JPH08246102A - Multilayered high tensile strength steel excellent in fatigue strength in weld zone - Google Patents

Abstract

PURPOSE: To produce a multilayered high tensile strength steel excellent in fatigue strength in a weld zone by constituting the steel of surface layers of the front and rear faces of a steel plate of a specific composition consisting of C, Si, Mn, P, S, Nb, V, Ti, W and Fe and also regulating the carbon equivalent of the inner layer to a specific value. CONSTITUTION: This multilayered steel is constituted of surface layers, each having a thickness of >=0.1mm from the surface and the rear surface of a steel plate and also <50% of plate thickness, and an inner layer sandwiched between the surface layers. At this time, the surface layer has a composition containing, as essential components, 0.015-0.20%, by weight, C, 0.1-2.0% Si, 0.3-2.0% Mn, <=0.05% P, and <=0.05% S, further containing one or >=2 kinds among 0.05-1.0% Nb, 0.05-2.0% V, 0.05-1.0% Ti, and 0.10-2.0% W, satisfying 0.5<=6Nb+3V+6Ti+2 W<=6.0, having a carbon equivalent Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+ V)/5+Nb/3 satisfying 0.20<=Ceq<=0.35, and also having the balance Fe with inevitable impurities. Further, the inner layer has a carbon equivalent, Ceq satisfying 0.35<=Ceq<=0.70.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for members that require high fatigue strength of welded parts and high base metal strength in the fields of shipbuilding, offshore structures, bridges, construction machinery and the like. The present invention relates to a multi-layer high-strength steel having excellent fatigue strength of a welded part having a base material strength of 490 to 780 MPa.

[0002]

2. Description of the Related Art With the increase in size of structures, reduction of the weight of structural members has become an important issue in recent years, and in order to realize this, the tensile strength of steel used for structures has been increasing. . However, in the fields of ships, offshore structures, bridges, construction machinery, and the like, repeated loads are applied during the period of use. Therefore, consideration must be given to preventing fatigue failure in such structures. Since the site where fatigue failure is most likely to occur is the welded part, there is a strong demand for improving the fatigue strength of the welded part.

To date, a great deal of research has been conducted on the factors governing the fatigue strength of welds and the improvement of fatigue properties. The improvement of the fatigue properties of welds is achieved by grinding and grinding the weld bead final layer and remelting the toe. By reducing the stress concentration by improving the shape of the weld toe such as shaping the shape,
Most of the improvements were due to mechanical factors, such as those due to the generation of compressive stress at the weld toe such as shot peening treatment (JP-A-59-110490, JP-A-1-33).
No. 01823 publication). Further, the effect of reducing residual stress by post-welding heat treatment has been well known.

On the other hand, there has been proposed a method of improving the fatigue strength of the welded portion by the composition of the steel material without using the above-mentioned special construction and post-weld heat treatment. JP-A-3-26464
No. 6, in order to improve stretch flangeability, fatigue characteristics, and resistance weldability by obtaining a specific structure by adding Nb and Ti, C: 0.01 to 0.2.
%, Mn: 0.6 to 2.5%, Si: 0.02 to 1.5
%, And Nb: 0.01 to 0.1% and Ti:
Disclosed is a high-strength steel sheet excellent in stretch flangeability and the like, which contains at least one element selected from the group consisting of 0.01 to 0.1%.

In Japanese Patent Laid-Open No. 63-282240, T
By obtaining a specific structure by limiting the ratio of i and N, the yield ratio is 0.7 or less and the fatigue strength / tensile strength ratio is 0.5 or more for the purpose of C: 0.03.
~ 0.1%, Mn: 0.5-2.0%, Si≤1.5
%, P ≦ 0.15%, Ti ≦ 0.05% and Ti ≦ 3.
Disclosed is a high-strength hot-rolled steel sheet having excellent fatigue properties, which is composed of 42 × N and B: 0.0002 to 0.001%.

In Japanese Patent Laid-Open No. 57-60052, Ce is used.
By limiting the relationship between the q value and the tensile strength of the base material, the weld heat affected zone (Heat Affected Zone:
For the purpose of not softening (hereinafter abbreviated as HAZ) small, causing no local deformation even during processing after welding, and not lowering fatigue strength, C: 0.02 to 0.15%, S
i: 2.0% or less, Mn: 0.1 to 2.0%, Nb:
Including 0.005 to 0.05%, Ceq = C + Si / 2
Ceq defined by 4 + Mn / 6 and tensile strength TS of base material
Discloses a high-strength steel for automobile wheels, which has a relationship of TS ≦ 25 + 100 Ceq.

