively exert these effects, Si is contained in an amount of 0.01% or more. There must be. However, if too much, the toughness will be adversely affected, so 2.5% was set as the upper limit. The more preferable content rate of Si is 0.1.
The range is from 1.0 to 1.0.

Mn: 0.01 to 3.0% Mn acts as a deoxidizing agent during melting as well as Si, and acts as a strengthening element.
% Or more must be contained. However, if it is too large, the amount of inclusions increases, and especially machinability deteriorates, so it must be suppressed to 3.0% or less. The more preferable content of Mn is in the range of 0.1 to 2.5%.

Al: 0.005 to 0.1% Al is effective for preventing coarsening of austenite crystal grains and for fixing N to prevent embrittlement due to strain aging. It is effectively exhibited by containing 0.005% or more. However, their effects saturate at 0.1%.

The essential components of the steel material according to the present invention are as described above, and the balance is iron and inevitable impurities. In addition to the above components, Cu: 0.1-3.0%, Ni: 0.1-0.1%. 3.0
%, Cr: 0.1 to 3.0%, Mo: 0.05 to 1.0
%, Nb: 0.01 to 0.2%, V: 0.01 to 0.5
%, Ti: 0.01 to 0.2% and B: 0.0003
By incorporating at least one selected from the group consisting of 0.005%, it is possible to further improve the physical properties.

That is, Cu, Ni, Cr, and Mo are effective not only for improving hardenability but also for improving strength, toughness, and corrosion resistance. Particularly, in the case of thick-walled materials and large-diameter materials, those effects are effective. Remarkably exerted, and such effects are Cu, Ni, Cr
Are effectively exhibited by containing 0.1% or more and Mo by 0.05% or more, respectively. However, such an effect is 3.0% for Cu, Ni and Cr, and 1.
Saturate at 0%. Nb and V are each 0.01
% Is effective in preventing coarsening of austenite grains and improving hardenability, and also has an effect of increasing strength and toughness. Further, in order to enhance hydrogen embrittlement resistance by precipitation of carbonitride, It works effectively. However, these effects saturate at 0.2% of Nb and 0.5% of V, so that addition of more than that is economically wasteful.

Ti fixes nitrogen in austenite,
In addition to improving the hardenability by adding B, it has the effect of improving the hydrogen embrittlement susceptibility by the precipitation of Ti carbonitride, and these effects are effectively exhibited by containing 0.01% or more. However, if it exceeds 0.2%, huge nitrides and carbides are formed to deteriorate the toughness.

B is effective for improving the hardenability of the hypoeutectoid steel, and particularly effective for improving the hardenability when the austenite grain size is small. And such an effect is most effectively exhibited when the content of B is 0.00
It is when it is set in the range of 03 to 0.005%.

The steel material according to the present invention contains trace amounts of P, S, N, O, etc. as unavoidable impurities. Of these, P and N segregate at the grain boundaries to promote embrittlement of the steel material. When contained in a large amount, the hydrogen embrittlement resistance is significantly deteriorated, so it is desirable to suppress the content to 0.02% or less.

The structure of the steel material satisfying the above-mentioned compositional requirements after hot rolling or after the subsequent heat treatment varies variously depending on the cooling conditions, the size of the material and the like. However, according to the results of the research conducted by the present inventors, the steel having the above composition has a small amount of carbon, so that when the pro-eutectoid ferrite is contained in the structure, sufficient strength cannot be exerted, and such low composition It was clarified that the main constituent of the structure must be martensite, bainite, or a mixed structure thereof in order to secure sufficient strength even with the carbon content.
Further, among the metallographic structures, a lath-shaped structure containing austenite in a volume ratio of 1 to 30% between the laths is
It was found that the retained austenite acts as an absorption site for hydrogen and has an excellent effect in improving the hydrogen embrittlement resistance, as well as preventing crack propagation due to hydrogen.

As described above, in the present invention, the composition of the steel material is specified, and the structure after hot rolling or after hot forging or heat treatment is made into martensite or bainite or a mixed structure thereof, and more preferably , The metal structure has a lath shape, and the volume ratio between the laths is 1 to 30%.
By including the retained austenite of
It has become possible to provide a high-strength steel material having improved hydrogen embrittlement resistance and exhibiting extremely excellent delayed crack resistance even without quenching / tempering treatment.

Next, the manufacturing conditions of the steel material having the above metal structure will be described. In carrying out the present invention,
A steel material satisfying the above component composition is subjected to continuous cooling or isothermal transformation after rolling. When cooling after hot rolling, depending on the thickness of the rolled material and the amount of alloying elements added, water,
Continuous cooling in oil or air or a cooling medium having a cooling capacity close to these, or isothermal transformation in lead, a salt bath or a fluidized bed or a cooling medium having a cooling capacity close to them, the main constituent of the structure is martensite Alternatively, bainite or a mixed structure thereof is used.

