JP2003080385A - Laser-welded steel structural member and steel material used in its steel structural member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel structural member having a laser welding part excellent in fatigue strength property and steel material used in its steel structural member. SOLUTION: In this laser-welded steel structural member, the value of weighted average of welding metal side vicat hardness in the region where the distance from the bonded part after laser welding is within 1.6 mm on the position of 1 mm depth from the surface of base material is <=1.6 times of the value of weighted average of welding heat affecting part side vicat hardness in the region where the distance is within 1.6 mm. Therein, the value of weighted average of the vicat hardness in the region where the distance from the bonded part is within 1.6 mm denotes the value calculated by the following expression when the test is performed with a test force of <=9.807N; 0.4×HV10.4 +0.3×HV10.8 +0.2×HV11.2 +0.1×HV11.6 , therein, the Vickers hardness on the position where the distance from the bonded part is L mm is HV1L.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser welded steel structural member and a steel material used for the steel structural member. More specifically, the present invention relates to a steel structural member having a laser welded portion having excellent fatigue strength characteristics and a steel structural member thereof. Steel materials used for.

[0002]

2. Description of the Related Art Various steel structures such as bridges, ships, offshore structures, buildings, and tanks, especially their welds are often the starting point of fracture, and various mechanical properties such as strength, toughness, fatigue properties, etc. It is required to have excellent properties, and the steel materials used for the above steel structures are generally required to have excellent weldability in terms of structural safety and economical efficiency.

Among the mechanical properties required for the steel structure, particularly the fatigue property may lead to the destruction of the entire structure even with a relatively small stress equal to or lower than the yield stress. This is an important characteristic. It should be noted that the fatigue strength characteristic of the welded portion in the steel structure is often inferior to that of the base metal, and is therefore extremely important.

The characteristics of the weld, which consists of the weld metal and the heat affected zone, are influenced by the welding method, that is, the energy supply source.

Conventionally, as an energy supply source for welding,
Electrical energy, chemical energy, mechanical energy, ultrasonic energy, and light energy are known, and as typical welding methods of respective energy sources,
There are arc welding, resistance welding, gas welding, thermite welding, friction welding, ultrasonic welding, laser welding, etc.

Among the above welding methods, laser welding is capable of focusing energy to an extremely high density. Therefore, the amount of heat input for welding is remarkably lower than that of other welding methods, and it is possible to suppress deformation due to welding to be small. Therefore, in addition to joining thin steel sheets, laser welding has recently been expanding its application to joining thick steel sheets in fields such as shipbuilding and construction machinery, and "laser processing" refers to cutting thin steel sheets and thick steel sheets. It is being used for. Since laser processing is a non-contact cutting method, it can be applied even when processing distortion is likely to occur, and since the trajectory of the laser spot can be arbitrarily drawn, it can be used for various shapes.

As with other conventional welding methods, it is extremely important to secure high fatigue strength characteristics for a steel structure and its welded joint after laser welding.

As a technique for improving the fatigue strength of the welded portion in the case of laser welding, Japanese Patent Application Laid-Open No. 11-58060 discloses "a jig for laser butt welding and a welded portion structure by laser butt welding". However, the technique proposed in the above publication is related to the laser welding construction technique, and in order to improve the fatigue strength of the welded portion, it is necessary to use a special laser butt welding jig, which is not always a general technique. Is hard to say.

Japanese Patent Publication No. 5-14782 discloses a technique relating to "a steel plate for laser processing having excellent fatigue characteristics". However, the technique proposed in this publication relates to a steel plate for laser processing which is excellent in fatigue strength of a laser cutting portion which is simply a free surface. Therefore, even if this technique is applied to laser welding, the fatigue strength of the laser welded portion cannot always be increased.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to provide a steel structural member having a laser welded portion excellent in fatigue strength characteristics and a steel material used for the steel structural member. To provide.

[0011]

The gist of the present invention is to provide a laser-welded steel structural member as set forth in (1) to (3) below.
The steel material used for the steel structural member shown in (4) and (5).

(1) At a position 1 mm deep from the surface of the base metal, the distance from the bond portion after laser welding is 1.6 mm.
The weighted average value of the Vickers hardness on the weld metal side in the area within 1.6 mm is 1.6 times or less the weighted average value of the Vickers hardness on the welding heat affected zone in the area within 1.6 mm. Steel structural member.

Here, the distance from the bond portion is 1.6.
The weighted average value of Vickers hardness in the region within mm is the following when the Vickers hardness at the position where the distance from the bond portion is D (mm) is HV1 _D when tested with a test force of 9.807 N or less ( 1) Indicates the value represented by the formula.

Weighted average value = 0.4 × HV1 _0.4 + 0.3 × HV1 _0.8 + 0.2 × HV1 _1.2 + 0.1 × HV1 _1.6 (1)

(2) Steel is% by mass, C: 0.003 to
0.07%, Si: 0.1-0.6%, Mn: 0.3-
The laser-welded steel structural member according to (1) above, which contains 2.0%, Al: 0.01 to 0.1%, and the balance being Fe and impurities.

(3) Steel is mass%, C: 0.003 to
0.07%, Si: 0.1-0.6%, Mn: 0.3-
2.0%, Al: 0.01 to 0.1% is contained, and further, the first group: Cr: 0.01 to 0.15%, Ni: 0.01
.About.0.15%, Cu: one or more of 0.1 to 0.35%, second group: V: 0.01 to 0.07%, Nb: 0.01 to
One or more of 0.06%, the third group: Ti: 0.003 to 0.015%, Ca: 0.0.
At least one of 0005 to 0.006%, including at least one group of, and the balance consisting of Fe and impurities,
The laser-welded steel structural member according to (1) above, wherein Mo in the impurities is 0.08% or less.

(4) Laser welding was performed under the conditions of laser output L (W (watt)), thermal efficiency η, and welding speed v (cm / sec), and after laser welding at a position at a depth of 1 mm from the base metal surface. The weighted average value of the Vickers hardness on the weld metal side in the region within 1.6 mm from the bond portion is 1 of the weighted average value of the Vickers hardness on the weld heat affected region in the region within 1.6 mm. Thickness h less than 6 times
(Cm) Steel material used for steel structural members, in mass%
C: 0.003 to 0.07%, Si: 0.1 to 0.
6%, Mn: 0.3-2.0%, Al: 0.01-0.
A steel material containing 1%, the balance consisting of Fe and impurities, Mo in the impurities being 0.08% or less, and the value represented by the following (2) being 100 or less.

{3.91 × 10 ⁶ × (Lη) ^−1.2 × (vh) ^1.2 } / (e ^{−24 (H P1 −0.2)} +1) (2), where , HP1 is a value represented by the following formula (3) with the element symbol being the content of the alloying element in mass%.

