JPH08291363A - Welded joint with high fatigue strength and its production - Google Patents

Abstract

PURPOSE: To improve the fatigue strength of a welded joint by forming the structure of the heat affected zone of a welded joint of a steel, containing specific amounts of C, Si, Mn, Al, and Fe, so that it is composed of lath-like or granular ferrite and cementite and further contains martensite under specific conditions. CONSTITUTION: The heat affected zone of a welded joint of a steel, having a composition consisting of, by weight, 0.02-0.35% C, 0.02-2.5% Si, 0.3-3.5% Mn, 0.002-0.1% Al, and the balance essentially Fe, has a structure composed of lath-like or granular ferrite and cementite and further containing martensite of <=800nm maximum size by 0.2-5% area ratio. This structure can be obtained by subjecting the heat affected zone to heating up to a temp. of inequality I at <=0.5 deg.C/s temp. rise rate and then to a temp. not higher than the Ms point at a cooling rate of inequality II. In the inequalities, each symbol of element represents its content (wt.%), and HR, TH, and CR mean temp. rise rate ( deg.C/s), heating temp. ( deg.C), and cooling rate ( deg.C/s), respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high fatigue strength welded joint of a steel material used as a member in a structure and a heat treatment method for increasing the fatigue strength of the welded joint.

[0002]

2. Description of the Related Art In structures such as ships, offshore structures, bridges, and land structures that are subject to fluctuating loads, fatigue cracks are generated and propagated from welded parts and structural stress concentration parts, causing the structure to break. Becomes Therefore, for example, Japanese Patent Publication Sho 54-3
Various techniques have been taken for avoiding stress concentration during construction in design as seen in Japanese Patent No. 0386. However, such a method has drawbacks such as great restrictions on the design and construction of the structure and an increase in the weight of the structure. Therefore, a method of increasing the fatigue strength of the steel itself without relying on the design and construction. Has been desired. As a method for increasing the fatigue strength of such a steel material, there are Japanese Patent Publication No. 3-61748 and Japanese Patent Laid-Open No. 3-291355. Since the former technique is limited to application to steel having a relatively high C content, its application is limited to strength members such as automobile parts, and it is difficult to apply it to the above-mentioned structures. The latter technique has a drawback in that the method of use is largely restricted because it has an inhomogeneous material in the plate thickness direction.

On the other hand, as an example of a mixed structure steel containing martensite, there is a mixed structure steel of ferrite and martensite as shown in JP-A-57-108241, but this steel is basically It is a so-called thin plate, and although it has been shown to have excellent strength-elongation balance,
There is no effect of improving fatigue strength. Examples of similar steels include JP-A-57-1375422 and JP-A-58-6.
No. 937, etc. Further, as an example of a steel plate having a complex structure containing martensite, Japanese Patent Laid-Open No. 58-93814 is known.
Japanese Patent Laid-Open No. 62-174322 and Japanese Laid-Open Patent Publication No. 62-174322 all aim at a low yield ratio and have no effect of improving fatigue strength.

Most of the fatigue fracture accidents of actual structures occur from the welded joint portion, and it is necessary to increase the fatigue strength of the welded joint rather than the fatigue strength of the steel itself. For this reason, a steel material has been reported that enhances the fatigue strength of the weld heat affected zone, as disclosed in JP-A-5-345928. However, in this method, the rate of temperature rise during heat treatment is low,
Since a coarse structure is likely to be formed, there is a drawback that the fatigue strength is not improved so much.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for increasing the fatigue strength of a welded joint made of steel by controlling the metallographic structure of the welded joint.

[0006]

In order to solve such a problem, the present invention defines a state of the metallographic structure of a welded joint so as to provide a high fatigue strength welded joint which is not restricted by a base material composition and a welding method. It is intended to provide a heat treatment method for increasing the fatigue strength of a welded joint. That is, the gist is as follows: (1)% by weight C: 0.02% to 0.35% Si: 0.02% to 2.5% Mn: 0.30% to 3.5% Al: 0. 002% to 0.10% The heat-affected zone of a welded joint of steel with the balance being Fe and unavoidable impurities, consisting of lath-like or granular ferrite and cementite, and having an area ratio of 0.2% or more and 5% or less. A welded joint with high fatigue strength, which contains martensite having a maximum diameter of 800 nm or less.

