JPH0967643A - Welded joint with excellent fatigue strength and welding method thereof - Google Patents

Abstract

(57) [Abstract] [Problem] When used in a steel structure in which fatigue fracture is a problem, high fatigue strength could not be obtained without requiring special consideration in design and construction, but it is possible. A welded joint and a welding method thereof are provided. SOLUTION: For a steel material in which an additive element is specified, the welding heat is heated to a temperature of Ac ₁ point or more and less than the melting point by preheating in an appropriate temperature range during welding and further limiting the welding heat input. The affected area contains residual austenite in a specific volume ratio, and the fatigue-induced fatigue cracking of fatigue cracks is generated by causing plastic deformation of the retained austenite under fatigue loading to expand and to generate compressive residual stress in the HAZ near the weld toe. Improves fatigue strength by suppressing generation / propagation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structural steel (thick steel plate, hot rolled steel) for which higher fatigue strength is required than before in the industrial fields of ships, offshore structures, construction machinery, bridges, building structures and the like. The present invention relates to a welded joint (turn welded joint, fillet joint, butt joint) of steel plates, steel pipes, etc., and a welding method thereof.

[0002]

2. Description of the Related Art Generally, the fatigue strength of a base material for structural steel plates increases as the strength of the base material increases, but the fatigue strength of a welded joint (hereinafter referred to as the joint fatigue strength) increases even if the base material strength is increased. It was confirmed that it would not improve. Therefore, the joint fatigue strength of structural high-strength steel is almost the same as that of structural low-strength steel, and in the case of steel structures where fatigue fracture is a problem,
The design strength could not be increased even if high-strength steel was used. Under such circumstances, various methods for improving joint fatigue strength have been studied.

Among these, as a method of utilizing the transformation for improving the characteristics of the weld metal or the weld heat affected zone (hereinafter referred to as HAZ) of the welded joint, Japanese Patent Laid-Open Nos. 54-130451 and 60-68175 are known. There is one described in the official gazette. Further, as long as it is not limited to welded joints, there are some inventions of steel materials containing retained austenite for cold and hot rolled steel sheets, such as JP-A-64-79345, JP-A-63-42690, and JP-A-63-42690. 64-62420, JP-A-1-242721, JP-A 1-23
No. 0715 publication. All of these are intended to improve ductility represented by press formability.

[0004]

Among the prior arts, Japanese Patent Application Laid-Open No. 54-130451 relates to characteristic improvement utilizing transformation.
What is disclosed in the publication is to cool and transform the weld metal welded with an austenitic welding rod having an Ms point (martensitic transformation point) below room temperature to a temperature below room temperature for transformation expansion. However, the material whose Ms point is below room temperature is
Since it is an expensive material containing a large amount of i, Cr and the like, and requires additional work for cooling to Ms point below room temperature after welding, it leads to a great increase in cost. Also, Ni,
Only the materials that can be welded with welding materials containing a large amount of Cr, etc. are applicable, and other materials cannot be applied.

The invention disclosed in Japanese Patent Laid-Open No. 60-68175 is intended only for 13Cr martensitic stainless steel, and is a martensitic welding containing a base metal and a Ni content of 10% or less. In addition to the need to use rods, post-weld heat treatment is also required, which also increases the cost significantly. Further, JP-A-64-79345, JP-A-63-42690, and JP-A-64-62420 relating to cold / hot rolled steel sheets containing retained austenite.
Japanese Patent Laid-Open No. 1-224271 and Japanese Patent Laid-Open No. 1-221.
The inventions described in Japanese Patent No. 30715 are all aimed at improving ductility represented by press formability.
When these steel plates are welded by the normal welding method, HAZ
Has a martensite or bainite structure, and residual austenite does not occur. Therefore, the fatigue strength of the welded joint of the cold / hot rolled steel sheet containing retained austenite is exactly the same as that of the conventional steel sheet.

