JPH09227983A - High strength steel excellent in delayed fracture resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel having high strength of >=1100MPa tensile strength and furthermore excellent in delayed fracture resistance. SOLUTION: This steel has a compsn. contg., by weight, 0.20 to 0.50% C, 0.2 to 2.0% Si, one ore two kinds among Mn, Ni, Cr and Mo respectively by 0.2 to 3.0% one or >= two kinds among Al, Nb, V and Ti respectively by 0.01 to 0.3%, <=0.02% Ni, and the balance Fe with inevitable impurities, in which the main structure is composed of tempered martensite, and the precious austenitic grain boundary is applied with a ferritic layer having 0.1 to 10μm thickness. Its tensile strength is regulated to 1100 to 1500MPa.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to high strength bolts, screws,
The present invention relates to high strength steel applicable to structural members such as springs, shafts and reinforcing plates, and machine parts.

[0002]

2. Description of the Related Art Although high strength bolts with a tensile strength of 1100 MPa or more cause delayed fracture even when used in a natural environment, it is required to reduce the weight of structures and mechanical parts by increasing the strength. Its use is restricted. Delayed fracture is a phenomenon mainly caused by a trace amount of hydrogen invading the steel from the environment atmosphere due to corrosion of the steel, and becomes more remarkable as the strength of the steel increases.

It is a usual method to obtain a high strength steel by using a martensitic structure. Conventionally, it has been empirically empirically found that addition of Si, Ca or the like to a PC steel bar is effective for improving delayed fracture resistance. It is known that addition of Ni, Ni reduces hydrogen penetration. Further, the delayed fracture morphology in the martensite structure is often due to the cracking of the former austenite crystal grain boundaries, so it is considered that the grain boundary strength is reduced.
The reduction of impurity elements such as S and S and the miniaturization of crystal grains are also attempted.

However, the mechanism of delayed fracture has not been clarified yet, and these countermeasures are not drastic. In particular, in practical steel, the non-uniformity of materials such as segregation of alloying elements is unavoidable, so that the alloy component alone is not a reliable measure.

Further, it has been attempted to use bainite as the main structure instead of martensite, and it is known that maraging steel having a low carbon has excellent evasion fracture resistance. However, in these cases, it is unavoidable that the alloy component is increased in order to obtain high strength, resulting in inevitably high cost.

[0006]

As described above, as the strength of steel is increased, delayed fracture is likely to occur.
Considering cost, practically there is a limit to strengthening, and the actual situation is that strengthening is regulated like high strength bolts. Therefore, there is a need for a technology for producing steel with a tensile strength of 1100 MPa or more and excellent delayed fracture resistance at a relatively low cost. The present invention provides a means for improving delayed fracture resistance while maintaining the main structure of steel as martensite which is effective for increasing strength and is easily obtained.

[0007]

In order to solve the above problems, the present inventor has noticed that the delayed fracture mode of martensitic steel is old austenite grain boundary cracking.

According to a study of temper embrittlement, impurity elements such as P, S and As are easily segregated in the former austenite grain boundaries, which lowers the fracture strength. It is also known that alloying elements are concentrated from ferrite to untransformed austenite at the stage where ferrite precipitates during the solidification process of steel and the cooling process such as the weld heat affected zone. From these findings, the present inventor has paid attention to utilizing the precipitation of ferrite to reduce the impurity concentration in the former austenite grain boundary.

That is, according to the present invention, when austenite is transformed into martensite by firing, a thin ferrite phase is preliminarily precipitated in austenite grain boundaries,
The technical idea is to reduce the amount of impurity elements in the austenite grain boundaries. In the conventional strengthening technology, it is a general guideline to increase the tempering temperature by making the structure uniform martensite in order to improve the toughness of the steel, and to avoid the precipitation of ferrite that reduces the strength of the steel. Is common sense.

The present inventor first applied the technical idea of the present invention of precipitating a thin ferrite phase to the austenite grain boundaries to the mechanical structural chromium steel SCR2 which has been conventionally used for high strength parts. However, if the cooling rate from austenite is slowed down, or if ferrite is precipitated at the austenite grain boundaries by isothermal transformation, the growth rate is high and ferrite becomes coarse, and it is practically difficult to maintain the strength of steel. there were.

Therefore, it has been found that it is an essential requirement that the amount of grain boundary ferrite, which also has the effect of lowering the strength of steel, be kept to the minimum necessary for cleaning the austenite grain boundaries. Therefore, the present inventor studied conditions for precipitating the minimum amount of grain boundary ferrite for steels having various components.