(2) In% by weight C: 0.02% to 0.35% Si: 0.02% to 2.5% Mn: 0.30% to 3.5% Al: 0.002% to 0 10% After heating the heat-affected zone of a welded joint of steel consisting of the balance Fe and unavoidable impurities at a temperature rising rate of 0.5 ° C./s or more to the temperature range of equation (1), A heat treatment method for increasing fatigue strength of a welded joint, which comprises cooling to a Ms point or lower at a cooling rate to be satisfied. 710-11Mn-17Ni + 28Si + 100V + 31Mo + 13W + 20 × HR ≦ T H ≦ 730-11Mn-17Ni + 28Si + 100V + 31Mo + 13W + 30 × HR ............... (1) where each element name the content (wt%), HR the heating rate (° C. / s ) And T _H indicate heating temperature (° C.). 1 / [C + Mn / 6 + Si / 24 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15] ≦ CR ≦ 50 / [C + Mn / 6 + Si / 24 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15] ………… (2) , Each element name indicates the content (wt%), and CR indicates the cooling rate (° C / s).

(3) One or two of Nb: 0.002% to 0.10% and Ti: 0.002% to 0.10% in terms of weight% (1) or ( 2) The high fatigue strength welded joint and the heat treatment method for the welded joint as described above.

(4) Cu: 0.05% to 3.0% and Ni: 0.05% by weight
10.0%, Cr: 0.05% to 10.0%, Mo:
0.05% to 3.5%, Co: 0.05% to 10.0
%, W: 0.05% to 2.0% of one type or two or more types of high fatigue strength welded joints and welded joints according to any one of (1) to (3). Heat treatment method.

(5) V: 0.002% to 0.
10% is contained, The high fatigue strength welded joint of any one of (1)-(4), and the heat treatment method of a welded joint.

(6) B: 0.0003% by weight
It is characterized by containing 0.0025% (1) ~
(5) A high fatigue strength welded joint according to any one of (5) and a heat treatment method for the welded joint.

(7) REM: 0.002% to 0.10%, Ca: 0.000% by weight
3% -0.0030% of 1 type or 2 types are contained, It exists in the high fatigue strength welded joint of any one of (1)-(6), and the heat treatment method of a welded joint.

There is a fatigue limit as an index showing fatigue strength,
In general, the fatigue limit tends to increase uniformly as the tensile strength or yield strength obtained in a tensile test increases. The fatigue limit of ordinary steel has been reported as a function of the yield strength, such as the following formula (Takahashi et al., Proc. Of the Japan Society of Mechanical Engineers, No. 38).
Vol. 310, 1972, p. 1154). σ ₀ = 0.382 σ _y +23.9 where σ ₀ : fatigue limit (kgf / mm ² ), σ _y : yield strength (kgf / mm ² ). Therefore, normally, if σ _y is determined, only the fatigue limit greater than the value predicted from this equation can be obtained. However, the applicable range of this formula is σ _y ≧ 45 kgf / mm ² .

Lath-like ferrite is a lath-like ferrite structure observed when observing the quenched structure with a transmission electron microscope. Since it is subjected to heat treatment such as tempering, solid solution C precipitates as carbides. Cementite and ferrite are separated. As shown in FIG. 1, cementite which exists between ferrite laths in a layered form means cementite precipitated along the ferrite laths. For the fraction of the metal structure, the area ratio was measured based on a photograph taken by a transmission electron microscope of a sample taken from a thick steel plate.
The diameter of martensite was defined as the maximum diameter of each martensite.