It is an object of the present invention to provide a welded joint having improved joint fatigue strength and a welding method thereof, without adding an expensive element and without requiring special post-treatment. is there.

[0007]

In order to solve the above problems, the present invention provides a steel material in which an additive element is specified by preheating at the time of welding to limit the heat input of welding.
AZ contains retained austenite, and the retained austenite is expanded by plastically induced transformation under fatigue loading to generate compressive residual stress in the HAZ near the weld toe, thereby suppressing the occurrence and propagation of fatigue cracks. It was made based on a new idea.

That is, the gist of the present invention is as follows. (1) In weight%, 0.10% ≦ C ≦ 0.45%, 0.5
% ≤ Si ≤ 3.0%, 0.2% ≤ Mn ≤ 2.5%, the balance consisting of Fe and unavoidable elements, and the effect of welding heat when heated to a temperature of Ac ₁ point or more and less than the melting point. A welded joint with excellent fatigue strength, characterized in that the part contains 5 to 20% by volume of retained austenite, and the balance is composed of one or more of ferrite, pearlite, bainite and martensite.

(2) In% by weight, further 0.02% ≦ P ≦
0.20%, 0.01% ≦ Ni ≦ 2.5%, 0.01%
≤ Cr ≤ 2.5%, 0.1% ≤ Mo ≤ 4.0%, 0.0
05% ≦ Ti ≦ 1.0%, 0.0001% ≦ B ≦ 0.0
1%, 0.005% ≦ Nb ≦ 1.0%, 0.005% ≦
1 out of V ≦ 2.0% and 0.001% ≦ Al ≦ 0.1%
A welded joint excellent in fatigue strength as set forth in (1) above, which contains one or more kinds.

(3) Tensile strength of 590 MPa or more and 950 MPa
A welded joint excellent in fatigue strength as set forth in (2) above, which uses a structural steel of less than MPa as a base material. (4) In% by weight, 0.10% ≦ C ≦ 0.45%, 0.5
% -Si ≤ 3.0%, 0.2% ≤ Mn ≤ 2.5%, with the balance being 350 to 500 ° C on the welding line when welding structural steel composed of Fe and inevitable elements. After preheating,
Covered arc welding or gas shielded arc welding is performed with a heat input of 3300 to 20000 J / cm, and after welding 1
A method for welding a welded joint having excellent fatigue strength, which comprises cooling for 5 seconds or more.

(5) 0.10% ≦ C ≦ 0.4 in% by weight
5%, 0.5% ≦ Si ≦ 3.0%, 0.2% ≦ Mn ≦
When welding structural steel containing 2.5% and the balance of Fe and inevitable elements, 350 to 50 on the welding line
After preheating to 0 ° C, covered arc welding or gas shielded arc welding is performed with a heat input of 3300 to 20000 J / cm, and 40 to 100 ° C / se 15 seconds to 15 minutes after the completion of welding.
A method for welding a welded joint having excellent fatigue strength, which comprises cooling with water at a rate of c from room temperature to 200 ° C.

(6) 0.10% ≦ C ≦ 0.4 in% by weight
5%, 0.5% ≦ Si ≦ 3.0%, 0.2% ≦ Mn ≦
2.5%, 0.02% ≦ P ≦ 0.20
%, 0.01% ≦ Ni ≦ 2.5%, 0.01% ≦ Cr ≦
2.5%, 0.1% ≦ Mo ≦ 4.0%, 0.005% ≦
Ti ≦ 1.0%, 0.0001% ≦ B ≦ 0.01%,
0.005% ≦ Nb ≦ 1.0%, 0.005% ≦ V ≦
Fatigue strength as set forth in (4) or (5) above, wherein structural steel containing one or more of 2.0% and 0.001% ≦ Al ≦ 0.1% is welded. Excellent welding method for welded joints.