[0019]   HP1 = C + (Si / 50) + (Mn / 20) + (Mo / 30) (3).

(5) Laser welding was performed under the conditions of laser output L (W (watt)), thermal efficiency η, and welding speed v (cm / sec), and after laser welding at a position 1 mm deep from the base metal surface. The weighted average value of the Vickers hardness on the weld metal side in the region within 1.6 mm from the bond portion is 1 of the weighted average value of the Vickers hardness on the weld heat affected region in the region within 1.6 mm. Thickness h less than 6 times
(Cm) Steel material used for steel structural members, in mass%
C: 0.003 to 0.07%, Si: 0.1 to 0.
6%, Mn: 0.3-2.0%, Al: 0.01-0.
1%, further, the first group: Cr: 0.01 to 0.15%, Ni: 0.01
.About.0.15%, Cu: one or more of 0.1 to 0.35%, second group: V: 0.01 to 0.07%, Nb: 0.01 to
One or more of 0.06%, the third group: Ti: 0.003 to 0.015%, Ca: 0.0.
At least one of 0005 to 0.006%, including at least one group of, and the balance consisting of Fe and impurities,
A steel material in which Mo in impurities is 0.08% or less and the value represented by the following (4) is 100 or less.

{3.91 × 10 ⁶ × (Lη) ^−1.2 × (vh) ^1.2 } / (e ^{−24 (H P2-0.2)} +1) (4), Here, HP2 is a value represented by the following formula (5) with the element symbol being the content of the alloying element in mass%.

[0022]   HP2 = C + (Si / 50) + {(Mn + Cu + Cr) / 20} + (Ni / 6 0) + (V / 20) + Ca + (Mo / 30) ... (5).

The thermal efficiency η is an index showing how much energy irradiated by the laser is input into the steel material to be treated, and is a ratio to the output energy.

In order to obtain a steel structural member having a laser welded portion having excellent fatigue characteristics, the present inventors first investigated the fatigue crack initiation site in laser welding.

As a result, it was revealed that even in the case of laser welding, fatigue cracks occur at the weld toe (hereinafter simply referred to as "toe") as in the case of other conventional welding methods. Note that the fatigue crack initiation site in the welded joint becomes the toe, because the toe is the shape change portion and the strain is concentrated, and the shape factor and the boundary between the heat-affected zone of the base metal and the weld metal are the bonds. This is due to the material factor that the material is inhomogeneous because it is a part of the part.

Next, the present inventors examined the relationship between the welded joint hardness distribution and the fatigue characteristics in the case of laser welding. This is because laser welding has very little welding heat input compared to other conventional welding methods and the cooling rate immediately after welding is very fast.Therefore, when laser welding is performed, depending on the composition system of the steel due to the large cooling rate. The hardness of the weld metal is much higher than the hardness of the base metal, and the hardness distribution near the bond, which is the boundary between the weld metal and the heat-affected zone, is significantly different from other conventional welded joints. Therefore, we are trying to clarify the effect.

That is, a large number of various types of steel applicable as structural steels are melted on a laboratory scale to obtain steel sheets, and various welding methods including laser welding are actually applied to produce welded joints, The width of the test part is 4 by machining from this joint.
0 mm, parallel length of test part is 250 mm, grip part width is 70
A shaft fatigue test piece having a length of 700 mm and a total length of 700 mm was sampled, and a fatigue test was performed under a condition of a load ratio R of 0.1 with a sine waveform (sin waveform) under axial load limitation to evaluate the joint fatigue strength. .

As a result, the following matters were first clarified.

(A) In the case of the same material steel, in the conventional welding method such as carbon dioxide arc welding, when the tensile strength level of the steel sheet is changed, the base material fatigue strength of the steel sheet itself is improved according to the tensile strength. There is no difference in the fatigue strength of the steel sheets depending on the strength level of the steel sheet.

(B) A clear steel material dependency is recognized in the fatigue strength of the joint after laser welding.

Then, the phenomenon in which the fatigue strength of the welded joint depends on the steel material in the case of laser welding was examined in detail, focusing on the material or shape heterogeneity of the joint portion caused by the welding process. As a result, the following new findings were obtained.

(C) The hardness distribution in the vicinity of the bond portion, which is the boundary between the weld metal and the weld heat affected zone, greatly affects the stress distribution in the fatigue crack initiation portion, and the hardness distribution in the vicinity of the bond portion is It is affected by the chemical composition of the base steel.

(D) Fatigue crack initiation and fatigue strength are determined by the shape of the vicinity of the bond portion and the material of this portion.

Then, the hardness distribution (that is, strength distribution) of the laser welded portion is further reflected, and the stress-strain relationship is appropriately input according to the distance from the bond portion, that is, the tensile strength and hardness of the steel sheet are preliminarily set. Was prepared as a master curve, the tensile strength was estimated from the hardness value, and based on the tensile strength value, uniform elongation and yield stress were set and elasto-plastic finite element analysis was performed. . As a result, the following matters were found.

(E) When the change in hardness in the vicinity of the bond is large, the strain concentration in the sudden change in hardness is large, and fatigue cracks are easily generated. Therefore, the more uniform the hardness distribution, the higher the fatigue strength of the laser-welded structural member.

(F) The strain distribution at the fatigue crack initiation point is determined by the strength distribution (hardness distribution) in the vicinity of the fatigue crack initiation point in the joint, except for the shape factor.

(G) Strain concentration at the crack initiation portion that determines the fatigue crack initiation life is determined by the hardness distribution at a position at a depth of 1 mm from the surface of the base material that is not affected by various processes, decarburization, oxide film, etc. Correlates with. That is, a weighted average value of hardness with the distance from the bond portion as a parameter in the region within 1.6 mm of the weld metal side after laser welding at the above position and the base material side from the bond portion, that is, the weld heat affected zone. Side 1.6
The ratio of the hardness to the weighted average value with the distance from the bond portion as a parameter in the region within mm greatly influences the strain concentration of the crack generation portion, and when the ratio is 1.6 or less, the laser welded structural member Has good fatigue strength.

Then, further, at a position at a depth of 1 mm from the surface of the base metal, a weighted average value of Vickers hardness on the weld metal side in a region within a distance of 1.6 mm from the bond portion after laser welding is: Study to identify the laser welding conditions and the combination of steel materials for obtaining a welded structural member having 1.6 times or less the weighted average value of the Vickers hardness on the weld heat affected zone in the region of 1.6 mm or less. went.

As a result, the following matters were found.

