JP2002003984A - Structure having laser or electron beam welded joint excellent in fatigue strength, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve mechanical properties by attaining a laser or electron beam welded joint preventing the occurance of an excessively hardened and a softened zone. SOLUTION: The structure having a laser welded joint has 4-20 mm steel plate thickness and also has a composition consisting of, by mass, 0.005-0.15% C, 0.01-0.8% Si, 0.2-2.0% Mn, 0.001-0.2% Al, <=0.02% N, <=0.01% P, <=0.01% S, further <=4.5 mass%, in total, of one or more elements among Ni, Cr, Mo, Cu, V, Nb, Ti, Ca, B and Mg, and the balance iron with inevitable impurities. Moreover, this structure has fine-grained ferritic structure of <=3 μm average circle-equivalent diameter in both regions positions each at a depth of >=5% of plate thickness in a plate-thickness direction from the surface and the rear surface, respectively, and the width of the heat affected zone in a weld zone is <=1.6 mm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser or electron beam welding joint and a method for producing the same, and more particularly, to a laser or electron beam welding joint having excellent mechanical properties such as fatigue strength, static strength and toughness. And a method for manufacturing the same.

[0002]

2. Description of the Related Art It is no exaggeration to say that the reliability of a welded structure is substantially determined by the characteristics of the welded portion. Therefore, much attention has been paid to improving the characteristics of the welded portion. For example, regarding the fatigue properties of welds, recently, Journal of the Japan Welding Society, Vol. 18, No. 1, 141-14.
On page 5, a method of reducing the transformation temperature of the weld metal by welding using a highly alloyed welding material and reducing the residual stress in the weld by utilizing the expansion due to transformation after the completion of welding is reported. ing. However, since this method uses a welding material to which a large amount of expensive alloying elements are added, the cost of the welding material is increased, and there is a problem in terms of economy.

[0003] Further, as another method for improving the fatigue properties of the welded portion, the softening of the weld heat affected zone is suppressed by adjusting the components of the materials to be joined, or the maximum hardness is reduced to reduce the weldability. There is known a method of flattening the hardness distribution of a portion and reducing the concentration of strain on a relatively softened portion. As a specific method of flattening the hardness distribution of the welded portion, a hardenable element such as C is added to suppress the softening of the weld heat affected zone, or to reduce the maximum hardness. Generally, a method of relatively reducing the hardness difference between the softened portion and the highest hardness by reducing the hardenable element. However, although the former method of adding a hardenable element such as C is effective in suppressing the softening of the heat affected zone, the hard structure such as martensite increases and the maximum hardness becomes too high, resulting in toughness. Is reduced. In addition, the latter method of reducing the quenchable element is effective for lowering the maximum hardness of the heat affected zone, but the softened portion expands, making it difficult to secure the required strength of the welding joint. The problem arises.

[0004] Therefore, the conventional softening suppression of the weld heat affected zone or the reduction of the maximum hardness by adjusting the base metal component can improve the fatigue strength, but has a problem of deteriorating either strength or toughness. .

On the other hand, a steel sheet having excellent fatigue properties has been proposed by controlling the composition of the steel sheet and controlling the structure by controlled rolling cooling. For example, Japanese Patent Application Laid-Open No. 6-49593 discloses a steel sheet having a predetermined range from the front and back layers of the steel sheet. Discloses a thick steel plate for a welded structure having a texture having a predetermined aspect ratio (major axis / minor axis), and JP-A-6-49587 discloses that a predetermined range of hardness from the front and back layers of the steel sheet is equal to the internal thickness. A high fatigue-strength steel plate for a welded joint having a structure higher than a predetermined level is disclosed.

However, when welding a steel sheet whose structure is controlled as disclosed in Japanese Patent Application Laid-Open No. 6-49593, the original steel sheet structure disappears due to welding heat not only in the weld metal but also in the heat affected zone in the vicinity. As a result, the original properties of the steel sheet cannot be maintained. Also, Japanese Patent Application Laid-Open No. 6-49
The steel sheet disclosed in Japanese Patent No. 587 is a thick steel sheet for fillet welding to be welded to a surface layer of a steel sheet. Although the fatigue strength can be improved by delaying the fatigue crack generated from the part, it cannot be applied to butt welding of a general steel sheet, and there is a problem of versatility.