[0013]

The present invention will be described in detail below. First, the reasons for limiting the components of the structural steel in the welded joint and welding method of the present invention will be described. C has the effect of concentrating in austenite and keeping its Ms point below room temperature. As a result, it is possible to obtain retained austenite that expands due to plastic-induced transformation under fatigue loading. The C content is preferably low from the viewpoint of weldability and toughness, but if it is less than 0.10%, it is impossible to obtain retained austenite (5 to 20% in volume ratio) enough to improve fatigue strength. For that purpose, 0.10% or more is required. On the other hand, the amount of C is 0.4
If it exceeds 5%, the toughness of bainite, which is a structure other than retained austenite in the heat-affected zone of welding, is significantly deteriorated, making it unusable for practical use. For this reason, the C content is set to 0.10 to 0.45%.

Si has the effects of improving strength, suppressing the precipitation of carbides, and promoting the concentration of C in austenite, thereby stabilizing austenite and increasing the amount of retained austenite. For this, at least 0.5% is required. On the other hand, if added excessively, the toughness deteriorates, so it is necessary to set it to 3.0% or less. Therefore, the Si amount is set to 0.5 to 3.0%.

Mn is an austenite stabilizing element,
Further, the strength is improved similarly to C and Si. In order to secure the amount of retained austenite, Mn needs to be balanced with C and Si, but at least 0.2% is required. On the other hand, when the amount of Mn exceeds 2.5%, not only the expected effect is saturated, but also the toughness is significantly deteriorated. Therefore, the Mn amount is set to 0.2 to 2.5%.

P, Ni, C added and added as necessary
The contents of r, Mo, Ti, B, Nb, V and Al are regulated for the following reasons. P, Ni, Cr, Mo, T
i, B, Nb, and V are all components that improve the joint fatigue characteristics, and are considered to affect the HAZ and improve the fatigue strength. In this respect, these components are components having the same effect, and in the present invention, it is desirable to include one or more of these components. However, any excessive addition deteriorates the steel sheet material such as toughness and susceptibility to welding cracks, so P is 0.02% or more and 0.20% or less, Ni is 0.01% or more and 2.5% or less, and Cr is 0.0
1% or more and 2.5% or less, Mo is 0.1% or more and 4.0% or less, Ti and Nb are 0.005% or more and 1.0% or less, B is 0.0001% or more and 0.01% or less, V is 0.005%
It is desirable to set it to 2.0% or less. In addition, since P is an element inevitably contained in the steel material in an amount of less than 0.02% in the ordinary steelmaking process, in order to improve the joint fatigue strength as compared with the conventional welded joint, 0.02% or more is added. is necessary.

Al is required to be 0.001% or more for deoxidation, but if it is added in excess of 0.1%, the inclusions in the steel become too much and the toughness decreases, so 0.1% is required. The upper limit. Next, the reasons for limitation in the welding process will be described. The preheating on the welding line according to the present invention reduces the temperature to 350-50.
Set to 0 ° C. The reason for preheating at this temperature is to delay the cooling rate in the heat-affected zone due to the subsequent welding only in this temperature region, promote the isothermal transformation in HAZ, and concentrate C to austenite to make it stable. The bainite transformation is dominant during the isothermal transformation, and C is accompanied by the formation of bainite.
Will be enriched in austenite. Preheating temperature is 3
If the temperature is lower than 50 ° C., the HAZ heated to a temperature of Ac ₁ point or higher by welding is cooled to a temperature below the martensitic transformation start temperature, so that martensitic transformation occurs and retained austenite cannot be obtained. If the preheating temperature exceeds 500 ° C., the amount of bainite with low strength increases and the production of pearlite becomes remarkable, so that retained austenite cannot be secured.

During welding, heat input is 3300 to 200
00 J / cm. If the welding heat input exceeds 20,000 J / cm, the cooling rate becomes slow, and the structure of HAZ consists only of ferrite and pearlite, and retained austenite cannot be obtained. The cooling rate needs to be at least higher than the cooling rate at which bainite starts to be formed, in consideration of the added element. Also, 3300 J / cm
If the welding heat input is less than 1, stable arc welding cannot be performed and a sound welded joint cannot be obtained. For this reason, the heat input was set to 3300 J / cm or more and 20000 J / cm or less.