In Japanese Patent Laid-Open No. 6-207245, for the purpose of suppressing fatigue crack initiation due to residual stress of compression and delaying fatigue crack propagation by producing a large number of microcracks at the tips of fatigue cracks. 3% or more of Ni is added to a region of 0.2 mm or more from the front and back surfaces of the steel plate and 25% or less of the plate thickness, and is composed of grains having the same crystal orientation from the front and back surfaces of the steel plate to 5% or more of the plate thickness. The colony aspect ratio is 4 or more and the uniaxial diameter is 5μ
A multi-layer steel sheet having a structure of m or less and having excellent fatigue properties is disclosed.

In Japanese Patent Laid-Open No. 147840/1993, the inside of the steel sheet has a soft hardness distribution, so that the surface layer is C: 0 for the purpose of improving fatigue strength by surface layer hardening without impairing press workability. 0.01-0.15%, S
i: 0.05 to 2.0%, average hardness: 140 to 200,
The inner layer is C: 0.1% or less, Si: 0.05 to 0.5%,
A composite steel sheet having an average hardness of 50 to 130 and excellent in fatigue strength and a method for manufacturing the same are disclosed.

[0009]

Of these, JP-A-59-110490 and JP-A-1-3018 are among these.
According to Japanese Patent No. 23, it is necessary to carry out a special construction after welding, and it is impossible to improve the fatigue characteristics with welding. The post-welding heat treatment method is also not preferable because the number of steps is increased and the welding work becomes complicated. Also, the effect is limited. The steel sheets disclosed in JP-A-3-264646 and JP-A-63-282240 are mainly used for automobile wheel rims and discs, but the target is not the welded portion but the base metal. There
There is no description regarding the fatigue characteristics of welds.

The steel sheet disclosed in Japanese Unexamined Patent Publication No. 57-60052 relates to a high-strength steel in which the fatigue strength is not lowered by reducing the softening of the heat-affected zone of the welding, and like the steel of the present invention, HAZ. Nb is not added in order to strengthen and improve fatigue strength, but the basic idea is different. Moreover, there is no specific description regarding fatigue strength.
Further, the amount of Nb added is very small, and the component range is different from the amount of Nb added required in the present application.

The steel sheet disclosed in Japanese Unexamined Patent Publication No. 6-207245 suppresses the occurrence of fatigue cracks due to residual stress of compression by adding Ni in an amount of 3% or more. Fatigue crack initiation is not suppressed by precipitation strengthening and solid solution strengthening, but the basic idea is different. The present invention is also different from the present invention in that the crystal orientation and the microstructure in the range of 5% or more of the plate thickness from the front and back surfaces of the steel plate are defined. Further, it is unknown whether the fatigue strength of HAZ is improved.

The steel sheet disclosed in Japanese Patent Laid-Open No. 3-147840 is the same in that the surface layer improves the fatigue strength, but this is to improve the fatigue strength by hardening the surface layer. The basic idea is different, not due to precipitation strengthening or solid solution strengthening. Further, it is not clear whether the fatigue strength of the welded portion is improved. The present invention does not improve the fatigue characteristics by additional welding work for realizing the reduction of stress concentration and the reduction of welding residual stress, but by controlling the structure of the weld heat affected zone as welded, the steel sheet It is an object of the present invention to provide a multi-layer high-strength steel capable of obtaining a high base metal strength while improving the fatigue strength of the welded part.

[0013]

The main principle of the present invention for solving the above problems can be summarized as follows. That is, in the surface layer of the steel sheet, not only the HAZ structure in which fatigue cracks often occur is precipitation strengthened by carbides and nitrides, but also the matrix of the HAZ structure is solid-solution strengthened to improve the fatigue properties of the welded part. On the other hand, in the inner layer portion of the steel sheet, a high base metal strength is secured by using a component having a carbon equivalent higher than that of the surface layer, and a multi-layer high tensile steel having excellent fatigue strength of the welded portion is obtained.