[0015]

The present invention will be described in detail below. The technical idea that forms the basis of the present invention is as follows. Tensile strength 6
Steel plates of 0 kg / mm ² or more are usually manufactured by quenching and tempering or by direct quenching after rolling and tempering.
The metal structure is often martensite, bainite, or a mixed structure thereof. In addition, steel having a tensile strength of 50 kg / mm ² or more and less than 60 kg / mm ² is usually produced as-rolled or by normalizing or by accelerated cooling after rolling, and its metallographic structure is ferrite-pearlite or ferrite-bainite. Often has a mixed structure. These steels have relatively high fatigue strength because of their high strength, but as expected from the above equation, if the yield strength is the same, the fatigue strength is almost the same regardless of the constituent system. Therefore, even if the metal structure was controlled, it was not possible to obtain higher fatigue strength with the same yield strength.

However, the present inventors have found that a steel having a structure in which a small amount of fine martensite is dispersed has remarkably high fatigue strength. The metallographic structure of this steel is basically a steel consisting of ferrite and cementite, and in particular, it is a fine lath-like ferrite with a width of about 0.5 micron and its internal or lath boundary like the structure after ordinary quenching-tempering. The effect is more remarkable when cementite is dispersed in. Normally, in the metal structure after quenching-tempering, martensite as transformed remains decomposed into carbides such as ferrite and cementite, but it is not recognized, but by heating above a prescribed temperature and then rapidly cooling, lath ferrite + cementite When the martensite in an area ratio of 0.5% or more is contained in the structure consisting of, the fatigue strength is remarkably increased. It is considered that this is because many dislocations exist around the finely dispersed martensite as it is, and the stress concentration at the crack tip is relaxed when the fatigue crack progresses.

The inventors of the present invention have made a detailed investigation of the chemical composition of steel, the metallurgical structure of the welded joint, and the heat treatment conditions after welding to obtain the same, based on the above-mentioned new discoveries concerning the high fatigue strength of thick steel plates. As a result, the inventors have found a high fatigue strength welded joint and a method for manufacturing the same as set forth in claims 1 to 7. The reasons for limiting the chemical composition and the metal structure will be described in detail below. First, the reasons for limiting the components of the steel of the present invention will be described.

C is an element effective for strengthening the welded joint, and if it is less than 0.02%, sufficient strength cannot be obtained.
On the other hand, if its content exceeds 0.35%, the weldability is deteriorated.

Si is effective as a deoxidizing element and as a strengthening element for welded joints, but if the content is less than 0.02%, it is not effective. On the other hand, if it exceeds 2.5%, the surface quality of the welded joint is impaired.

Mn is an element effective in strengthening the welded joint, and if it is less than 0.30%, a sufficient effect cannot be obtained. On the other hand, if its content exceeds 3.5%, the workability of the welded joint is deteriorated.

Al is added as a deoxidizing element. 0.0
If the content is less than 02%, there is no effect, and if it exceeds 0.1%, the surface quality of the welded joint is impaired.

Both Ti and Nb function effectively in terms of grain refinement and precipitation hardening by adding a small amount, but if the addition amount is too small, the effect cannot be obtained. In order to deteriorate the toughness of the part, the addition amount of both Nb and Ti is Ti: 0.002% to 0.10%, Nb: 0.00
It is limited to the range of 2% to 0.10%.

Cu, Ni, Cr, Mo, Co and W are all elements for improving the hardenability of the welded joint. In the case of the present invention, the strength of the steel can be increased by the addition thereof, but if the addition amount is small, the effect of improving the hardenability cannot be obtained, and addition of an excessive amount impairs weldability. : 0.05% to 3.0%, Ni: 0.05%
~ 10.0%, Cr: 0.05% ~ 10.0%, Mo:
0.05% to 3.5%, Co: 0.05% to 10.0
%, W: limited to the range of 0.05% to 2.0%.

V is effective in increasing the strength of the welded joint by precipitation hardening, but if the addition amount is small, the effect cannot be obtained, and if the addition amount is excessive, the toughness of the welded joint is impaired. The addition amount is limited to the range of 0.002% to 0.10%.