The present invention is limited to the covered arc welding or the gas shielded arc welding because of the necessity of preheating and the limitation of the arc welding capable of welding the heat input amount. Allow 15 seconds or more to cool after welding, or finish welding 15
Water cooling shall be performed after a second to 15 minutes. After welding 15
The reason for leaving for a second or longer is to promote bainite transformation in the welding heat affected zone heated to a temperature of Ac ₁ or higher and lower than the melting point during this period, and to concentrate C in austenite and stabilize austenite. The above-mentioned welding heat affected zone is
The maximum heating temperature varies depending on the distance from the melting boundary, but after welding for a few seconds to a dozen seconds, it is cooled to around 500 ° C. However, the welded joint is 350 ~
Since it has been preheated to 500 ° C., when it reaches a temperature in the vicinity of this, the cooling rate becomes extremely slow, and the preheating temperature is maintained even if constant temperature treatment such as post heat is not performed. If the standing time is less than 15 seconds, the progress of bainite transformation is insufficient and austenite in which C is not sufficiently enriched becomes martensite during cooling to room temperature, and has a structure similar to that of a joint obtained by a normal welding method. Therefore, the joint fatigue strength is not improved. On the other hand, if the standing time exceeds 15 minutes, the bainite transformation proceeds too much and the retained austenite decreases, so cooling to 200 ° C or less at a cooling rate of 40 to 100 ° C / sec secures the retained austenite. It has been found to be more effective.
If the cooling rate is lower than 40 ° C./sec, it is impossible to completely stop the bainite transformation, and conversely 1
When the cooling rate is more than 00 ° C./sec, the residual deformation of the welded joint is large and the shape becomes unusable for practical use. Therefore, the cooling rate was set in the above range. Further, even when cooling is stopped at a temperature higher than 200 ° C, bainite transformation cannot be stopped completely and residual austenite is reduced, so that room temperature to 200 ° C
It is preferred to cool to a temperature between. However, even if it is left as it is, it does not impair the improvement of the joint fatigue strength.

That is, it is preheated to 350 to 500 ° C.,
A region of the joint welded with a heat input of 3300 to 20000 J / cm, which was heated to a temperature of Ac ₁ point or higher and lower than the melting point, was heated and quenched, and then subjected to a constant temperature treatment at 350 to 500 ° C. Become. Maximum temperature is Ac ₁ point to Ac
_The region heated to the temperature of ₃ points has a two-phase state of ferrite and austenite at the maximum temperature, and has an austenite single-phase state at the Ac ₃ point or higher, and has a structure as it is even when the temperature reaches the preheating temperature. When the temperature reaches near the preheating temperature, part of the austenite undergoes bainite transformation, but the C discharged by the formation of bainite concentrates in the remaining austenite, and the austenite is stabilized, so that it remains even when cooled to room temperature. To do. This is because the C concentration in the austenite at this time reaches 1.1% or higher, and the Ms point at that time becomes room temperature or lower. The thermal history of retained austenite formation in the present invention is schematically shown in FIG.

The present inventors have examined the relationship between the amount of retained austenite and the degree of improvement in fatigue strength. As a result, the volume ratio is 5 to 20.
It was found that the fatigue strength is particularly improved when the content of retained austenite is%. When the residual austenite content is less than 5%, the compressive residual stress generated even when the transformation expands due to the fatigue load is small and does not substantially contribute to the improvement of the fatigue strength. If the retained austenite content exceeds 20%, HA
The strength of Z is extremely reduced, and the plastic deformation is concentrated on HAZ, which causes the fatigue strength to be reduced.