[0007]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, the present invention controls the composition and structure of a steel sheet, and has a good penetration depth even at a low heat input without using a welding material. To provide a structure provided with a laser or electron beam welding joint having excellent mechanical properties such as fatigue strength, static strength, and toughness by using laser welding or electron beam welding, and a method for producing the same. Aim.

[0008]

The present invention achieves the above object, and the gist thereof is as follows.

(1) The thickness of the steel plate is 4 to 20 mm,
As a component, in mass%, C: 0.005 to 0.15%,
Si: 0.01 to 0.8%, Mn: 0.2 to 2.0%,
Al: 0.001 to 0.2%, N: 0.02% or less,
P: 0.01% or less, S: 0.01% or less, the balance being iron and unavoidable impurities, in the range of 5% or more of the sheet thickness in the sheet thickness direction from each of the front and back surfaces of the steel sheet, A structure provided with a laser welding joint, which has a fine-grained ferrite structure having an average equivalent circle diameter of 3 μm or less.

(2) Ni, Cr, Mo, Cu,
The laser welding joint according to the above (1), wherein one or more of V, Nb, Ti, Ca, B, and Mg are contained in a total amount of 4.5% by mass or less. Structure.

(3) The laser welded joint according to any one of (1) and (2), wherein a width of a heat affected zone of a welded portion of the laser welded joint is 1.6 mm or less. Structure.

(4) The thickness of the steel sheet is 10 to 50 mm, and as a component, in mass%, C: 0.005 to 0.15
%, Si: 0.01 to 0.8%, Mn: 0.2 to 2.0
%, Al: 0.001 to 0.2%, N: 0.02% or less, P: 0.01% or less, S: 0.01% or less, the balance being iron and unavoidable impurities. In the range of 5% or more of the plate thickness in the plate thickness direction from each of the front surface and the back surface, it has a fine-grained ferrite structure with an average equivalent circle diameter of 3 μm or less, and the width of the heat-affected zone of the welded portion is 1.6 mm.
A structure provided with an electron beam welding joint, characterized in that:

(5) Further, Ni, Cr, Mo, Cu,
The electron beam welding joint according to the above (4), wherein one or more of V, Nb, Ti, Ca, B, and Mg are contained in a total amount of 4.5% by mass or less. Structure.

(6) The laser welded joint according to (4) or (5), wherein the width of the heat affected zone of the welded portion of the electron beam joint is 1.6 mm or less. Structure.

(7) The thickness of the steel plate is 4 to 20 mm,
As a component, in mass%, C: 0.005 to 0.15%,
Si: 0.01 to 0.8%, Mn: 0.2 to 2.0%,
Al: 0.001 to 0.2%, N: 0.02% or less,
P: 0.01% or less, S: 0.01% or less, the balance being iron and unavoidable impurities, in the range of 5% or more of the sheet thickness in the sheet thickness direction from each of the front and back surfaces of the steel sheet, A method for manufacturing a structure provided with a laser welding joint, wherein a steel sheet having a fine-grained ferrite structure having an average circle equivalent diameter of 3 μm or less is laser-welded such that the width of the heat-affected zone is 1.6 mm or less.

(8) Ni, Cr, Mo, Cu,
The laser welding joint according to (7), wherein one or more of V, Nb, Ti, Ca, B, and Mg are contained in a total amount of 4.5% by mass or less. The method of manufacturing the structure.

(9) The thickness of the steel sheet is 10 to 50 mm, and as a component, in mass%, C: 0.005 to 0.15
%, Si: 0.01 to 0.8%, Mn: 0.2 to 2.0
%, Al: 0.001 to 0.2%, N: 0.02% or less, P: 0.01% or less, S: 0.01% or less, the balance being iron and unavoidable impurities. A steel sheet having a fine-grained ferrite structure with an average equivalent circle diameter of 3 μm or less in a range from the front surface and the back surface to 5% or more of the thickness in the thickness direction so that the width of the heat-affected zone is 1.6 mm or less. A method for manufacturing a structure provided with an electron beam welding joint, wherein the structure is subjected to electron beam welding.

(10) Ni, Cr, Mo, C
The electron beam welding joint according to (9), wherein one or more of u, V, Nb, Ti, Ca, B, and Mg are contained in a total amount of 4.5% by mass or less. A method for manufacturing a structure comprising:

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the technical concept of the present invention will be described.

Generally, laser welding and electron beam welding can be performed at a higher energy density than arc welding, so that welding can be performed without using a welding material, and even with a lower heat input than arc welding. Complete penetration welding can be performed.