(H) HVmax is the maximum Vickers hardness of the welded portion, and HVBM is the Vickers hardness of the base metal.
max-HVBM) / (width of welding heat affected zone in mm) "
By combining laser welding conditions and steel material components so that the value of is 100 or less, the distance from the bond portion after laser welding is within 1.6 mm at a position at a depth of 1 mm from the base metal surface. The weighted average value of the Vickers hardness on the weld metal side can stably and reliably achieve 1.6 times or less of the weighted average value of the Vickers hardness on the weld heat affected zone in the region within 1.6 mm. it can.
In the following description, the "width of the welding heat affected zone" may be referred to as the "HAZ width".

The present invention has been completed based on the above findings.

[0042]

BEST MODE FOR CARRYING OUT THE INVENTION Each requirement of the present invention will be described in detail below. In addition, "%" of the content of a chemical component means "mass%".

In the present invention, the hardness distribution in the vicinity of the bond portion after laser welding at a position at a depth of 1 mm from the surface of the base material is defined.

This is because the hardness value of the extreme surface layer of the steel material constituting the steel structural member is complicatedly affected by various processes, decarburization, oxide film, etc., but it is at a depth of 1 mm from the surface of the base material. This is because the hardness value at the position is not affected by the above-mentioned various processes, decarburization, oxide film and the like.

At a position 1 mm deep from the surface of the base metal, the weighted average value of Vickers hardness on the weld metal side in the region within 1.6 mm from the bond portion after laser welding is the weighted average value. When it exceeds 1.6 times the weighted average value of the Vickers hardness on the weld heat affected zone in the region of 6 mm or less, strain concentration occurs and fatigue cracking is likely to occur remarkably. Therefore, the weighted average value of the Vickers hardness on the weld metal side is defined as 1.6 times or less the weighted average value of the Vickers hardness on the weld heat affected zone. Here, when a weighted average value of Vickers hardness in a region within a distance of 1.6 mm from the bond portion is tested with a test force of 9.807 N or less, a position where the distance from the bond portion is D (mm). As mentioned above, the Vickers hardness at HV1 _D is the value expressed by the following equation (1).
Each coefficient in the equation is determined in consideration of the degree of influence on the strain concentration coefficient in the fatigue crack occurrence part.

Weighted average value = 0.4 × HV1 _0.4 + 0.3 × HV1 _0.8 + 0.2 × HV1 _1.2 + 0.1 × HV1 _1.6 (1).

In the present invention, the hardness measurement interval is 0.4 mm. This is because the hardness distribution by elasto-plastic finite element analysis, that is, the effect of the material strength distribution on the strain concentration at the fatigue crack initiation part was investigated in detail, and the measurement interval was set to 0. It is based on the fact that 4 mm is found to be sufficient. The hardness measurement may be performed at intervals of less than 0.4 mm, but it takes time and labor.

The test force in the Vickers hardness test was set to 9.
The test force of 807 N or less is larger than this.
When performing hardness measurement at 0.4 mm intervals, JIS Z 22
This is because the Vickers hardness test of No. 44-the case where the positional condition of the depression defined in the test method is not satisfied may occur. The test force may be any value as long as it is 9.807 N or less. For example, the test force of 0.09807 to 9.807 N described in Table 1 of JIS Z 2244 is preferable.

Next, the chemical composition of the material steel of the laser-welded steel structural member of the present invention is as follows.

C: 0.003 to 0.07% C is preferably contained in an amount of 0.003% or more in order to secure the strength of the steel structure. On the other hand, its content is 0.07%
If the hardness exceeds the above, the hardness of the weld metal becomes extremely high due to the rapid cooling after laser welding, which is characterized by a small heat input, and as a result, the above hardness regulations cannot be satisfied under normal laser welding conditions. There is. Therefore, the C content is preferably 0.003 to 0.07%. Note that C
The upper limit of the content is more preferably 0.05%.

Si: 0.1 to 0.6% Since Si has a deoxidizing effect, it is preferable that the Si content be 0.1% or more. On the other hand, if the content exceeds 0.6%, the fracture toughness value may decrease. Therefore, S
The content of i is preferably 0.1 to 0.6%. In addition,
The Si content is more preferably 0.25 to 0.5%.

Mn: 0.3 to 2.0% Mn is preferably contained in an amount of 0.3% or more in order to secure the strength of the steel structure and prevent solidification cracking during laser welding. . On the other hand, if the content exceeds 2.0%, laser welding may become difficult. Therefore, Mn
The content of is preferably 0.3 to 2.0%. In addition, M
More preferably, the n content is 0.5 to 1.8%.

Al: 0.01 to 0.1% Since Al has a deoxidizing effect, it is preferable that the content of Al is 0.01% or more. On the other hand, if the content exceeds 0.1%, the fracture toughness value may decrease or it may be difficult to secure the cleanliness of steel. Therefore, the content of Al is 0.01 to
It is better to set it to 0.1%. The upper limit of the Al content is more preferably 0.05%. A in the present invention
1 content means sol. Indicates the amount of Al (acid-soluble Al).

The material steel of the laser-welded steel structural member of the present invention may further contain one or more of the first to third groups in addition to the above-mentioned respective constituent elements. The effects and desirable contents of these alloying elements are as follows.

Cr: 0.01 to 0.15%, Ni: 0.
01 to 0.15%, Cu: 0.1 to 0.35% Cr, Ni, and Cu have an action of improving the toughness at a low temperature even in a small amount, and also have an action of improving the toughness of the weld heat affected zone. is there. Therefore, it may be contained for the purpose of ensuring toughness at a low temperature and toughness at a welded joint, but if the content of any of the above three elements is less than 0.01%, the above effect is difficult to obtain. On the other hand, if the content of Cr exceeds 0.15%, the content of Ni exceeds 0.15%, and the content of Cu exceeds 0.35%, not only the above effect is saturated but also the toughness deteriorates. It may be invited. Therefore, when one or more of Cr, Ni, and Cu are further added in addition to the above C to Al, the content of Cr is 0.01 to 0.15%,
The Ni content is preferably 0.01 to 0.15% and the Cu content is preferably 0.1 to 0.35%.

V: 0.01 to 0.07%, Nb: 0.0
1 to 0.06% Since V and Nb have the action of enhancing toughness and strength, these elements may be contained if it is desired to secure toughness and strength in the steel material. 0.01%
If it is less than the above, the above effect is difficult to obtain. On the other hand, if the content of V exceeds 0.07% and the content of Nb exceeds 0.06%, the toughness of the weld heat affected zone may deteriorate. Therefore,
When one or more kinds of V and Nb are added in addition to the above C to Al, the content of V is 0.01 to 0.0.
It is preferable that the content of Nb is 7% and the content of Nb is 0.01 to 0.06%.