That is, the gist of the present invention is as follows: (1) From the surface layer which is 1.0 mm or more from the front and back surfaces of the steel sheet and is less than 50% of the sheet thickness, and the inner layer sandwiched between these surface layers. In the multi-layer steel, the weight ratio of the surface layer is C: 0.015 to 0.20, Si: 0.1
2.0, Mn: 0.3 to 2.0, P: 0.05 or less,
S: 0.05 or less as a basic component, and Nb: 0.
05-1.0, V: 0.05-2.0, Ti: 0.0
5 to 1.0, W: 0.10 to 2.0, containing 1 type or 2 types or more, 0.5 ≦ 6Nb + 3V + 6Ti + 2W ≦
6.0, and the carbon equivalent (Ceq) is 0.2
Welding characterized by satisfying 0 ≦ Ceq (surface layer) ≦ 0.35, the balance consisting of Fe and inevitable impurities, and the inner layer having a carbon equivalent of 0.35 ≦ Ceq (inner layer) ≦ 0.70. Multi-ply high-strength steel with excellent fatigue strength in parts. However, Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr
+ Mo + V) / 5 + Nb / 3

(2) B: 0.00% by weight of the surface layer
03-0.0030 is contained, The multi-layer high-strength steel excellent in fatigue strength of the welded part as described in (1) above. (3) The surface layer is wt%, Ni: 0.10 to 3.0,
Cr: 0.10-2.0, Mo: 0.10-4.0, C
u: 0.10-1.5, 1 type (s) or 2 or more types are contained, The multilayer high-strength steel excellent in the fatigue strength of the welded part as described in said (1) or 2 characterized by the above-mentioned.

(4) The surface layer is weight% and Ca: 0.0
005-0.0050, REM: 0.0005-0.0
The high-strength multi-layer steel having excellent fatigue strength of the welded part according to any one of (1) to (3) above, which contains one or two kinds of 050. However, REM is a rare earth element.

[0017]

In the following (1) to (4), the main principle for improving the fatigue strength of the welded portion and obtaining a high base metal strength in the present invention will be explained. (1) First, the relationship between the additive elements in the steel sheet surface layer for improving the fatigue strength of the weld and the main principle of limiting the carbon equivalent will be described. The fatigue strength of welded joints is affected by the HAZ structure in addition to the stress concentration at the toe, initial defects, welding residual stress, plate thickness effect, etc., and fatigue cracks are generated from the HAZ on the steel plate surface and directed toward the center of the steel plate. In many cases, it propagates and finally breaks. Therefore, in order to improve the fatigue strength of the welded portion as it is welded, it is important to strengthen the HAZ structure and control the HAZ structure to prevent or suppress the occurrence of fatigue cracks from the steel sheet surface. .

Examples of the HAZ strengthening method include precipitation strengthening, solid solution strengthening, dislocation strengthening, and fine grain strengthening. Among these, dislocation strengthening and fine grain strengthening are caused by rearrangement of dislocations and coarsening of the structure due to rapid reheating and cooling during welding.
Does not contribute to strengthening the Z organization. However, when Nb, V, Ti and W are added in appropriate amounts, carbides and nitrides are generated and precipitation strengthened, and at the same time, the matrix is solid-solution strengthened, so that the HAZ structure can be strengthened.

These elements have the same effect in terms of precipitation strengthening and solid solution strengthening, but the addition amount and its effect are different, so the effect of the addition amount of these elements on fatigue strength is simply in the form of a sum. It cannot be represented. As a result of conducting many experiments and studying the contribution rate of each element to the fatigue strength, it has been found that the relationship of the addition amount in which these effects appear can be equivalently evaluated as K = 6Nb + 3V + 6Ti + 2W. Hereinafter, this relationship will be represented by a value K.

By adding one or more of these elements, the effect of improving the fatigue strength appears when the value of K becomes 0.5 or more. Therefore, the lower limit value is set to 0.5. Further, when the K value exceeds 6.0 and is added in a large amount, not only the effects of precipitation strengthening and solid solution strengthening are saturated, but also the hardenability becomes high as will be described later, so that the fatigue strength decreases conversely. To do. Therefore, the upper limit is set to 6.
It was set to 0. When the HAZ has a bainite structure, fatigue cracks are generated from inside the crystal grains.
By strengthening the AZ structure, it is possible to improve fatigue strength. However, when the area ratio of the martensite structure in the HAZ increases, fatigue cracks easily occur from the grain boundaries of the martensite structure, and the fatigue strength does not improve.