Further, from the examination results of the inventors of the present invention and the like, it was found that the softening portion of the weld heat affected zone near the weld metal portion can be significantly reduced in laser welding and electron beam welding as compared with arc welding. This is because laser welding and electron beam welding are more effective in controlling the microstructure of the steel sheet to improve welding joint characteristics such as strength, compared to welding methods in which the heat affected zone tends to soften, such as arc welding. This suggests that

On the other hand, laser welding and electron beam welding
Since the welding is performed with a lower heat input compared to arc welding, the cooling rate of the welded portion is high, a hard quenched structure such as martensite is easily generated, the maximum hardness is increased, and the weld joint toughness is reduced.

Therefore, the present inventors take advantage of the drastic reduction of the softened portion of the weld heat-affected zone, which is an advantage of laser welding and electron beam welding, and have the highest hardness due to the hardened structure, which is a disadvantage thereof. Solve the problem of growth,
We studied the manufacturing method of the structure with the welded joint excellent in the fatigue strength, static strength and toughness of the welded joint. Disadvantages of laser welding and electron beam welding, that is, a method of suppressing the increase in maximum hardness, which causes a decrease in toughness of the weld joint, and securing the required strength of the weld joint,
The study focused on improving the toughness by reducing the quenching component of the steel sheet and ensuring the strength by refining the structure.

FIG. 1 shows a hardness distribution diagram of a welding joint by laser welding or electron beam welding according to the present invention (left figure) and a welding joint by conventional arc welding (right figure).

The dotted line in the figure shows the hardness distribution when the quenching component is reduced and the steel material is welded as compared with the solid line steel plate.

From FIG. 1, it can be seen that laser welding or electron beam welding has a higher maximum hardness than arc welding and can greatly reduce or eliminate the softened portions that are lower in hardness than the base metal. This is because when welding at a high energy density such as laser welding or electron beam welding, welding can be performed with a low heat input and good penetration compared to welding methods such as arc welding. The width is greatly reduced, and on the other hand, the hardened structure is easily generated in the weld metal (the region where the base metal is melted and solidified by cooling) in the weld metal because the cooling speed becomes faster due to the low heat input. Conceivable.

FIG. 1 also shows that in laser welding or electron beam welding, only the maximum hardness can be suppressed in a state where there is no softened portion of the heat-affected zone by reducing the quenching component of the steel sheet. On the other hand, in arc welding, the maximum hardness is suppressed by reducing the quenching component of the steel sheet, but the softened part of the heat-affected zone is further expanded, which not only reduces the strength of the weld joint, but also causes fatigue cracks due to strain concentration. It is expected that the fatigue characteristics are rather deteriorated due to the expansion of the softened portion and the decrease in hardness, which are factors.

From the above results, it is possible to suppress the maximum hardness of the heat-affected zone and the formation of a softened portion by laser welding or electron beam welding of a steel material in which the quenching component is specified to be low, and the fatigue characteristics and toughness are good. It can be seen that a structure having a suitable welding joint can be obtained.

Laser welding or electron beam welding can significantly reduce the softened portion of the heat-affected zone as compared with arc welding. It turns out that the improvement of the strength by making the steel structure finer is effective.

That is, the present invention, based on the above findings,
Laser welding or electron beam welding is used as the welding joint, and the maximum hardness of the weld heat affected zone is suppressed by reducing the quenching component of the steel material, and the quenching component is reduced by refining the structure of the steel material. The technical idea is to suppress a decrease in the strength of the welding joint due to the above. Further, the structure provided with a welding joint means a steel material such as a steel plate, a building structure, or the like.

In the welding such as laser welding and electron beam welding, the mechanism by which the softened portion in the weld heat-affected zone of the welding joint can be significantly reduced as compared with arc welding or the like can be considered as follows.

That is, in these welding methods, since welding is performed using a concentrated heat source having a high energy density, complete penetration is achieved with a lower heat input than in arc welding or the like. Therefore, the shape of the weld bead becomes elongated, and the width of the weld heat affected zone tends to be narrower than that of arc welding with the same heat input. In particular, the softened region of the heat-affected zone is a region having a peak temperature of at most 900 to 750 ° C., and this region is much narrower in the high energy density welded portion than in the arc welded portion. The reason for this is that when comparing the heat input that achieves complete penetration at the time of welding, the required heat input for high energy density welding such as laser welding and electron beam welding is reduced to a fraction of that of arc welding. This is considered to be because the temperature region corresponding to the softened portion is significantly narrowed because the temperature can be reduced.