Ti: 0.003 to 0.015%, Ca:
0.0005 to 0.006% Since Ti and Ca have the effect of refining the structure of the weld heat affected zone, even if they are contained for the purpose of refining the structure of the weld heat affected zone to enhance mechanical properties. However, if the Ti content is less than 0.003% and the Ca content is less than 0.0005%, it is difficult to obtain the effect of refining the structure of the weld heat affected zone. on the other hand,
If the Ti content exceeds 0.015%, the toughness deteriorates, and if the Ca content exceeds 0.006%, CaO increases.
The content of inclusions becomes too large, and the cleanliness of the steel remarkably decreases. Therefore, in addition to the above C to Al, T
When one or more of i and Ca are added, the Ti content is preferably 0.003 to 0.015% and the Ca content is preferably 0.0005 to 0.006%.

The content of Mo as an impurity element is preferably set as follows.

Mo: 0.08% or less Mo promotes the formation of island martensite structure which adversely affects fracture toughness such as COD (crack opening displacement) characteristics,
In particular, if its content exceeds 0.08%, the proportion of island martensite structure may increase and the fracture toughness of the welded part may be significantly reduced. Therefore, the content of Mo as an impurity is preferably 0.08% or less.

Laser welding is performed under the conditions of the output L (W (watt)), thermal efficiency η, and welding speed v (cm / sec) of the present invention, and laser welding is performed at a position at a depth of 1 mm from the base metal surface. The weighted average value of the Vickers hardness of the weld metal side in the area within 1.6 mm from the subsequent bond portion is the weighted average value of the Vickers hardness of the weld heat affected zone in the area within 1.6 mm. Thickness h less than 1.6 times
The steel material used for the steel structural member of (cm) has a chemical composition in the range of C to Mo described above, and the above (2) or
It is preferable that the value represented by (4) satisfies 100 or less.

That is, the steel material used for the steel structural member having the above thickness h (cm) is C: 0.003 to 0.0 in mass%.
7%, Si: 0.1 to 0.6%, Mn: 0.3 to 2.0
%, Al: 0.01 to 0.1%, the balance consisting of Fe and impurities, and when Mo in the impurities is 0.08% or less, the value represented by (2) above is 100 or less. It is better to meet.

The steel material used for the steel structural member having the above thickness h (cm) is, in mass%, C: 0.003 to 0.07%,
Si: 0.1-0.6%, Mn: 0.3-2.0%, A
1: 0.01-0.1%, and further 1st group: Cr: 0.01-0.15%, Ni: 0.01
.About.0.15%, Cu: one or more of 0.1 to 0.35%, second group: V: 0.01 to 0.07%, Nb: 0.01 to
One or more of 0.06%, the third group: Ti: 0.003 to 0.015%, Ca: 0.0.
At least one of 0005 to 0.006%, including at least one group of, and the balance consisting of Fe and impurities,
If Mo in the impurities is 0.08% or less, the above (4)
It is preferable that the value represented by is 100 or less.

Hereinafter, the value represented by the above (2) or (4) is 10
The reason why it is preferable to satisfy 0 or less will be described.

In FIG. 2, the vertical axis represents the ratio (hardness ratio) of the weighted average value of the Vickers hardness of the weld metal side to the weighted average value of the Vickers hardness of the weld heat affected zone, and the horizontal axis represents “(HVmax − H
VBM) / (HAZ width in mm) ”is a diagram arranged by taking measured values. From this figure, "(HVmax-H
If the value of (VBM) / (HAZ width in mm) is 100 or less, the distance from the bond portion after laser welding is 1.6 mm or less at a position at a depth of 1 mm from the base metal surface. The condition that the weighted average value of the Vickers hardness on the weld metal side is 1.6 times or less than the weighted average value of the Vickers hardness on the weld heat affected zone in the region within 1.6 mm can be stably and reliably achieved. Therefore, it is clear that a welded structural member having excellent fatigue strength can be realized.

The above "(HVmax-HVBM) / (m
value of HAZ width in units of m) (that is, HVmax, HV
The specific value of the HAZ width in BM and mm) can be measured by
The measurement may be performed using a small laser-welded sample, but it requires a lot of time and labor. Therefore, it is extremely important to estimate the above value without performing actual laser welding.

Therefore, as a result of various investigations by the present inventors, the laser power L (W (watt)) and the thermal efficiency η as the laser welding conditions can be obtained by performing processing in accordance with the following procedures (A) to (E). If the welding speed v (cm / sec), the thickness h (cm) of the steel structural member, and the chemical composition of the steel material used for the member are known, “(HVma
x-HVBM) / (HAZ width in mm) ".

(A) As laser welding conditions, laser output L
(W), thermal efficiency η, welding speed v (cm / sec), and thickness h (cm) of the steel material to be treated are set.

(B) By assuming a two-dimensional heat flow,
The cooling time (second) and HAZ width (mm) in the temperature range of 00 to 500 ° C. are derived from the theoretical solution.

(C) From the cooling time (second) in the temperature range of 800 to 500 ° C. in (B) above and the chemical composition of the steel material to be treated (that is, the chemical composition of the base material), 800 to 500 ° C. (HVmax-HVB in the cooling time of 1 second in the temperature range of
Estimate the value of M).

(D) Using the estimated value of (HVmax-HVBM) in the cooling time in the temperature range of 800 to 500 ° C. for 1 second, (HVma) in the cooling time estimated from the welding conditions is used.
x-HVBM) is derived.

(E) The above (B) obtained by assuming a two-dimensional heat flow
The value of "(HVmax-HVBM) / (HAZ width in mm)" is calculated from the HAZ width of.

The above (A) to (E) will be described in detail below.

Procedure (A): Here, as laser welding conditions, laser output L (W), thermal efficiency η, welding speed v (c
m / sec) and the thickness h (cm) of the steel material to be treated.

Usually, in laser welding, a combination of laser output and welding speed is used so as to penetrate the thickness h (cm) of the steel material to be treated. For a specific combination, for example, those described in the explanation of Kitani (Pressure Technology, Vol. 36, No. 6, 1998, 460 to 465) may be used.

As described above, the thermal efficiency η is an index showing how much energy irradiated by the laser is put into the steel material which is the material to be treated, and indicates the ratio to the output energy. This thermal efficiency is generally affected by the shielding gas, and the welding and joining handbook (editor: Japan Welding Society,
(Publisher: Maruzen Co., Ltd., Date of issue: September 30, 1990)
Also show that the thermal efficiency η may be 60%. Here, the measured value of the cooling time (second) in the temperature range of 800 to 500 ° C and the cooling time in the temperature range of 800 to 500 ° C using the thermocouple by the two-dimensional heat flow theoretical analysis of (B) described later. From the correspondence between the two, the thermal efficiency η should be 0.5 when the focus amount is 2.5 mm or more, and when the focus amount is less than 2.5 mm,
It was found that 0.25 should be used as the thermal efficiency η. The "focus amount" refers to the distance between the focused point and the surface of the steel material that is the material to be processed when the laser light is focused into a conical shape.