As a structure other than retained austenite,
It has a mixed structure of one or more of ferrite, pearlite, bainite and martensite, but from the viewpoint of improving fatigue strength, it is preferable that the volume fraction of the ferrite structure having a large fatigue crack propagation resistance is as large as possible. .
However, other structures do not impair the effect of retained austenite. The amount of retained austenite was quantified by a method of obtaining from diffraction intensity using an X-ray diffractometer (3rd Edition Iron and Steel Handbook IV, Steel Materials, Testing and Analysis, edited by the Iron and Steel Institute of Japan).

In the present invention, the range of tensile strength is 59
It was set to 0 MPa or more and less than 950 MPa. Tensile strength is 59
When it is less than 0 MPa, the base material structure is ferrite + pearlite or ferrite + bainite and has a relatively low carbon content, so that the amount of retained austenite produced by welding is small and the effect of improving fatigue strength is smaller than that of a steel material of 590 MPa or more. Further, when the tensile strength is 950 MPa or more, weld cracking (hydrogen embrittlement cracking) due to hydrogen mixing during welding
Is likely to occur, and the toughness deteriorates, resulting in brittle fracture fatigue fracture, and improvement in fatigue strength cannot be expected.

[0024]

Examples For structures having a plate thickness of 20 mm, which have the components shown in Table 1, Table 2 (continued-1 in Table 1), Table 3 (continued-2 in Table 1) and Table 4 (continued-3 in Table 1) Turning welding and cross fillet welding were performed on steel to produce a turning weld joint and cross fillet weld joint. The chemical composition and mechanical properties of steel materials are shown in the table. In the table, YS is the yield stress of the steel material, TS is the tensile strength, and El. Indicates elongation at break. Furthermore, pre-heat temperature (Pre-heat Temp.), Welding heat input (H
I), the standing time after welding (T), the cooling rate after standing (R), and the average value (residual γ) of the amount of retained austenite in the HAZ coarse grain region / fine grain region / spherical pearlite region are also shown. FIG.
Further, the turn welded joint and the cross fillet welded joint having the shape and size of the test piece shown in FIG. 3 were subjected to a swing fatigue test (stress ratio R = 0) in the atmosphere at room temperature. The fatigue strength (nominal stress range) at which the breaking life is 2 million times is shown in the table. For comparison, examples of comparative welded joints in which the content of additional elements, welding conditions, etc. were changed outside the scope of the present invention are also shown.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

[Table 4]

Although the fatigue strength of the welded joint varies depending on the joint type, comparing the fatigue strengths of the same joint type, the welded joint of the present invention has improved fatigue strength as compared with the comparative welded joint. The present invention welded joints 1 to 8 are joints according to claim 1 or joints according to the method of claim 4, the present invention welded joints 9 to 16 are joints according to claim 1 or joints according to the method of claim 5, and joints according to the present invention 17 to 34. Is a joint according to claim 2 or a joint according to the method of claim 6. The HAZ of the welded joints of the present invention all contain residual γ in an amount of 5% or more. Other than that, the welded joints 1 to 8 of the present invention have a mixed structure of ferrite, pearlite and bainite, and the welded joints 9 to 34 of the present invention include bainite and martensite. The mixed organization of is the main organization. In the turn welded joint and the cross fillet welded joint, even the joint (No. 11) having the lowest fatigue strength among the welded joints of the present invention has improved the fatigue strength by about 30% as compared with the comparative welded joint.

[0030]

As described above, the welded joint of the present invention and the joint obtained by the welding method of the present invention are
Excellent fatigue characteristics of welded joints over a wide range regardless of joint type. Therefore, when used in a structure in which fatigue fracture is a problem, high fatigue strength can be stably obtained without requiring special consideration in design and construction, and the effect is industrially great.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a thermal history for generating retained austenite in the present invention.

FIG. 2 is an explanatory view of the shape and dimensions of a test piece of a turn welded joint in an example of the present invention, (a) is a plan view, and (b) is a cross-sectional view.