Therefore, in order to improve the fatigue strength of the welded portion, it is desirable to suppress the area ratio of the martensitic structure to 1% or less. Therefore, the carbon equivalent, which is an index of hardenability, is used to chemically analyze the steel material. The ingredients need to be further limited. When the effect of Nb is taken into consideration in the formula of IIW, the value of carbon equivalent can be shown by the following formula. Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr
+ Mo + V) / 5 + Nb / 3 When the value of the carbon equivalent exceeds 0.35, the area ratio of the martensite structure in the HAZ increases, so the upper limit of the carbon equivalent was set to 0.35. Also, the carbon equivalent is 0.2
When it is less than 0, the effect of improving the fatigue strength is saturated, but the strength of the surface layer portion decreases, and it becomes difficult to secure the strength of the entire steel sheet even if the strength of the base material is maintained in the inner layer. Two
It was set to 0.

(2) Next, a description will be given of the multi-layered structure for obtaining a high base metal strength, and the main principle of limiting the ratio of the steel sheet surface layer to the inner layer and the carbon equivalent of the steel sheet inner layer. If the carbon equivalent of the steel sheet is set to 0.35 or less in order to improve the fatigue strength of the welded portion, the steel sheet cannot be sufficiently strengthened.
It becomes difficult to secure a base material strength of Pa or more. Therefore, this problem can be solved by forming the steel sheet into multiple layers. That is, a multilayer steel sheet having a surface layer of a steel sheet capable of preventing or suppressing the fatigue cracks generated from the HAZ and improving the fatigue strength of the welded portion and an inner layer of the steel sheet capable of sufficiently securing the base material strength and the base material toughness is provided. It is possible to achieve both high fatigue strength of the welded part and high base metal strength. Also,
Even for 490 MPa class steel sheets, by increasing the carbon equivalent of the inner layer while keeping the carbon equivalent of the surface layer low, even if the controlled rolling / controlled cooling conditions are relaxed, both high fatigue strength of the weld and high base metal strength are achieved. it can.

Fatigue cracks are generated from the steel sheet surface of the welded portion where stress is concentrated. Therefore, the thickness of the surface layer is required to be 1.0 mm or more in order to suppress this generation. It was set to 0.0 mm. Also, the surface layer thickness is 50% of the plate thickness.
When it becomes the above, it becomes difficult to obtain high base metal strength, so the upper limit was made less than 50% of the plate thickness. Further, preferably from the viewpoint of securing the strength of the base material, 30% of the plate thickness
Is less than. Further, since the carbon equivalent needs to be 0.35 or more to secure the strength of the base material, the lower limit value is set to 0.35. On the other hand, the higher the carbon equivalent, the higher the base metal strength, but the lower the base metal toughness and weldability. To secure these, it is necessary to suppress the carbon equivalent to 0.7 or less. Was set to 0.70.

(3) Next, each component of the steel sheet surface layer limited in the present invention will be described. First, each element whose components are limited in claim 1 will be described. C is an element that increases the strength of the base material, and it is desirable to add a large amount thereof in order to increase the strength of the base material. However, addition of more than 0.20% of C reduces the toughness of the base metal as well as the weld. Therefore, the upper limit of C is set to 0.20%. Furthermore, it is preferable that the weldability is 0.
It is 10% or less. Further, if C is less than 0.015%, it becomes difficult to secure the strength of the base material, so the lower limit of C is set to 0.01
It was set to 5%.

Si is an element necessary for deoxidation at the time of melting, and when added in an appropriate amount, it solid-solution strengthens the matrix. Si
If less than 0.1%, the deoxidizing effect during melting decreases, so the lower limit was made 0.1%. If Si is added in excess of 2.0%, not only the hardenability increases, but also the toughness decreases. Therefore, the upper limit is set to 2.0%. Mn is an element that increases the strength of the base material without significantly reducing the toughness. If Mn is less than 0.3%, sufficient base metal strength cannot be obtained, and S embrittlement easily occurs, so the lower limit is 0.3%.
And Further, if Mn is contained in excess of 2.0%, not only the toughness of the welded portion is deteriorated but also the weldability and ductility are deteriorated, so the upper limit was made 2.0%.