Procedure (B): When performing energy beam welding such as laser welding in welding metallurgy (author: Fukuhisa Matsuda, publisher: Nikkan Kogyo Shimbun, date of issue: August 20, 1975) It has been clarified that the temperature distribution of can be accurately approximated by a two-dimensional heat flow. That is,
Assuming a two-dimensional heat flow, the temperature θ
The cooling rate CR "θ" (° C / sec) at (° C)
From the equation (7), the cooling time CT “θ1 to θ2” (seconds) from the temperature θ1 (° C.) to the temperature θ2 (° C.) is given.

In the above "welding metallurgy", λ is the thermal conductivity (cal / (cm · ° C. · second)) and c is the specific heat (cal /
(G · ° C.), ρ is density (g / cm ³ ), v is welding speed (cm / min), h is the thickness of the steel material (c)
m), L is the laser output (W), η is the thermal efficiency, Q is the heat input and corresponds to Lη, θ is the temperature (° C) for which the cooling rate is being sought, and θ1 and θ2 are the cooling times, respectively. The following equations are shown with the high temperature side (° C.) and the low temperature side (° C.) existing, and θ 0 as the initial temperature (or environmental temperature, (° C.)).

CR “θ” = 2πλcρ (vh / Q) ² (θ−θ0) ³ (6), CT “θ1 to θ2” = (4πλcρ) ⁻¹ (Q / vh) ² {(θ2− θ0) ⁻² − (θ1 −θ0) ⁻² } (7).

From the above equation (7), the cooling time CT (second) under arbitrary welding conditions can be easily calculated. Hashimoto et al., Based on this theoretical formula, in a paper (Journal of Japan Welding Society, Vol. 33, No. 10 (1964), pages 918 to 927) 800 to 50 in a welding heat affected zone at the time of electron beam welding.
Cooling time CT in the temperature range of 0 ° C "800-500"
For (seconds), the following equation (8) is proposed.

CT “800 to 500” = 3.8 × 10 ⁻² × {(EI · η) / (vh)} ² × {(500−θ0) ⁻² − (800−θ0) ⁻² } ·· (8), where EI indicates the output (W) of the electron beam, and other symbols are the same as those already described.

The heat conduction behavior of laser welding targeted by the present invention is very similar to the heat conduction behavior of electron beam welding described above. Therefore, for laser welding, see (8) above.
It may be considered that Eq. (9) similar to Eq.

CT “800 to 500” = 3.8 × 10 ⁻² × {(Lη) / (vh)} ² × {(500−θ0) ⁻² − (800−θ0) ⁻² } ... ( 9).

Here, CT “800 to 500” is the cooling time (second) in the temperature range of 800 to 500 ° C. in the welding heat affected zone at the time of laser welding. If the initial temperature (or environmental temperature) θ0 is 20 ° C, the following equation (10) is obtained. As mentioned above, L indicates the laser output (W).

CT “800 to 500” = 1.025 × 10 ⁻⁷ × {(Lη) / (vh)} ² (10).

From the equation (10), the focus amount is 2.5 m.
When the focus amount is less than 2.5 mm, the thermal efficiency η is set to 0.5 when m or more, and when the focus amount is less than 2.5 mm, the thermal efficiency η is set to 0.25 to cool in the temperature range of 800 to 500 ° C. It is possible to estimate the time (seconds).

On the other hand, the HAZ width in cm can be obtained from the isothermal line of a certain maximum ultimate temperature (° C.), and the HAZ width in mm is H in cm.
It is obtained by multiplying the AZ width by 10.

That is, when a two-dimensional heat flow is assumed, the distance y “θmax” (cm) between the isothermal line and the melting boundary line for a specific maximum ultimate temperature θmax (° C.) is the above-mentioned welding metallurgy. According to the study, it is expressed by the following equation (11).

(Θmax−θ0) ⁻¹ = {4.13cρh · y “θmax” / (Q / v)} + (θm −θ0) ⁻¹ (11), where θmax is the maximum temperature reached (° C.) and θm indicate the melting point temperature (° C.) of the steel material to be treated, and other symbols are the same as those already described.

By solving the above equation (11) for y "θmax" (cm), equation (12) is obtained. From this equation (12), the distance from the melting boundary line at a position where a certain set temperature θmax (° C) is reached can be obtained.

Yθ “max” = {(θmax−θ0) ⁻¹ − (θm−θ0) ⁻¹ } (Q / v) / (4.13cρh) (12).

For example, when estimating the HAZ width in mm, the regions where the maximum attainable temperature θmax is sandwiched between 1350 ° C. and 750 ° C. are the heat affected zone of the welding (that is, HA
Z), the melting point temperature θm is 1530 ° C., θ0 is 20 ° C.,
cρ is 3.9 cal / (c as described in the above-mentioned welding metallurgy.
m ³ · ℃), first, HAZ in cm unit
The width can be calculated using the following equation (13). Then, the HAZ width in cm unit calculated using the equation (13) is 1
The HAZ width in cm can be obtained by multiplying by 0.

HAZ width = y “750” −y “1350” = [{(730) ⁻¹ − (1510) ⁻¹ } − {(1330) ⁻¹ − (1510) ⁻¹ }] (Q / v) /(16.11h)=6.180×10 ⁻⁴ (Q / v) / (16.11h) = 3.836 × 10 ⁻⁵ Q / (vh) = 3.836 × 10 ⁻⁵ (Lη) / (Vh) (13).

Based on the procedures (A) and (B) described above,
The laser output L is 5 to 30 kW and the thermal efficiency η is 0.5 and 0.
6. The welding speed v is 50 to 250 cm / min, that is, 0.
800 to 500 ° C. in the case of 83 to 4.2 cm / sec and the thickness h of the steel material to be treated of 0.2 to 1.7 cm
Tables 1 to 5 show examples of the results of calculation of the cooling time (second) and the HAZ width (mm) in the temperature range of.

The HAZ width obtained by calculation is 0.8 m.
In the case of m or less, it was judged that the fusion zone did not penetrate in the thickness direction of the steel material, and the analysis result was not shown in the table. Further, when penetration welding can be performed with a low laser output, irradiation with a high output laser cannot be considered in actual construction. Therefore, for the laser output exceeding 5 kW, only the analysis result of the condition that welding cannot be performed below the output is shown.

[0095]

[Table 1]

[0096]

[Table 2]

[0097]

[Table 3]

[0098]

[Table 4]

[0099]

[Table 5]

The following items (1) to (3) are apparent from Tables 1 to 5 showing examples of the results calculated based on the above procedures (A) and (B).