FIG. 3 is an explanatory view of the shape and dimensions of a test piece of a cross fillet welded joint in an example of the present invention, (a) a plan view, (b).
Shows a cross-sectional view.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B23K 31/00 B23K 31/00 B C21D 9/50 101 C21D 9/50 101B C22C 38/04 C22C 38 / 04 38/54 38/54 (72) Inventor Jun-ichi Kobayashi 20-1 Shintomi, Futtsu-shi Nippon Steel Co., Ltd. Technology Development Division (72) Inventor Yuuo Seiya 20-1 Shintomi, Futtsu-shi Nippon Steel Co., Ltd. Company Technology Development Division

Claims (6)

[Claims] 1. By weight%, 0.10% ≦ C ≦ 0.45%, 0.5% ≦ Si ≦ 3.0%, 0.2% ≦ Mn ≦ 2.5%, and the balance A weld heat affected zone, which is composed of Fe and an unavoidable element and is heated to a temperature of Ac ₁ point or higher and lower than the melting point, contains 5 to 20% by volume of retained austenite, and the balance is ferrite, pearlite,
A welded joint excellent in fatigue strength, characterized by being composed of one or more of bainite and martensite. 2. In% by weight, further 0.02% ≦ P ≦ 0.20%, 0.01% ≦ Ni ≦ 2.5%, 0.01% ≦ Cr ≦ 2.5%, 0.1% ≦ Mo ≦ 4.0%, 0.005% ≦ Ti ≦ 1.0%, 0.0001% ≦ B ≦ 0.01%, 0.005% ≦ Nb ≦ 1.0%, 0.005% ≦ V ≦ 2.0% 0.001% ≦ Al ≦ 0.1% One or more kinds are contained, The welded joint excellent in fatigue strength according to claim 1. 3. A tensile strength of 590 MPa or more and 950 MP or more.
The welded joint excellent in fatigue strength according to claim 2, characterized in that a structural steel of less than a is used as a base material. 4. By weight%, 0.10% ≦ C ≦ 0.45%, 0.5% ≦ Si ≦ 3.0%, 0.2% ≦ Mn ≦ 2.5%, and the balance When welding structural steel composed of Fe and unavoidable elements, after preheating to 350 to 500 ° C. on the welding line, 3300 to 20000
A method for welding a welded joint having excellent fatigue strength, which comprises performing covered arc welding or gas shielded arc welding with a heat input of J / cm and allowing to cool for 15 seconds or more after the welding is completed. 5. By weight%, 0.10% ≦ C ≦ 0.45%, 0.5% ≦ Si ≦ 3.0%, 0.2% ≦ Mn ≦ 2.5% are contained, and the balance is When welding structural steel composed of Fe and unavoidable elements, after preheating to 350 to 500 ° C. on the welding line, 3300 to 20000
Covered arc welding or gas shielded arc welding is performed with a heat input of J / cm, and 40 to 15 seconds to 15 minutes after the completion of welding.
A method for welding a welded joint having excellent fatigue strength, which comprises water cooling at a rate of 100 ° C / sec to a temperature of room temperature to 200 ° C. 6. By weight%, 0.10% ≦ C ≦ 0.45%, 0.5% ≦ Si ≦ 3.0%, 0.2% ≦ Mn ≦ 2.5%, and 0 0.02% ≤ P ≤ 0.20%, 0.01% ≤ Ni ≤ 2.5%, 0.01% ≤ Cr ≤ 2.5%, 0.1% ≤ Mo ≤ 4.0%, 0.005 % ≦ Ti ≦ 1.0%, 0.0001% ≦ B ≦ 0.01%, 0.005% ≦ Nb ≦ 1.0%, 0.005% ≦ V ≦ 2.0% 0.001% ≦ Al The method for welding a welded joint excellent in fatigue strength according to claim 4 or 5, wherein structural steel containing one or more of 0.1% or less is welded.