As a method of precipitating a grain boundary ferrite layer from austenite during cooling, a method of heating to austenite temperature range and then martensite by continuous cooling, and controlling the cooling rate at that time,
After heating to Osutenaito temperature range, and continuous consolidation cooled to a temperature at which a predetermined Feraito layer is deposited, a method of placing baked from the temperature in water or oil, after heating to the austenite temperature region, one sputum A ₃ temperature below isothermal transformation diagram We examined methods such as rapid cooling to a temperature above the nose temperature and holding.
In each case, the thickness is 0.1 to 10 at the austenite grain boundary.
The purpose was to quench in water or oil immediately after the μm ferrite layer was deposited.

It has been found that the method can be selected depending on the composition of the steel, considering the easiness of the operation in the actual process, because the transformation characteristics also change when the composition of the steel changes. For example, it is preferable to use the isothermal transformation when the ferrite precipitation start time is about 5 seconds and is relatively short, and when the steel has a high burntability, the following method can be used. However, in any of the methods, it was most important to delay the growth rate of ferrite in order to stably thin the ferrite precipitated at the austenite grain boundaries and reduce the required minimum amount.

The present inventor has further found that the growth rate of ferrite is suppressed by the addition of trace elements, and the thickness of
We have succeeded in stably depositing 1 to 10 μm. Then, the inventors have invented the high-strength steel of the present invention which has the effect of improving delayed fracture resistance while maintaining high strength.

That is, according to the present invention, the tensile strength of steel is 1100M.
The composition is designed so that the growth of the ferrite phase is suppressed in the manufacturing process of precipitating the ferrite phase in the grain boundaries while controlling the structure to be Pa or more.

That is, in the present invention, C: 0.20 to 0.
50, Si: 0.2 to 2.0, one or more of Mn, Ni, Cr and Mo 0.2 to 3.0, Al, Nb and V, respectively.
And one or more of Ti 0.01 to 0.3
0, N: 0.02 or less, the remainder is a component system consisting of Fe and unavoidable impurities, the main structure is tempered martensite, and a ferrite layer with a thickness of 0.1 to 10 μm is deposited on the former austenite grain boundaries. This is a high-strength steel of 1100MPa to 1500MPa with improved delayed fracture resistance.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The strength of martensite is mainly determined by the amount of C. 1100 MPa to 1500 MPa targeted by the present invention
C content was 0.2 to 0.50% in order to obtain the tensile strength of.
In addition, the hardenability of steel is necessary to make a mechanical part having a thickness of, for example, 10 mm or more, such as a bolt or a mechanical part, a martensitic structure, and in the present invention, the toughness of the martensitic structure is improved in the process. Therefore, it is necessary to secure the strength when tempering is performed after quenching. M
n, Ni, Cr, and Mo are added for the purpose of delaying the ferrite transformation start time, and one kind or two or more kinds are made 0.2 to 3.0% respectively. Mn also has an effect as a deoxidizer for steel.

The amount added is determined by the hardenability required for the size of the product and the transformation characteristics. The lower limit of the addition amount is provided to ensure hardenability, and the upper limit is provided to prevent the ferrite transformation from starting too late and causing trouble in the actual process as the alloy cost increases.

Si is required as a deoxidizing material for steel in an amount of 0.2% or more, but it can be used as a reinforcement for steel. However, if too much, the toughness deteriorates, so 2.0% was made the upper limit. Al,
Nb, V and Ti act as a deoxidizing agent for steel, and precipitate as fine carbides or nitrides. These fine protrusions suppress the growth of ferrite precipitated at the austenite grain boundaries prior to the martensitic transformation,
It has the effect of stabilizing the manufacturing process conditions for keeping the ferrite layer thin. Therefore, 0.01% is set as the lower limit for the addition amount, and if it is too large, the toughness of the steel deteriorates.
The upper limit was 3%.

N is necessary for precipitating Al, Nb, V, Ti and the like as nitrides, but if it is too much, it causes difficulty in steelmaking and deteriorates the toughness of the steel. The upper limit was set. The impurities such as P, S, Sn and As segregate in the former austenite grain boundaries, and it is desirable to inevitably reduce them in order to increase the delayed fracture susceptibility.

The feature of the present invention is the combination of the composition system and the structure of steel, and the structure is also controlled by the manufacturing process conditions. The characteristic structure is that the main structure is tempered martensite and ferrite is thinly precipitated at the former austenite grain boundaries. The purpose of the ferrite layer of the present invention formed at the austenite grain boundaries is to reduce the impurity concentration in the austenite grain boundaries, but on the other hand, when the amount of ferrite increases, the strength of the steel as a whole decreases. Therefore, the thickness of the ferrite is set to 0.1 to 10 μm.