Under practical welding conditions, the HAZ width is at most about 1.8 mm.

Under practical welding conditions, 800 to 50
The cooling time in the temperature range of 0 ° C. is approximately 1 to 2 seconds, and
It is sufficient to cover the range of 2-5 seconds.

As the laser output increases, the weldable plate thickness (that is, the thickness of the steel material to be treated) and the welding speed increase.

Depending on thermal efficiency, HAZ width, 800-5
Both the cooling time in the temperature range of 00 ° C. changes.

Procedure (C): Next, (HVmax − at an arbitrary cooling time t (sec) in the temperature range of 800 to 500 ° C.
In order to derive the value of (HVBM) (hereinafter simply referred to as (HVmax-HVBM)), the cooling time (H
Estimate the value of (Vmax-HVBM) (sometimes referred to as (HVmax "1" -HVBM)).

The inventors conducted a preliminary experiment to find out the laser output L (W (watt)) and the thickness h of the steel material to be treated.
(Mm) and welding speed v (cm / sec) in various combinations, a condition was found that the cooling time in the temperature range of 800 to 500 ° C was 1 second, and based on this, laser welding was performed on various steel materials. Then, the value of (HVmax "1" -HVBM) was measured.

As a result, the value of (HVmax-HVBM) at the cooling time of 1 second in the temperature range of 800 to 500 ° C, that is, (HVmax "1" -HVBM) depends on the chemical composition of the steel material to be treated. It turned out to be uniquely determined. Further, the cooling time in the temperature range of 800 to 500 ° C is 1
The value of (HVmax "1" -HVBM) in seconds is (1
It has been newly found that the equation (4) can be approximated with a high correlation.

(HVmax “1” −HVBM) “800 to 500” = 300 / (e ^{−24 (HP −0.2)} +1) (14).

Here, HP is the above-mentioned formula (5) (or (3) where the element symbol is the content of the alloying element in mass%.
Expression).

Procedure (D): Next, (HVmax − at an arbitrary cooling time t (sec) in the temperature range of 800 to 500 ° C.
HVBM) value is derived.

As already described in the procedure (B), under practical welding conditions, the cooling time in the temperature range of 800 to 500 ° C. is approximately 1 to 2 seconds, and 0.2 to 5 seconds. It is enough to cover the range.

The results of experiments conducted by the present inventors by varying the laser output L (W (watt)), the thickness h (mm) of the steel material to be treated, and the welding speed v (cm / sec). , 80
When the cooling time t in the temperature range of 0 to 500 ° C is in the range of 0.2 to 5 seconds, the horizontal axis represents the cooling time on a logarithmic scale, and the vertical axis represents the value of (HVmax-HVBM). All the plots of the experimental results can be accurately approximated by a straight line descending to the right, and it is clear that the straight line has the same slope of -10 regardless of the chemical composition of the steel material to be treated. Became.

As a result of the above, (HVmax "1" -HVBM)
From the relationship between the cooling time and hardness described above, an arbitrary cooling time t in the temperature range of 800 to 500 ° C.
(Second) (where t is 0.2 to 5), (HVmax
The value of −HVBM) can be accurately estimated.

The value of (HVmax-HVBM) at an arbitrary cooling time t (second) within the cooling time range of 0.2 to 5 seconds thus obtained is shown below as an empirical formula.

[0115] (HVmax -HVBM) = (HVmax "1" ^{-HVBM) t -0.1 ··· (15)} .

Combining the equations (14) and (15), the element symbol is the content of the alloying element in% by mass, which has already been described.
HP corresponding to formula (5) (or formula (3)) and 800-500 ° C
Cooling time t (second) in the temperature range of
(HVmax-HVBM) can be expressed by the following equation (16).

(HVmax-HVBM) = {300 / (e- ^{24 (HP-0.2)} +1)} t ^{- 0.1} (16).

Procedure (E): As described above, if the two-dimensional heat flow is assumed, the HAZ width in cm unit shown in equation (13) can be accurately estimated. Therefore, equation (16) and (13) From the formula, “(HVmax-HVBM) / (HAZ in mm unit
The value of “width)” can be derived by the following equation (17).

(HVmax−HVBM) / (HAZ width in mm) = {3.91 × 10 ⁶ × (Lη) ^−1.2 × (vh) ^1.2 } / (e- ^{24 ( HP-0 2)} +1) ... (17).

FIG. 3 shows the relationship between the measured value of (HVmax-HVBM) / (HAZ width in mm) and the calculated value obtained by the equation (17). From this figure, it is clear that the two agree very well in the present condition range.

As a result, for example, when the conditions of laser welding are determined, the composition of the steel material can be determined from the required joint fatigue strength, and conversely, it should be secured based on the steel material to be applied. It is also possible to select laser welding conditions for fatigue strength.

Next, the present invention will be described in more detail by way of examples.

[0123]

EXAMPLES Steels having the chemical compositions shown in Tables 6 to 8 were melted in a test furnace by a usual method.

[0124]

[Table 6]

[0125]

[Table 7]

[0126]

[Table 8]

Next, these steels were made into steel pieces having a thickness of 80 to 120 mm by ordinary hot forging, and then 900 to 1
After heating to 100 ° C, hot rolling, plate thickness 18-12m
finished to m.

The steel plate obtained by hot rolling was subjected to butt laser welding at room temperature to produce a joint as it was, or after flat-face grinding on both sides to a plate thickness of 16 to 3 mm.

That is, a carbon dioxide gas laser (CO ₂ laser) or a yttrium aluminum garnet laser was produced by processing an I-shaped groove on the above-mentioned steel plate having a thickness of 3 to 18 mm and pressing it completely with no gap. (YA
(G laser) was irradiated from one side to produce a joint. Tables 9 and 10 show the details of the laser welding conditions. As described above, the “focus amount” in these tables means the distance between the focused point and the surface of the steel sheet when the laser beam is focused into a conical shape. “Presence or absence of addition of filler” means whether or not a welding material, which is a co-material of the base steel plate, is inserted into the laser irradiation portion in the formation of the welding bead. In addition, from the welded joints having the respective joint numbers in Tables 9 and 10, at least six fatigue test pieces described below were produced for each joint number.

[0130]

[Table 9]

[0131]

[Table 10]

From the butt-welded joint produced as described above, a butt-axial fatigue test piece was sampled by machining so that the longitudinal direction of the test piece was perpendicular to the welding direction. The dimensions of the axial fatigue test piece are 18 to 3 mm with the plate thickness being the plate thickness of the steel plate, and the width of the test portion is 40 mm.
mm, the width of the grip portion is 70 mm, and the length is 700 mm.