Next, the details of the present invention will be described.

The reasons for limiting the steel components of the welding joint of the present invention are as follows. In addition,% shown below means mass%.

C is an element which enhances hardenability most among the elements added to the steel sheet. Also, like laser welding,
When the cooling rate is high, the microstructure becomes almost 100% martensite near the fusion line of the heat-affected zone, but the hardness itself of this martensite is determined by the content of the interstitial solid solution element represented by C. Is almost determined by Therefore, suppressing the C content is the most effective means for suppressing the maximum hardness of the heat-affected zone. This is exactly the same for the hardness of the so-called weld metal, in which the base material is once melted and solidified during welding. In particular, in laser welding and electron beam welding, since no filler material is used, the components of the weld metal and the base metal are the same. Therefore, it is possible to suppress the hardness of the weld metal by suppressing the C content of the steel sheet. In the present invention, the C content is defined as 0.005 to 0.15%. If the C content is excessively reduced, it is not possible to secure a sufficient strength of the welded joint by using the grain refinement of the steel sheet structure described later. Therefore, the lower limit is set to 0.005%. If the upper limit of 0.15% is added more than this, the hardness of the fusion wire becomes too high, and even if the base metal strength is improved by refining, the hardness distribution is not flattened and the fatigue of the weld joint is reduced. It is specified because characteristics cannot be improved and deformation resistance during rolling becomes too large.

Si is an element to be effectively used as a deoxidizing element, and its content is set to 0.01 to 0.8%. S
The lower limit of 0.01% of i was set as the minimum value at which this deoxidizing effect was obtained. The upper limit of 0.8% was set because excessive addition of Si lowers the workability of the steel material, increases the hardenability, and causes deterioration in the toughness of the welded portion.

Mn is an element to be effectively utilized as a component for improving the strength of a steel material. Further, securing the strength by Mn can reduce the amount of C to be added to the steel material. On the other hand, as described above, since the hardness of 100% martensite is determined only by C, Mn should be used effectively in order to suppress the maximum hardness of the weld heat affected zone. In the present invention, the content is specified to be 0.2 to 2.0%.
The lower limit of 0.2% was set as the minimum value at which the effect of securing strength and suppressing the addition of C was obtained. However, excessive Mn addition lowers the two-phase region rolling temperature too much and increases the deformation resistance, so the upper limit was made 2.0%.

Al and N are added for refining steel by solid solution and precipitation in the rolling process, in addition to refining steel by Al nitride, but there is no effect when the added amount is small, and there is no effect. Since the addition deteriorates the toughness of the steel material, the content of Al is set to 0.001 to 0.2%, and the content of N is set to 0.02% or less.

P and S are components treated as impurities in the present invention. However, if these elements are present in excess, the toughness of the steel material is degraded.
It was set to 0.01% or less.

The above are the basic components of the steel material in the welded joint of the present invention. Further, Ni, Cr, Mo, Cu, V, and N, depending on the required properties such as the strength and toughness of the steel material.
b, Ti, Ca, B, Mg can be added. However, if these component elements are added excessively, the deformation resistance during the two-phase region rolling for achieving the grain refinement of the steel material structure described later increases, and problems such as an increase in the rolling load occur. It is necessary to regulate the upper limit of the total content of elements to 4.5%.

Next, the reasons for limiting the steel structure of the welding joint of the present invention will be described.

In the present invention, the quenching component elements added to the steel sheet to reduce the maximum hardness of the welded portion are reduced, and the reduction in the strength of the welded joint caused by the reduction in the microstructure of the steel sheet is considered. Technical philosophy. In order to secure the required strength of the welded joint in the component system in which the quenching component is reduced, in order to secure the required strength of the welded joint, the average equivalent circle diameter in the range of 5% or more of the thickness in the thickness direction from each of the front and back surfaces of the steel plate. It is necessary that a fine-grain ferrite-based structure of 3 μm or less exists.

The existence range of the fine-grained ferrite-based structure is defined as 5% or more of the sheet thickness in the sheet thickness direction from each of the front and back surfaces of the steel sheet. This is because it is not possible to secure a sufficient strength in a component system having a low quenching component that does not deteriorate the toughness and reduces the maximum hardness of the weld heat affected zone.