B is an element that improves the hardenability of steel. In the case of the present invention, the addition thereof can increase the strength of the welded joint, but if the addition amount is small, the hardenability does not improve, and excessive addition increases the precipitates of B and impairs the toughness of the welded joint. Therefore, its content is 0.0003%
To 0.0025%.

Although REM and Ca are effective in detoxifying S,
If the addition amount is small, S remains harmful, and excessive addition impairs toughness. Therefore, REM: 0.002% to 0.10.
%, Ca: added in the range of 0.0003% to 0.0030%.

Next, the limiting conditions of the metal structure in the present invention will be described. As described above, the present invention basically comprises a mixed structure of ferrite and cementite, and contains martensite in an area ratio of 0.2% or more and 5% or less. If the structure is mainly a structure in which carbides such as cementite are not precipitated, such as martensite as quenched, the hardness is excessive and the fatigue properties are poor. The effect of ferrite is recognized in both granular and lath shapes, but it is particularly effective when the ferrite shape is lath with a width of about 0.5 micron or less, as is seen when tempering martensite. It is remarkable. Further, when the shape of cementite is also arranged in layers between the ferrite laths and the area ratio is 1% or more, the fatigue strength is remarkably increased. However, if it exceeds 40%, the toughness may deteriorate. The effect obtained when the cementite is arranged in layers between the ferrite laths is recognized even when the main structure is a pearlite structure.

The reason why the fatigue strength is increased in this way is that there are many dislocations around the finely dispersed martensite that has been transformed, and the stress concentration at the crack tip when the fatigue crack progresses is relaxed. is there. The development of fatigue cracks is suppressed if a certain amount of as-quenched hard martensite is contained, but the fatigue strength increases remarkably when it exists at each boundary of fine laths. Further, when cementite is arranged in layers at each boundary of fine laths and martensite is dispersed, fatigue crack growth resistance is further increased because fatigue crack growth is suppressed by a synergistic effect. If the area ratio of martensite is 0.2% or less, the effect is small,
If it is 5% or more, the toughness is impaired, so the range is set to 0.2% or more and 5% or less. Further, the finer the martensite, the more remarkable the above effect, so the maximum diameter of martensite is set to 800 nm or less.

In order to obtain the above metallographic structure regardless of the composition of steel, plate thickness and welding method, the welded joint is rapidly heated to a predetermined temperature range after welding and then rapidly cooled again. . If the heating rate HR during heating is insufficient, martensite grows coarsely and its maximum diameter is 80
It exceeds 0 nm. Therefore, the lower limit of the heating rate is 0.5
Limited to ° C / s. On the other hand, if the heating temperature range is too low, the amount of martensite will be insufficient because the alloy will not sufficiently undergo reverse transformation to austenite. On the other hand, if it is too high, the amount of martensite becomes excessive, and the maximum diameter of each martensite also increases. Since the amount and size of martensite are influenced by the heating rate and the heating temperature range, the heating rate and the heating temperature range are limited within the range of the formula (1). If the cooling rate after heating is less than the lower limit of the expression (2), the amount of martensite is insufficient because austenite does not sufficiently transform into martensite. If the upper limit of the expression (2) is exceeded, the amount of martensite becomes excessive, so the cooling rate is set within the range of the expression (2).

[0030]

EXAMPLES Next, the present invention will be described in detail based on examples. First, the steel having the components shown in Table 1 was subjected to T-shaped fillet welding under the welding conditions shown in Table 2, and this welded joint was subjected to heat treatment under the conditions shown in Table 3. A fatigue test piece shown in FIG. 2 was prepared from this welded joint. This was subjected to repeated tensile loads of different magnitudes at a stress ratio of 0 to obtain an SN diagram.
In Table 3, the metal structure after heat treatment and the fatigue limit obtained by the fatigue test are described. The fraction of the metallographic structure was measured based on a photograph taken with a transmission electron microscope of a sample taken from the welded joint.