It has been experimentally sought that this range is effective for improving delayed fracture characteristics, and in addition to controlling the thickness of the ferrite layer in the actual process, maintaining the strength stably. Is what you need. Further, in order to obtain a predetermined strength and to improve the toughness after quenching, a tempering process corresponding to a normal quenching / tempering process is performed.

[0023]

【Example】

1) Sample steel A in Table 1 was heated at 1050 ° C for 1 hour, then quenched in a lead bath at 550 ° C, held for 30 seconds, water-quenched, and further 350 ° C.
Then, it was tempered for 1 h. As shown in FIG. 1, the main structure of the metal structure is martensite, and a ferrite layer having a thickness of about 5 μm is deposited at the former austenite grain boundaries. Tensile strength of this sample is 1303MPa, elongation is 12%
Met.

For comparison, same-strength steel at 1050 ° C. for 1 h
After heating, water quenching and tempering at 420 ℃, tensile strength 1285M
Pa, elongation 41% was obtained. No precipitation of ferrite was observed at the former austenite grain boundaries in the comparative material.

Two samples of the present invention and the comparative material were tested at 20% at 50 ° C.
A constant load delayed fracture test was conducted in an ammonium thiocyanate aqueous solution. The result is shown in FIG. Obviously, by precipitating the ferrite layer on the grain boundaries of the former austenite, the delayed fracture resistance was greatly improved by increasing the fracture limit stress and increasing the fracture time at constant stress.

[0026]

[Table 1] Composition of test steel (% by weight)

2) The steel B to be tested in Table 1 was heated at 1050 ° C. for 1 hour, quenched in a lead bath at 600 ° C., kept for 30 seconds, and then water-baked.
Further, tempering treatment was performed at 330 ° C. for 1 hour. The main structure of the metal structure is martensite, and a ferrite layer having a thickness of about 3 μm is precipitated at the former austenite grain boundaries. The tensile strength of this sample was 1352 MPa and the elongation was 15%.

For comparison, steel of the same composition was heated at 1050 ° C. for 1 h.
After heating, quenching with water and tempering at 420 ℃, tensile strength 1421M
Pa, elongation 11% was obtained. No precipitation of ferrite was observed at the former austenite grain boundaries in the comparative material.

The two samples were subjected to a constant load delayed fracture test in a 20% ammonium thiocyanate aqueous solution at 50 ° C. The result is shown in FIG. Apparently, the delayed fracture resistance was significantly improved by precipitating the ferrite layer on the former austenite grain boundaries.

3) The test steel C in Table 1 was heated at 1100 ° C. for 10 minutes, continuously cooled at a cooling rate of 6 ° C./s to 570 ° C., water-quenched at that temperature, and further tempered at 250 ° C. for 1 hour. Was done. The main structure of the metal structure is martensite,
A ferrite layer with a thickness of approximately 8 μm is deposited at the former austenite grain boundaries. The tensile strength of this sample is 1380MPa,
The growth was 13%.

For comparison, steels of the same composition were heated at 1100 ° C. for 10 minutes, water-quenched and tempered at 350 ° C. to obtain a tensile strength of 1380M.
Pa, elongation 11% was obtained. No precipitation of ferrite was observed at the former austenite grain boundaries in the comparative material.

The two samples were subjected to a constant load delayed fracture test in a 20% ammonium thiocyanate aqueous solution at 50 ° C. FIG. 4 shows the result. Apparently, the delayed fracture resistance was significantly improved by precipitating the ferrite layer on the former austenite grain boundaries.

[0033]

According to the present invention, a steel having a high tensile strength of 1100 MPa or more and excellent delayed fracture resistance can be manufactured at low cost.

[Brief description of drawings]

FIG. 1 is a diagram of a metallurgical structure of a sample steel A in which ferrite is precipitated in a prior austenite grain boundary.

FIG. 2 is a diagram showing a delayed fracture characteristic of Sample Steel A.

FIG. 3 is a diagram showing a delayed fracture characteristic of Sample Steel B.

FIG. 4 is a diagram showing a delayed fracture characteristic of a test steel C.

Claims (1)

[Claims] 1. C: 0.20-0.50, Si: 0.2-
2.0, one or more of Mn, Ni, Cr and Mo are 0.2 to 3.0 respectively, one or more of Al, Nb, V and Ti are 0.01 to 0.30 respectively, and N: 0.02 or less, and the balance is inevitable with Fe. It consists of impurities, the main structure is tempered martensite, and the thickness is 0.1 at the former austenite grain boundaries.
Has a ferrite layer of ~ 10μm and a tensile strength of 1100 MPa
A high-strength steel with excellent delayed fracture resistance characterized by a pressure of up to 1500 MPa.