The grain size of the fine-grain ferrite-based structure is set to 3
The reason specified below μm is that if the particle size exceeds 3 μm,
Reduces the maximum hardness of the heat affected zone of laser welds or electron beam welded joints even when the entire thickness of the steel material is the same grain size, ensuring sufficient strength in a low quenching component system where toughness does not deteriorate. Because they cannot do it.

In order to attain the above-mentioned fine-grain ferrite-based structure, for example, a necessary amount of the ferrite structure during the temperature raising process is reduced by a method such as repeated hot rolling while cooling between rolling passes. What is necessary is just to add a process and prevent the reverse transformation to austenitization. According to this method, dislocations introduced into the processed ferrite by hot rolling are recovered and rearranged, and ultrafine ferrite can be obtained. Therefore, during hot rolling, the front and back surfaces of the thick steel plate are water-cooled and once transformed into ferrite, and then the sensible heat at the center of the plate thickness, where the temperature hardly decreases even by cooling, is applied to the front and back surfaces. Rolling is further performed while raising the temperature of the ferrite structure, and finally the ferrite structure in the specific thickness range on the front and back surfaces can be controlled to 3 μm or less.

Further, in the present invention, there is no problem that the required properties of the present invention are hindered even if the fine grain ferrite-based microstructure contains an unavoidable microstructure such as pearlite, bainite and martensite. It is necessary that the grain size of the structure is 3 μm or less.

Next, the reason why the plate thickness of the welding joint of the present invention is limited will be described.

In the present invention, as a condition for reducing the softened portion of the welding heat affected zone of the welding joint, a high energy density laser welding or an electron beam that can achieve complete penetration by low heat input welding compared to arc welding or the like is used. Although welding is used, even when welding is performed using such a concentrated heat source having a high energy density, the width of the weld bead and the weld heat affected zone increases with an increase in the plate thickness, and the softened portion cannot be reduced. Since the upper limit of the sheet thickness at which the softened portion cannot be reduced differs between laser welding and electron beam welding depending on heat source characteristics such as energy density, it is necessary to define the sheet thickness in each welding method.

When laser welding is used in the present invention, the thickness is set to 4 to 20 mm.

If the upper limit of the plate thickness of 20 mm exceeds the upper limit of the plate thickness, the width of the weld bead and the weld heat affected zone becomes too wide even if a concentrated heat source of laser welding is used, and the width of the softened zone cannot be reduced. Since the improvement of the fatigue characteristics cannot be obtained, it is specified. When the lower limit of the sheet thickness is 4 mm is less than the lower limit of the sheet thickness, the temperature drop at the center of the sheet thickness becomes large due to the cooling during the hot rolling of the steel sheet, and the cooling during the hot rolling of the cooling between the rolling passes. Since the effect of increasing the temperature by the sensible heat of the central part of the steel sheet in the front and back regions of the steel sheet cannot be used later, as a result, the fine grain ferrite-based structure of the present invention cannot be generated, so that it is specified.

In the case where electron beam welding is used in the present invention, the thickness is set to 10 to 50 mm.

Since electron beam welding has a higher energy density than laser welding, it can be applied to thick plates. However, electron beam welding requires a vacuum in the vicinity of the welded portion, so that the welding workability must be lower than that for laser welding. The lower limit of the plate thickness is 10
The mm was set to the minimum thickness at which the advantages of applying electron beam welding can be obtained in consideration of welding workability. In addition, if the upper limit of the plate thickness of 50 mm exceeds the upper limit plate thickness, even in high energy density welding such as electron beam welding, the width of the welding heat affected zone becomes too wide, and the softened portion can be reduced. Stipulate to be eliminated.

Next, the reason why the width of the heat affected zone of the welding joint of the present invention is limited will be described.

When the laser welding or the electron beam welding according to the present invention is applied, the welded portion is affected by the weld metal, which is a region where the base material is once melted and then solidified by cooling, and the heat input from the weld. The microstructure of the material can be broadly divided into a weld heat affected zone, which is a region having a different structure. Since the weld metal of the present invention does not use a welding material, its component composition is almost the same as that of the base metal, but laser welding or electron beam welding has a low heat input and a high cooling rate compared to other welding methods. This is a region where a hard quenched structure is easily generated in a cooling and solidifying process after melting, and the hardness is high.