As shown in Table 3, when the same steel was welded under the same welding conditions and then subjected to the heat treatment of the present invention,
Fine martensite is dispersed and the fatigue limit is remarkably increased as compared with the case where it is not. Also, SN of FIG.
Even if they are compared in the diagrams, S-N depends on the presence or absence of the heat treatment of the present invention
The diagrams can be clearly separated, and it can be seen that the fatigue strength of the welded joint of the present invention is significantly increased.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

[Table 4]

[0036]

[Table 5]

[0037]

As described above, according to the present invention, a welded joint having a high fatigue strength can be obtained by controlling the metal structure, and a steel material optimal for use as a structural member can be provided. .

[Brief description of drawings]

FIG. 1 is a schematic diagram of a transmission electron micrograph of a quenched structure.

FIG. 2 is a welded joint fatigue test piece.

FIG. 3 is an example of an SN diagram.

[Explanation of symbols]

 1 Martensite lath 2 Interlath layered cementite lath 3 Retained austenite 4 Intragrain cementite

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/14 C22C 38/14 38/54 38/54

Claims (7)

[Claims] 1. C: 0.02% to 0.35% Si: 0.02% to 2.5% Mn: 0.30% to 3.5% Al: 0.002% to 0. 10% The heat-affected zone of a welded joint of steel with the balance Fe and unavoidable impurities, consisting of lath-shaped or granular ferrite and cementite, and having an area ratio of 0.2% to 5% and a maximum diameter of 800 nm or less. A welded joint with high fatigue strength, which is characterized by containing martensite. 2. C: 0.02% to 0.35% Si: 0.02% to 2.5% Mn: 0.30% to 3.5% Al: 0.002% to 0. 10% After the heat-affected zone of the welded joint of steel consisting of Fe and unavoidable impurities is heated to the temperature range of equation (1) at a temperature rising rate of 0.5 ° C / s or more, equation (2) is satisfied. A heat treatment method for increasing fatigue strength of a welded joint, which comprises cooling to a Ms point or lower at a cooling rate. 710-11Mn-17Ni + 28Si + 100V + 31Mo + 13W + 20 × HR ≦ T H ≦ 730-11Mn-17Ni + 28Si + 100V + 31Mo + 13W + 30 × HR ............... (1) where each element name the content (wt%), HR the heating rate (° C. / s ) And T _H indicate heating temperature (° C.). 1 / [C + Mn / 6 + Si / 24 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15] ≦ CR ≦ 50 / [C + Mn / 6 + Si / 24 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15] ………… (2) , Each element name indicates the content (wt%), and CR indicates the cooling rate (° C / s). 3. The composition according to claim 1 or 2, characterized by containing one or two of Nb: 0.002% to 0.10% and Ti: 0.002% to 0.10% by weight. High fatigue strength welded joint and heat treatment method for welded joint. 4. By weight%, Cu: 0.05% to 3.0%, Ni: 0.05% to 10.0%, Cr: 0.05% to 10.0%, Mo: 0.05%. ~ 3.5%, Cr: 0.05% to 10.0%, Mo: 0.05% to 3.5%, Co: 0.05% to 10.0%, W: 0.05% to 0 0.5% of 1 type (s) or 2 or more types are contained, The high fatigue strength welded joint of any one of Claims 1-3, and the heat treatment method of a welded joint. 5. V: 0.002% to 0.10.
% Is contained, The high fatigue strength welded joint according to any one of claims 1 to 4, and the heat treatment method for the welded joint. 6. B: 0.0003% to 0.0 by weight.
025% is contained, The high fatigue strength welded joint of any one of Claims 1-5, and the heat treatment method of a welded joint. 7. REM: 0.002% to 0.10%, Ca: 0.000 in% by weight.
3% -0.0030% of 1 type or 2 types are contained, The high fatigue strength welded joint of any one of Claims 1-6, and the heat treatment method of a welded joint.