The fatigue test was carried out in a room temperature atmosphere using a closed loop electrohydraulic fatigue tester, that is, by mounting the above axial force fatigue test piece on a tester with a dynamic load capacity of ± 500 kN by a hydraulic chuck. , And the load ratio R was 0.1 sine wave (sin wave). At this time, the fatigue rupture life was measured using the load range (that is, the maximum load-minimum load) ΔP as an experimental parameter. The nominal stress range Δσ was obtained from the load range ΔP and the nominal cross-sectional area that does not include extra, and an SN curve was created from the fatigue fracture life of each fatigue test piece for each joint number.

In the above-mentioned test, the repetition rate is set to 3 to 8 Hz according to the test load and the displacement amount, and the fatigue cracks generated on the surface of the test piece propagate while forming a fracture surface to control the load of the tester. The time when it became difficult was defined as the fatigue fracture life.

Although the fatigue test cutoff condition based on the number of repetitions was set to 3 × 10 ⁶ times as a standard, the test was continued on some test specimens without being cut off at the above number of repetitions.

The SN curve created as described above was almost linear, and no clear bending was observed.
The fatigue strength Δσw of the joint was evaluated by the time strength when the number of repetitions was 2 × 10 ⁶ .

The fatigue strength Δσw for each joint number is shown in Tables 11 to 11.
13 shows. In addition, in all of the specimens, the fatigue crack generation position was the weld toe.

[0138]

[Table 11]

[0139]

[Table 12]

[0140]

[Table 13]

Regarding the welded joints having the respective joint numbers shown in Tables 9 and 10, at a position at a depth of 1 mm from the surface of the base metal,
From the bond portion after laser welding to the weld metal side at a 0.4 mm pitch to 1.6 mm, and from the bond portion after laser welding to the welding heat affected zone side at a 0.4 mm pitch to 1.6 mm, test force 9 Vickers hardness HV1 _D (D
= 0.4, 0.8, 1.2 and 1.6) were measured.

Table 11 shows the HV1 _D measurement results, the weighted average value represented by the equation (1), and the ratio of the weighted average value on the weld metal side to the weighted average value on the welding heat affected zone. ~
13 is also shown. FIG. 1 shows an example of the influence of the ratio of the weighted average value on the weld metal side and the weighted average value on the weld heat affected zone side on the fatigue strength Δσw for each joint number. In Tables 11 to 13 and FIG. 1, the ratio of the weighted average value on the weld metal side to the weighted average value on the welding heat affected zone side is indicated as “hardness ratio”. Further, in Tables 11 to 13, the weighted average value is indicated as “AV”.

From Tables 11 to 13 and FIG. 1, there is an extremely strong correlation between the "hardness ratio" and the fatigue strength Δσw of the joint, and the "hardness ratio" is 1.6 or less. 1-9, 11-14, 18, 20, 22, 24, 27,
In the case of 28, 30 to 32, 36, 37 and 40, Δσ
A large joint fatigue strength of more than 200 MPa is obtained with w.

With respect to the base materials with the respective joint numbers in Tables 9 and 10, the tensile properties, toughness, cleanliness and weldability in room temperature atmosphere were also investigated.

For the tensile test piece, the No. 1A test piece described in JIS Z 2201 was used for the base material steel sheet having a plate thickness of more than 6 mm, and the same No. 5 test described in JIS Z 2201 was used for the base material steel sheet having a plate thickness of 6 mm or less.
No. 4 test piece was sampled and the yield strength in room temperature atmosphere was measured.

From the center of the base material having a plate thickness of 10 to 18 mm, a full-size Charpy V-notch test piece having a width of 10 mm described in JIS Z 2202 was similarly prepared.
The width described in JIS Z 2202 is 5 m from the center of the base material of m and the center of the base material with plate thickness of 3 mm and 4 mm, respectively.
A subsize Charpy V-notch test piece with m and a width of 2.5 mm was sampled, an impact test was performed, and a fracture surface transition temperature was measured to investigate the toughness.

Furthermore, for a base material having a plate thickness of 18 to 3 mm,
A block-shaped test piece was sampled from the center of the plate thickness, and a microscopic observation test of non-metallic inclusions was performed.

Further, each base material was laser-welded at room temperature without being preheated, and the weldability was examined by whether or not weld cracking occurred.

On the other hand, with respect to the welded joints having the respective joint numbers shown in Tables 9 and 10, the maximum hardness of the welded portion, the toughness of the welded joint, and the position 1 mm away from the weld fusion line (fusion line) on the base metal side (so-called " HAZ 1 mm ") was also investigated.

First, a short bead was placed on a block-shaped steel plate, the Vickers hardness HV was measured with a test force of 9.807 N, and the maximum weld hardness was investigated.

Next, the evaluation of the toughness of the welded joint was carried out by "HAZ1m
The test piece was processed so that "m" was the bottom of the V notch. Regarding the size of the test piece, a full-size Charpy V-notch test piece with a width of 10 mm and a width of 5 mm and a width of 2.5 mm described in JIS Z 2202, which is the same as the case of the base material, is used according to the thickness of the base material. The sub-size Charpy V-notch test piece was subjected to the above impact test, and the fracture surface transition temperature was measured to investigate the toughness.

Further, a block-shaped sample was cut out from the steel plate, embedded in a resin, and mirror-polished by a usual method, and then 3
% Corrosion with nital and microscopic observation were performed to measure the ferrite crystal grain size.

In Tables 14 and 15, the above-mentioned test results are summarized and shown by reference numerals.

"A", "○", and "X" in the "strength" column of the base materials in Tables 14 and 15 are 35 at yield strengths of 350 MPa or more and 294 MPa or more in room temperature atmosphere.
It is less than 0 MPa and less than 294 MPa.

“◎”, “○” in the “Toughness” column of the base material,
“X” indicates that the 50% fracture surface transition temperature was less than −30 ° C., less than −30 ° C. and less than 0 ° C., and 0 ° C. or more, respectively.

“○” and “x” in the “cleanliness” column of the base material are
It was evaluated based on the "microscopic examination method according to the standard drawing" described in JIS G 0555, and all the inclusions of A type, B type, C type and D type are ASTM standard diagrams of Annex A of JIS G 0555 mentioned above. In the case of the number of 1.5 or less, "○" was given, and in other cases, "x" was given.

[0157] "O" and "x" in the "weldability" column of the base material indicate that no weld cracks occurred and weld cracks occurred, respectively.

On the other hand, “X” and “◯” in the “maximum hardness” column of the welded joints in Tables 14 and 15 indicate that there are 400 or more parts in Vickers hardness (HV).
It was shown that V was less than 400.

“O” and “x” in the “toughness” column of the welded joints indicate that the 50% fracture surface transition temperatures were below 0 ° C. and above 0 ° C., respectively.