On the other hand, the hardness of the weld heat affected zone decreases with increasing distance from the weld metal in the direction of the base metal. A softened portion, which is softer and has the lowest hardness, is formed. The softened portion is a region in which the prior austenite grain size is smaller than that of the other heat affected zones because the highest temperature is lower than the other heat affected zones. Therefore, if the width of the weld metal and the weld heat-affected zone, that is, the width of the weld zone, is constant, the width of the weld heat-affected zone can be reduced by increasing the width of the weld metal. The softened portion existing in the vicinity of the base material therein can be reduced or substantially eliminated.

Based on the above findings, in the present invention, the width of the weld heat-affected zone is reduced to 1.6 mm or less in order to improve the fatigue strength characteristics by reducing the softened portion of the welding joint and to suppress the reduction in strength. Defined in The upper limit of 1.6 mm is specified because if the upper limit is exceeded, the effects of fatigue strength characteristics and strength reduction due to the formation of softened portions become significant.

As a method of controlling the width of the welding heat affected zone to the above range, for example, in the case of electron beam welding, an appropriate magnetic field is applied to the welding beam under the same welding heat input condition, The width of the weld heat affected zone can be controlled by adjusting the width of the weld metal while oscillating the beam.

In the case of laser welding, it is possible to oscillate the welding beam, but it is more difficult than in the case of electron beam. An alternative method is to use a suitable lens to control the laser beam so that it is oblong in the direction perpendicular to the weld line.
The same effect as in the case of oscillation can be obtained, and the width of the heat affected zone can be controlled to be narrow.

FIG. 2 is a schematic view (cross-sectional view) of the weld heat affected zone and the weld metal when the oscillation of the welding beam is not performed (upper figure) and when the oscillation is performed (lower figure) in electron beam welding. Indicated. The case where the oscillation of the welding beam in FIG. 2 is not performed (upper figure) and the case where it is performed (lower figure)
As is clear from the comparison, the position of the boundary (dotted line) between the weld heat-affected zone and the base metal, that is, the entire width of the weld zone (weld metal + weld heat-affected zone) is constant, but oscillation was performed. In this case (lower figure), the width of the weld heat-affected zone can be reduced by increasing the width of the weld metal. That is, under the same welding heat input, an appropriate magnetic field is applied to the welding beam, and the width of the weld metal is increased while oscillating the welding beam, whereby the width of the welding heat affected zone can be reduced.

[0060]

EXAMPLES The effects of the present invention will be described below with reference to examples and comparative examples of the present invention.

A steel sheet having a component composition in the range specified in the present invention shown in Table 1 was manufactured under the manufacturing conditions shown in Table 2, and the final sheet thickness was in the range of 10 to 75 mm. In Table 2, steel plate numbers 1 to 13 and 20 are obtained by performing controlled rolling by cooling between rolling passes with strict production conditions, and steel plate numbers 14 to 19 have heating temperatures of about 12
Hot rolling in the temperature range of 00 ° C, 900 to 1000 ° C,
Thereafter, normal rolling for air cooling was performed. Table 3 shows the thickness, structure and properties of the obtained steel sheet. In Table 3, the steel sheet Nos. 1 to 12 of the present invention have a fine-grain ferrite-based structure having an average grain size defined by the present invention in the range from the front and back layers of the steel sheet defined by the present invention to the thickness direction. A steel sheet having a total thickness strength of 440 MPa or more, and the steel sheet number 13 of the comparative example has an average grain size of the ferrite-based structure out of the range specified in the present invention, and the strength of the steel sheet total thickness is low. In the steel sheet No. 20 of the example, the front and back layer structure is within the scope of the present invention, but the sheet thickness is outside the scope of the invention. Further, the steel sheet numbers 14 to 19 of the comparative examples are steel sheets manufactured under the above-described normal rolling conditions, and the average grain size of the steel sheet structure is about 50 μm,
Out of the range of the present invention, the strength of the entire steel sheet was low.

[0062]

[Table 1]

[0063]

[Table 2]

Next, these steel plates were butt-welded by laser welding or electron beam welding.

Since the power required for laser welding and electron beam welding differs depending on the plate thickness, the power was adjusted according to the plate thickness as follows. In case of laser welding, 10kW, 1m / min up to 14mm thickness
And up to 18mm, 15kW, 1m / min, 23m
Up to m, welding was performed at 20 kW and 0.8 m / min.

In the case of electron beam welding, the thickness is 2
In the case of 5 mm or less, 150 mA, 150 kV, 60 c
m / min, and when the plate thickness is 40 to 50 mm, 200
In the case of mA, 160 kV, 50 cpm and a plate thickness of 75 mm, welding was performed at 210 mA, 150 kV, 16 cm / min.