Further, after the austenitizing treatment by the usual hot rolling treatment, the above-mentioned continuous cooling or isothermal transformation treatment is carried out depending on the thickness of the rolled material, the added amount of alloying elements, etc.
The main body of the structure can be martensite, bainite, or a mixed structure thereof. Further, tempering treatment can be applied to both the hot-rolled material and the heat-treated material as long as the dislocation density is not extremely reduced. Furthermore, in order to further improve the hydrogen embrittlement resistance, it is preferable to adopt a cooling rate such that a structure having a lath shape and a structure containing retained austenite in a volume ratio of 1 to 30% between the laths can be obtained. .

Next, examples of the present invention will be shown. However, the present invention is not limited to the following examples, and
It is within the technical scope of the present invention to make appropriate modifications and implement them within a range that is compatible with the spirit of the preceding and following statements.

[0023]

[Examples] Steel materials having the compositions shown in Table 1 were melted and forged by a conventional method to prepare a material for rolling. The conventional steel in Table 1 is a heat-treated steel that is used after being subjected to quenching / tempering treatment after hot rolling or hot forging. Table 2 shows the tensile strength and hydrogen embrittlement resistance of the rolled material obtained by hot rolling to a thickness of 15 to 30 mm and then water cooling. However, the hydrogen embrittlement property was evaluated by applying a stress to a test material having a notch coefficient of 10 in pure water and by delaying the fracture strength for 100 hours.

[0024]

[Table 1]

[0025]

[Table 2]

As is clear from Tables 1 and 2, the example steels satisfying the requirements of the present invention have delayed fracture resistance equivalent to or higher than that of the conventional steel and the comparative steel,
The effect is particularly remarkable on the high strength side. Of the steel materials shown in Tables 1 and 2, No. With respect to the steel materials Nos. 3, 14, and 20, the tensile strength and the delayed fracture strength when the microstructure and the manufacturing conditions were changed were examined, and the results shown in Table 3 were obtained.

[0027]

[Table 3]

As is clear from Table 3, the steel material of the present invention has excellent delayed fracture resistance in any of rolled material, hot rolled material and heat treated material. Further, it can be seen that when the microstructure is lath-shaped, it has superior delayed fracture resistance compared at the same tensile strength level as compared with the case of a granular structure.

[0029]

EFFECT OF THE INVENTION The present invention is constituted as described above, and the composition of the steel material is specified, and the main body of the structure after hot rolling, hot forging or heat treatment is martensite or bainite of low carbon content or By forming a mixed structure thereof, more preferably a structure having a lath shape and containing retained austenite in the range of 1 to 30% by volume between the laths, a conventional steel can be obtained even without heat treatment for quenching and tempering. Therefore, a steel material having excellent hydrogen embrittlement resistance can be provided.

Claims (6)

[Claims] 1. C: 0.01 to 0.3% (hereinafter referred to as weight% unless otherwise specified), Si: 0.01 to 2.5.
%, Mn: 0.01 to 3.0% and Al: 0.005
A high-strength steel excellent in hydrogen embrittlement resistance, characterized by containing 0.1% by weight, the balance being iron and inevitable impurities, and having a structure of martensite, bainite, or a mixed structure thereof. 2. As another element, Cu: 0.1 to 3.0
%, Ni: 0.1 to 3.0%, Cr: 0.1 to 3.0
%, Mo: 0.05 to 1.0%, Nb: 0.01 to 0.
2%, V: 0.01 to 0.5%, Ti: 0.01 to 0.
The high-strength steel according to claim 1, which contains at least one element selected from the group consisting of 2% and B: 0.0003 to 0.005%. 3. The high-strength steel according to claim 1, wherein P: 0.02% or less and S: 0.02% or less. 4. The high-strength steel according to any one of claims 1 to 3, wherein the microstructure is lath-like and contains 1-30% by volume of retained austenite between the laths. 5. A steel material having the composition defined in any one of claims 1 to 3 is used, which is subjected to hot rolling or subsequent hot forging or heat treatment, followed by continuous cooling or isothermal transformation. A method for producing high-strength steel excellent in hydrogen embrittlement resistance, characterized in that the main constituent of the structure is martensite, bainite, or a mixed structure thereof. 6. The method according to claim 5, wherein the microstructure is a lath-like structure, and the retained austenite is contained between the laths in a volume ratio of 1 to 30%.