"A" and "○" in the "particle size" column of the welded joint indicate that the ferrite average crystal grain size is 5 μm or less and 5 μm, respectively.
It shows that it was more than m and 25 μm or less.

[0161]

[Table 14]

[0162]

[Table 15]

From Tables 14 and 15, it can be seen that the properties of the base material and the welded joint are influenced by the chemical composition. That is, the above-mentioned elements of the first group, Cr, Ni, and Cu, improve the toughness of the base metal and the toughness of the weld zone. However, when the content is excessive, the toughness is rather decreased. Further, V and Nb, which are the elements of the second group described above, can enhance the toughness and strength, but it is recognized that when they are contained in excess, the toughness of the welded portion is lowered. Further, Ti and Ca, which are the elements of the third group described above, can make the metal structure of the HAZ fine, but if Ti is contained excessively, the toughness decreases and C
It is recognized that if a is contained excessively, the cleanliness of the steel decreases.

[0164]

According to the present invention, since a steel structural member having a laser welded portion having excellent fatigue strength characteristics and a steel material used for the steel structural member can be obtained, it is possible to improve the function such as reducing the weight of the entire structure. Can be promoted.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of the influence of the ratio of the weighted average value on the weld metal side and the weighted average value on the weld heat affected zone side on the fatigue strength Δσw of the joint investigated in the examples.

[Fig. 2] Ratio of the weighted average value of Vickers hardness on the weld metal side to the weighted average value of Vickers hardness on the weld heat affected zone, and (HVmax-HVBM) / (HAZ width in mm)
It is a figure which shows the correlation with the value of.

FIG. 3 (HVmax-HVBM) / (HAZ in mm)
It is a figure which shows the relationship between the measured value of (width) and the calculated value calculated by (17) formula.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Tomoya Fujiwara             4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture             Sumitomo Metal Industries, Ltd. (72) Inventor Ichiro Seta             4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture             Sumitomo Metal Industries, Ltd. (72) Inventor Tomoya Kawabata             Sumitomo Metal Industries, No. 3, Hikari, Oshima, Kashima City, Ibaraki Prefecture             Kashima Steel Works Co., Ltd. F-term (reference) 4E068 DB01

Claims (5)

[Claims] 1. A position at a depth of 1 mm from the surface of the base material,
The weighted average value of the Vickers hardness on the weld metal side in the region within 1.6 mm from the bond portion after laser welding is the weighted average of the Vickers hardness on the weld heat affected region in the region within 1.6 mm. A laser welded steel structural member having a value of 1.6 times or less. Here, the weighted average value of Vickers hardness in the region where the distance from the bond portion is within 1.6 mm means that the distance from the bond portion is D (mm) when tested with a test force of 9.807 N or less. The Vickers hardness at the position is HV1 _D , and indicates the value expressed by the following equation (1). Weighted average value = 0.4 × HV1 _0.4 + 0.3 × HV1 _0.8 + 0.2 × HV1 _1.2 + 0.1 × HV1 _1.6 (1). 2. Steel in mass%, C: 0.003 to 0.07
%, Si: 0.1 to 0.6%, Mn: 0.3 to 2.0
%, Al: 0.01 to 0.1%, and the balance being Fe and impurities. The laser-welded steel structural member according to claim 1. 3. Steel in mass%, C: 0.003 to 0.07
%, Si: 0.1 to 0.6%, Mn: 0.3 to 2.0
%, Al: 0.01 to 0.1%, and further, the first group: Cr: 0.01 to 0.15%, Ni: 0.01
.About.0.15%, Cu: one or more of 0.1 to 0.35%, second group: V: 0.01 to 0.07%, Nb: 0.01 to
One or more of 0.06%, the third group: Ti: 0.003 to 0.015%, Ca: 0.0.
The laser welding according to claim 1, further comprising at least one group of 0005 to 0.006%, the balance being Fe and impurities, and Mo in the impurities being 0.08% or less. Steel structural members. 4. Laser welding is performed under the conditions of laser output L (W (watt)), thermal efficiency η, and welding speed v (cm / sec).
At a position at a depth of 1 mm from the surface of the base metal, a weighted average value of Vickers hardness on the weld metal side in a region within a distance of 1.6 mm from the bond portion after laser welding is within the region of 1.6 mm. Thickness h which is 1.6 times or less of the weighted average value of Vickers hardness on the welding heat affected zone side in
(Cm) Steel material used for steel structural members, in mass%
C: 0.003 to 0.07%, Si: 0.1 to 0.
6%, Mn: 0.3-2.0%, Al: 0.01-0.
A steel material containing 1%, the balance consisting of Fe and impurities, Mo in the impurities being 0.08% or less, and the value represented by the following (2) being 100 or less. {3.91 × 10 ⁶ × (Lη) ^−1.2 × (vh) ^1.2 } / (e ^{−24 (HP 1−0.2)} +1) ... (2), where HP1 is It is a value represented by the following formula (3) where the element symbol is the content of the alloying element in mass%. HP1 = C + (Si / 50) + (Mn / 20) + (Mo / 30) (3). 5. Laser welding is performed under the conditions of laser output L (W (watt)), thermal efficiency η, and welding speed v (cm / sec).
At a position at a depth of 1 mm from the surface of the base metal, a weighted average value of Vickers hardness on the weld metal side in a region within a distance of 1.6 mm from the bond portion after laser welding is within the region of 1.6 mm. Thickness h which is 1.6 times or less of the weighted average value of Vickers hardness on the welding heat affected zone side in
(Cm) Steel material used for steel structural members, in mass%
C: 0.003 to 0.07%, Si: 0.1 to 0.
6%, Mn: 0.3-2.0%, Al: 0.01-0.
1%, further, the first group: Cr: 0.01 to 0.15%, Ni: 0.01
.About.0.15%, Cu: one or more of 0.1 to 0.35%, second group: V: 0.01 to 0.07%, Nb: 0.01 to
One or more of 0.06%, the third group: Ti: 0.003 to 0.015%, Ca: 0.0.
One or more of 0005 to 0.006%, including one or more groups, the balance consisting of Fe and impurities, Mo in the impurities is 0.08% or less, and is represented by the following (4). Steel with a value of 100 or less. {3.91 × 10 ⁶ × (Lη) ^−1.2 × (vh) ^1.2 } / (e ^{−24 (HP 2−0.2)} +1) ^··· (4), where HP2 is It is a value represented by the following equation (5), where the element symbol is the content of the alloying element in mass%. HP2 = C + (Si / 50) + {(Mn + Cu + Cr) / 20} + (Ni / 60) + (V / 20) + Ca + (Mo / 30) (5).