After the welding was completed, a fatigue test piece was sampled from the joint portion, and the front and back surfaces were finished to a smooth surface by machining so that the bead shape on the front and back surfaces did not affect the results of the fatigue test.

A fatigue test was performed using these test pieces. The comparison of the fatigue characteristics was performed between the fatigue life (cycle) when the stress amplitude was 400 MPa (10 Hz) and the fatigue limit.

Table 4 shows the width of the weld heat affected zone of the welded joint subjected to laser welding and electron beam welding, and the maximum hardness and fatigue properties of the weld heat affected zone. In Table 4, L indicates laser welding, and EB indicates electron beam welding.

As can be seen from the comparison of the maximum hardness of the weld in Table 4, it can be understood that the maximum hardness of the present invention example is almost the same level as that of the comparative material even if the total thickness strength of the steel material is high. This means that even if the steel material strength is increased, it is possible to suppress the maximum hardness of the welded portion to the same level as in the past, and this is preferable in terms of the mechanical properties of the welded portion.

In Table 4, the test numbers of the comparative examples: 17 to
No. 22 is a steel plate having a steel plate number: 14 to 19, which is manufactured under the normal rolling conditions shown in Table 3 and which is out of the range specified by the microstructure of the present invention, is produced by laser welding or electron beam welding, and is originally made of a welded joint. Due to the low hardness of the material, there was no part (softened portion) that was softer than the base metal in the heat affected zone, but the strength of the base material was low, so fatigue at 400 MPa was observed. 0.5 life
× 10 ⁵ or less, and the fatigue limit was 260 MPa or less.

Test No. 16 of the comparative example is a controlled rolled material as shown in Table 3, in which the average grain size of the ferrite-based structure deviates from the range specified by the present invention to a higher value (21 mm), and the steel plate strength is lower. : Laser welding of the steel sheet of No. 13 to produce a welded joint, in which the strength and hardness of the base material were low, and sufficient fatigue characteristics were not obtained as compared with the examples of the present invention. Test No. 23 of the comparative example is a control rolled material shown in Table 3, and the front and back layer structure is within the scope of the present invention. The welding joint is manufactured by beam welding, and the width of the weld heat affected zone is widened even when electron beam welding is performed under various oscillation conditions because the plate thickness is too thick. This occurred, and strain was concentrated on the softened portion, resulting in deterioration of fatigue characteristics.

Test Nos. 4, 7, and 11 of the comparative examples are the control rolled materials shown in Table 3, and the front and back layer structures are within the scope of the present invention. However, since the width of the weld heat-affected zone is out of the range of the present invention, as shown in Table 4, the weld heat-affected zone is softer than the fine-grained layer on the front and back surfaces, that is, A softened portion was generated, fatigue characteristics were deteriorated, and all fatigue limits did not reach 300 MPa. Test No. 14 of Comparative Example
Is a rolled control material shown in Table 3, and the front and back layer structure is within the scope of the present invention. However, in order to apply laser welding to a sheet thickness of 23 mm, a steel sheet having a thicker steel sheet number: 11 is laser-welded. Therefore, the width of the heat-affected zone cannot be suppressed to 1.6 mm or less even when the laser beam shape is horizontally elongated to control the width of the heat-affected zone, and the hardness is 175. Soft parts were generated, and the fatigue strength characteristics became low.

On the other hand, Test Nos. 1 to 3, 5
6, 8 to 10, 12, 13, and 15 are control rolled materials shown in Table 3, and the component composition and the front and back layer structure are within the scope of the present invention. A welded joint produced by electron beam welding, with a welded joint strength of 440 Mpa or more and 400 MPa
Fatigue life exceeds 0.5 × 10 ⁵ ,
The fatigue limits were all 300 MPa or more, and good weld joint strength and fatigue characteristics were obtained.

[0075]

[Table 3]

[0076]

[Table 4]

[0077]

As described above, the present invention has made it possible to improve the mechanical properties of a laser welding and an electron beam welding joint. By using the present invention, the base material strength is increased,
In addition, the heat affected zone can be suppressed, and the fatigue characteristics can be improved. In particular, laser welding is a process that is expected to benefit from the increased power in the future and expand its application to heavy industry. In view of these facts, the present invention, which can improve the mechanical properties of a weld that determines the reliability of a structure, has a great industrial advantage.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram comparing the hardness distribution of a welded portion in laser welding and electron beam welding with the hardness of an arc welded portion.