The smaller the content of P, the more preferable it is, and if it is added in excess of 0.05%, it segregates at the grain boundaries of the base material to cause grain boundary embrittlement.
The toughness of AZ decreases. Therefore, the upper limit is set to 0.05%. The lower the S content, the more preferable it is, and if it exceeds 0.05%, A-based inclusions become prominent and the toughness of the base metal and the welded portion is impaired.
It also reduces the ductility in the plate thickness direction. Therefore, the upper limit is 0.0
It was set to 5%. As described above, Nb, V, Ti, and W all have the effect of improving fatigue strength by the synergistic effect of precipitation strengthening due to the formation of carbides and nitrides and solid solution strengthening of the matrix. As the minimum amount of addition that can expect this effect, 0.05%, 0.05%, 0.05%,
0.10% was made into the lower limit. Further, excessive addition lowers the toughness of the welded portion and deteriorates the weldability. Therefore, the upper limits of the respective addition amounts are 1.0%, 2.0%, 1.0%,
It was set to 2.0%.

In the present invention, in addition to the above elements,
The following elements may be contained in the steel sheet, and each element whose composition is limited to claims 2, 3, and 4 will be described below. B is
Increases the hardenability of grain boundaries and suppresses the generation of grain boundary proeutectoid ferrite and ferrite side plates that are the source of cracks when the HAZ structure is a bainite structure, and improves the fatigue strength of welds. . The addition amount of B is 0.000.
If it is less than 3%, the effect of increasing the grain boundary hardenability is small, so the lower limit was made 0.0003%. Also, 0.003
If the content exceeds 0%, B nitrides and borocarbides are precipitated to lower the toughness, so the upper limit was made 0.0030%.

Ni not only increases the strength of the base material, but also significantly improves the toughness. The lower limit of the amount added to obtain the effect was set to 0.1%. The effect is saturated even if added over 3.0%, so the upper limit was made 3.0%. Cr has the effect of improving the base material strength and toughness, has the effect of forming carbides and nitrides to strengthen the HAZ structure, and also improves fatigue strength. To obtain these effects, addition of 0.10% is necessary. Also, 2.0
Even if added in excess of%, the effect is saturated and conversely the weldability is impaired. Therefore, the lower limit is 0.1% and the upper limit is 2.0%.
And

Mo has the effect of not only improving the strength of the base metal but also improving the toughness, and acts similar to Cr in that it forms carbides and nitrides. The lower limit was 0.10% as the amount of addition of the effect, and the upper limit was 4.0% as the amount of addition of the effect. Cu has the effect of improving the strength of the base material, and does not generate carbides, but improves fatigue strength by solid solution strengthening. 0.1%
If it is not added above, the effect is not obtained, and if it exceeds 1.5%, it causes solidification cracking of the slab, so the lower limit is set to 0.
1% and the upper limit value was 1.5%.

Ca has the effect of fixing the sulfide which is the source of fatigue crack generation and improving the ductility. Addition amount is 0.0
If 005% or less, the effect cannot be expected, and 0.00
If it exceeds 50%, the toughness is lowered. Therefore, the lower limit is set to 0.
0005% and the upper limit of 0.0050%. REM
Has the same effect as Ca in that it fixes sulfide, which is the source of fatigue crack generation, and improves ductility. It is considered that any element of REM has the same effect as long as it is a rare earth element, and among these, La and Ce are particularly representative. In order to exert the effect of REM addition, it is necessary to add 0.0005% or more in total, and even if 0.0050% or more is added, the effect is saturated and it is not economical. Therefore, the lower limit is 0.0
005% and the upper limit value was 0.0050%.

(4) Finally, a preferable range is shown as a component of the steel plate inner layer. The components of the steel sheet inner layer are not particularly limited, but C: 0.015 to 0.20, Si: 0.1
2.0, Mn: 0.3 to 2.0, P: 0.05 or less,
S: 0.05 or less, or further Nb:
0.005-1.0, V: 0.005-2.0, T
i: 0.005-1.0, W: 0.10-2.0, B
: 0.0003 to 0.0030, Ni: 0.10
3.0, Cr: 0.10 to 2.0, Mo: 0.10
4.0, Cu: 0.10 to 1.5, Ca: 0.0005
~ 0.0050, REM: 0.0005-0.0050
It is desirable to contain one kind or two or more kinds.