FIG. 2 In laser welding or electron beam welding,
It is the conceptual diagram which showed the influence of the oscillation in the width | variety of a weld metal and the width | variety of a welding heat affected zone.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) // B23K 103: 04 B23K 103: 04 (72) Inventor Toshihiko Koseki 20-1 Shintomi, Futtsu Nippon Steel Corporation F-term in Technology Development Division, Inc. (reference) 4E066 CA14 CB04 4E068 DA00 DA14 DB01

Claims (10)

[Claims] 1. The steel sheet has a thickness of 4 to 20 mm, and contains C: 0.005 to 0.15% by mass, Si:
0.01-0.8%, Mn: 0.2-2.0%, Al:
0.001-0.2%, N: 0.02% or less, P: 0.
S: 0.01% or less, S: 0.01% or less, the balance being iron and unavoidable impurities, and the average equivalent circle diameter in a range of 5% or more of the thickness in the thickness direction from each of the front and back surfaces of the steel plate. A structure provided with a laser welding joint, characterized by having a fine-grained ferrite structure of 3 μm or less. 2. Ni, Cr, Mo, Cu, V, N
The structure provided with a laser welding joint according to claim 1, wherein one or more of b, Ti, Ca, B, and Mg are contained in a total amount of 4.5% by mass or less. 3. The structure provided with a laser welding joint according to claim 1, wherein a width of a heat affected zone of a welding portion of the laser welding joint is 1.6 mm or less. 4. The steel sheet has a thickness of 10 to 50 mm, and contains C: 0.005 to 0.15% by mass% as a component.
i: 0.01 to 0.8%, Mn: 0.2 to 2.0%, A
l: 0.001 to 0.2%, N: 0.02% or less, P:
0.01% or less, S: 0.01% or less, the balance being iron and unavoidable impurities, the average circle in the range of 5% or more of the thickness in the thickness direction from each of the front and back surfaces of the steel plate. A structure provided with an electron beam welding joint, which has a fine-grain ferrite-based structure having an equivalent diameter of 3 µm or less and a width of a heat-affected zone of a welded portion is 1.6 mm or less. 5. Ni, Cr, Mo, Cu, V, N
The structure having an electron beam welding joint according to claim 4, wherein one or more of b, Ti, Ca, B, and Mg are contained in a total amount of 4.5% by mass or less. . 6. The structure provided with a laser welding joint according to claim 4, wherein a width of a heat-affected zone of a welding portion of the electron beam joint is 1.6 mm or less. 7. The steel sheet has a thickness of 4 to 20 mm, and contains C: 0.005 to 0.15% by mass% and Si:
0.01-0.8%, Mn: 0.2-2.0%, Al:
0.001-0.2%, N: 0.02% or less, P: 0.
S: 0.01% or less, S: 0.01% or less, the balance being iron and unavoidable impurities, and the average equivalent circle diameter in a range of 5% or more of the thickness in the thickness direction from each of the front and back surfaces of the steel plate. A method for producing a structure having a laser welding joint, wherein a steel sheet having a fine grain ferrite structure of 3 μm or less is laser-welded so that the width of the heat-affected zone is 1.6 mm or less. 8. Further, Ni, Cr, Mo, Cu, V, N
The structure having a laser welding joint according to claim 7, wherein one or more of b, Ti, Ca, B, and Mg are contained in a total amount of 4.5% by mass or less. Production method. 9. The steel sheet has a thickness of 10 to 50 mm, and contains C: 0.005 to 0.15% by mass% as a component.
i: 0.01 to 0.8%, Mn: 0.2 to 2.0%, A
l: 0.001 to 0.2%, N: 0.02% or less, P:
0.01% or less, S: 0.01% or less, the balance being iron and unavoidable impurities, the average circle in the range of 5% or more of the thickness in the thickness direction from each of the front and back surfaces of the steel plate. A method for manufacturing a structure having an electron beam welding joint, wherein an electron beam is welded to a steel sheet having a fine-grained ferrite structure having an equivalent diameter of 3 μm or less so that the width of the heat-affected zone is 1.6 mm or less. 10. Ni, Cr, Mo, Cu, V,
The structure having an electron beam welding joint according to claim 9, wherein one or more of Nb, Ti, Ca, B, and Mg are contained in a total amount of 4.5% by mass or less. Manufacturing method.