If the multi-layer steel sheet shown in the present invention is used, high base metal strength can be obtained in any welded joint such as T-time fillet welded joint, cross fillet welded joint, lap welded joint and butt welded joint. It is possible to improve the fatigue strength of the welded portion while maintaining the above. Further, the present invention uses a welding method by gas shielded arc welding such as arc welding using an inert gas (MIG), arc welding using a carbon dioxide gas or a mixed gas (MAG), and tungsten arc welding (TIG). In particular, it is effective in case of welding, but covered arc welding (SMAW) and submerged arc welding (SA
Even in a welded joint using a welding method such as W), the fatigue strength can be improved.

[0033]

EXAMPLES Examples of the present invention will be described below. In order to investigate the influence of each element, steels A to J were used as examples of the present invention.
10 steel types, 4 steel types of steels K to N as comparative examples, 14 in total
Steel type multi-layer slab was melted. Continuous casting and explosion-bonding of two steel plates are conceivable as the manufacturing method of the multi-layer slab,
Here, a continuous casting method is used in which a molten steel having a chemical composition of the surface layer and a molten steel of an inner layer which are separately melted are simultaneously cast to obtain a multilayer steel sheet. In Table 1, the chemical composition of the surface layer and the inner layer of the steel sheet, the carbon equivalent, and the value of 6Nb + 3V + 6Ti + 2W are shown as K. Here, any element belonging to the lanthanoids and actinides can be used as the REM, but here, La and Ce are particularly used, and the sum of them is set to RE.
The addition amount of M was set. The carbon equivalent (Ceq) was calculated by the formula used in claim 1. The underline in the table indicates that it is outside the scope of the claims of the present invention.

[0034]

[Table 1]

Table 2 shows the manufacturing method, the ratio of the surface layer, and the mechanical properties. As shown in this table, as hot rolled by steel,
By performing controlled rolling, controlled cooling, and tempering in combination, the plate thickness was rolled to 15 to 40 mm. Specifically, after holding the multi-layer slab at 1200 ° C. for 1 hour, the hot-rolled as-rolled sheet was rolled to a finished sheet thickness as it was, but otherwise, rough rolling was performed to a sheet thickness three times the finished sheet thickness, A
After waiting for a temperature of r ₃ points or more to a temperature not higher than the unrecrystallization temperature, rolling was performed to a plate thickness of 15 to 40 mm. In the case of only controlled rolling, it is air-cooled to the room temperature after the rolling is completed, and when it is combined with the controlled cooling, it is immediately water-cooled to 500 ° C after the rolling is completed and then air-cooled to the room temperature, and when the tempering is combined, after the rolling is completed. Immediately after cooling with water and cooling to room temperature, heat treatment was performed at 550 ° C. for 1 hour. The microstructure of this steel plate was observed, the total value of the plate thickness of the steel plate and the surface thickness of the front and back surfaces of the steel plate was measured, and the ratio of the surface layer to the plate thickness was calculated. Further, the yield stress, tensile strength, and elongation at break of the base material were measured by a tensile test. In the present invention example, steels A to D
Is a 490 MPa class steel, Steels E to I are 590 MPa class steels, and Steel J is a 780 MPa class steel.

[0036]

[Table 2]

Using this steel sheet, a T-shaped fillet welding fatigue test piece 1 shown in FIGS. 1 (A) and 1 (B) was prepared. In the figure, 2 is a flat plate, 3 is a rib plate, and a corner portion 4 is formed by both plates, and the corner portion is welded. 5 is a weld metal. The shape of the test piece 1 is a = 50 mm, b = 200 mm, c = 15 mm, d
= 30 mm and e = 15 mm. Since the T-shaped fillet welded joint has a plate thickness dependency and the plate thickness of the steel plate affects the fatigue strength of the welded joint, it is not possible to compare the fatigue strength of welded joints having different plate thicknesses. Therefore, the plate thickness is 15m
For steel plates thicker than m, grind one side to a plate thickness of 15 m
m, and the rib was welded to the non-ground side having the surface layer. Although various methods can be used as the welding method, carbon dioxide arc welding was used here and the welding heat input was 18 kJ / cm. A three-point bending fatigue test was performed on this test piece in the air at room temperature with a stress ratio R (minimum stress / maximum stress) = 0.1. Table 3 shows the fatigue strength of each welded joint. Here, the fatigue strength is 200 repetitions.
The stress range at ten thousand times was used.

[0038]

[Table 3]

Next, the fatigue test results of the welded joint will be described. In the joint 1, the amount of Nb added to the surface layer is 0.05% or more, the value of K is 0.50 or more, and the carbon equivalent of the surface layer is 0.35.
Hereinafter, the carbon equivalent of the inner layer is 0.35 or more,
It is a welded joint made of multi-layer high-strength steel A having a tensile strength of 490 MPa and satisfying the conditions of. As can be seen from the fatigue test results of the welded joint, the present invention example added Hb by adding Nb.
Since AZ is strengthened by precipitation strengthening or solid solution strengthening and the occurrence of fatigue cracks is suppressed, high fatigue strength of welded parts can be obtained. Further, even if the carbon equivalent of the surface layer portion is low, it is possible to obtain high base metal strength at the same time without performing controlled rolling and controlled cooling by forming a multilayer.

The joints 2 to 4 are also welded joints using the 490 MPa grade multi-layer high-strength steels B to D with V, Ti, and W added to the surface layers, and all have high fatigue strength due to the same effect. The joints 5 to 9 have one or more kinds of Nb, V, Ti and W added to the surface layer, or further B, Ni, Cr, Mo and C.
590 with one or more added u, Ca and REM
The multi-layer high-strength steels E to G of the MPa class and the joint 10 have the same 78
It is a welded joint made of 0 MPa class multi-layer high-strength steel H. The joint 5 has precipitation strengthening of Nb, V, Ti, and W and solid solution strengthening, and the joints 6 and 7 have an effect of suppressing grain boundary proeutectoid ferrite and ferrite side plates by B of the surface layer, and N.
Due to the effect of improving the fatigue strength and the strength of the base material by the formation of carbonitrides of i, Cr, Mo and Cu and solid solution strengthening,
The joints 8 to 10 achieve high fatigue strength of the weld and base metal strength due to the effect of suppressing the source of fatigue cracks caused by Ca and REM.

On the other hand, the joint 11 has Nb added to the surface 4
It is a welded joint that uses 90 MPa grade multi-layer high-strength steel K. In this joint, the addition amount of the strengthening element is less than the present range and the value of K is less than 0.50, the HAZ is not sufficiently strengthened, and fatigue cracking is relatively easy to occur. The fatigue strength of the joint is lower than that of the inventive example. Further, since the surface layer is 50% or more, the strength of the base material is insufficient. Fitting 12 is 590MP with Nb, V, Ti and W added to the surface layer
It is a welded joint using a-grade multi-layer high-strength steel L. Not only does this joint have a K value less than 0.50,
Since the thickness of the surface layer is less than 1 mm, the fatigue strength is not improved.

The joint 13 is a welded joint using a single layer steel M having the same chemical composition as the inner layer of the steel H. Although the base metal strength is high because there is no surface layer with a low carbon equivalent, the K value does not satisfy 0.5 because the number of reinforcing elements is small, and the fatigue strength of the welded joint is low. The joint 14 is a welded joint using single-layer steel N having the same chemical composition as the surface layer of steel H. Although the fatigue strength of welded joints is improved by strengthening HAZ, etc., the base metal strength does not reach the 780 MPa class, and multi-layering is effective in achieving both high fatigue strength and base metal strength of welded parts. I understand.

[0043]

As described in detail above, according to the present invention,
HAZ is added to the surface of steel sheet by adding Nb, V, Ti and W in appropriate amounts.
By strengthening precipitation strengthening and solid solution strengthening, and improving the fatigue characteristics by controlling the carbon equivalent and controlling the HAZ structure, while the inner layer of the steel sheet is a component system having high base metal strength and toughness, It has become possible to obtain multi-layer high-strength steel with excellent fatigue strength in the weld. The industrial significance of the steel of the present invention having such effects is extremely remarkable.

[Brief description of drawings]

FIG. 1 is a view showing a T-shaped fillet welding fatigue test piece according to the present invention.

[Explanation of symbols]

 1 T-shaped fillet welding fatigue test piece 2 Flat plate 3 Rib plate 4 Corner 5 Weld metal

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[Procedure amendment]

[Submission date] April 10, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0015

[Correction method] Change

[Correction content]

(2) The high-strength multi-layer steel having excellent fatigue strength of the welded part according to (1), characterized in that the surface layer contains B: 0.0003 to 0.0030 in weight%. . (3) The surface layer is wt%, Ni: 0.10 to 3.0,
Cr: 0.10-2.0, Mo: 0.10-4.0, C
u: 0.10-1.5, 1 type (s) or 2 or more types are contained, The multi-layer high-strength steel with the excellent fatigue strength of the welded part as described in said (1) or (2) .

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0020

[Correction method] Change

[Correction content]

By adding one or more of these elements, the effect of improving the fatigue strength appears when the value of K becomes 0.5 or more. Therefore, the lower limit value is set to 0.5. Further, when the K value exceeds 6.0 and is added in a large amount, not only the effects of precipitation strengthening and solid solution strengthening are saturated, but also the hardenability becomes high as will be described later, so that the fatigue strength decreases conversely. To do. Therefore, the upper limit is set to 6.
It was set to 0. When the HAZ has a ferrite or bainite structure, fatigue cracks occur in the crystal grains. Therefore, it is possible to improve fatigue strength by strengthening the HAZ structure by the above method. But HAZ
When the area ratio of the martensite structure in the alloy is high, fatigue cracks are likely to occur from the crystal grain boundaries of the martensite structure, and the fatigue strength is not improved.

[Procedure 3]

[Document name to be amended] Statement

[Name of item to be corrected] 0032

[Correction method] Change

[Correction content]

If the multi-layer steel sheet shown in the present invention is used, high base metal strength can be obtained in any welded joints such as T- shaped fillet welded joints, cross fillet welded joints, turn welded joints and butt welded joints. It is possible to improve the fatigue strength of the welded portion while maintaining the above. Further, the present invention uses a welding method by gas shielded arc welding such as arc welding using an inert gas (MIG), arc welding using a carbon dioxide gas or a mixed gas (MAG), and tungsten arc welding (TIG). In particular, it is effective in case of welding, but covered arc welding (SMAW) and submerged arc welding (SA
Even in a welded joint using a welding method such as W), the fatigue strength can be improved.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/58 C22C 38/58

Claims (4)

[Claims] 1. A multi-layer steel comprising a surface layer which is 1.0 mm or more from the front and back surfaces of a steel sheet and is an area of less than 50% of the plate thickness, and an inner layer sandwiched between these surface layers, wherein the surface layer is represented by weight%. , C: 0.015 to 0.20, Si: 0.1 to 2.0, Mn: 0.3 to 2.0, P: 0.05 or less, S: 0.05 or less as a basic component, Furthermore, Nb: 0.
05-1.0, V: 0.05-2.0, Ti: 0.05-1.0, W: 0.10-2.0, containing 1 type or 2 types or more, and 0.5 ≦ 6Nb + 3V + 6Ti + 2W ≦ 6.0, the carbon equivalent (Ceq) satisfies 0.20 ≦ Ceq (surface layer) ≦ 0.35, the balance consists of Fe and inevitable impurities, and the inner layer has a carbon equivalent of 0. 35 <Ceq (inner layer) <0.70 A multi-layer high-strength steel excellent in fatigue strength of a welded portion, characterized by: However, Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr
+ Mo + V) / 5 + Nb / 3 2. The multi-layer high-strength steel excellent in fatigue strength of a welded portion according to claim 1, wherein the surface layer contains B: 0.0003 to 0.0030 in weight%. 3. The surface layer is, by weight%, Ni: 0.10 to 3.0, Cr: 0.10 to 2.0, Mo: 0.10 to 4.0, Cu: 0.10 to 1. The multi-layered high-strength steel excellent in fatigue strength of a welded part according to claim 1 or 2, characterized in that it contains 5 or more kinds. 4. The surface layer contains, by weight, one or two of Ca: 0.0005 to 0.0050 and REM: 0.0005 to 0.0050. One of
A multi-layer high-strength steel excellent in fatigue strength of the welded part according to the item. However, REM is a rare earth element.