JP2004136312A - Thin steel sheet fillet welded joint having high fatigue strength, and method for improving its fatigue strength - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin steel sheet fillet welded joint having high fatigue strength and a method for improving the fatigue strength of the fillet welded joint, which is more excellent in the economical profit and the toughness of welding metal than before. <P>SOLUTION: In the thin steel sheet fillet welded joint having a welding end portion, the welded joint is composed of steel sheets having the thickness of 1.0-4.0 mm and the tensile strength larger than 590 MPa, and the welded bead having a welding metal having the depth of penetration smaller than 1/3 of the thickness of the thin steel sheet, the transformation starting temperature of 475-600°C from austenite to martensite or bainite, and the tensile strength larger than 590 MPa. Further, the welding end portion is recessed from the surface of the steel sheet by the amount larger than 0.03 mm and smaller than 1/4 of the thickness of the steel sheet over the range at least 10 mm from the end portions of the starting portion of the weld bead and the crater portion. <P>COPYRIGHT: (C)2004,JPO

Description

【０１２３】
【表２】

【０１２４】
【表３】

【０１２５】
【表４】

【０１２６】
【表５】

【０１２７】
【表６】

【０１２８】
【発明の効果】
以上のように、本発明によれば、溶接継手の疲労強度は従来継手より格段に向上させることが可能である。したがって、本発明は工業的価値がきわめて大きい発明である。
【図面の簡単な説明】
【図１】図１は、重ね継手部の概念図である。
【図２】図２は、継手Ａの試験片形状と疲労荷重負荷方向を説明した図である。
【図３】図３は、継手Ｂの試験片形状と疲労荷重負荷方向を説明した図である。
【図４】図４は、溶け込み深さを説明した概念図である。
【図５】図５は、溶接止端部のへこみ量を説明した図である。
【符号の説明】
１　スタート部
２　クレーター部
３　コーナー部
４　冶具
５　冶具
６　溶け込み深さ
７　へこみ量[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a fillet welding method for a steel sheet and a welded joint using the same, particularly applied to the manufacture of underbody parts for automobiles, and more particularly to a high fatigue strength welded joint for a thin steel sheet having a tensile strength of 590 MPa or more. And a fillet welding method capable of obtaining high fatigue strength.
[0002]
[Prior art]
Since fatigue cracks, which have a significant effect on the safety and reliability of welded steel structures, are likely to occur in welds, various methods for improving the fatigue properties of welds in welded steel structures have been studied in the past. .
[0003]
It has been known from the past that the most prone to fatigue cracks in welds is the weld toe, and that the main cause is stress concentration due to residual tensile stress that tends to occur at the weld toe. I have.
[0004]
Therefore, as a conventional method for improving the fatigue characteristics of a welded joint, a method of improving the weld toe shape by machining such as TIG tanning welding (decorative welding) or grinding after welding, and an improvement of the weld toe shape by peening or the like. And the simultaneous introduction of compressive residual stress. If these methods are not applied to the entire welding line, the fatigue strength of the entire joint cannot be ensured, the work load increases, and this method is not economically preferable. Recently, many techniques have been disclosed in the United States for peening using ultrasonic waves when performing peening, that is, ultrasonic peening techniques (for example, Patent Documents 1 to 3). In this technique, the efficiency of peening is greatly improved by using ultrasonic waves as compared with the conventional peening method, and thus, the peening work load can be expected to be reduced. However, in order to improve the fatigue strength of the entire welded joint, peening must be performed over the entire weld line, and the problem of increasing the number of work processes and increasing the economic load has not been resolved. Remains.
[0005]
On the other hand, recently, the composition of the welding material used in welding has been designed so that the transformation temperature of the weld metal is low, and compressive residual stress is introduced by utilizing the volume expansion accompanying transformation during welding, to thereby form a weld toe at the weld toe. A technique for reducing tensile residual stress and improving fatigue properties has been proposed (for example, Patent Document 4. Hereinafter, such welding materials are collectively referred to as low-temperature transformation welding materials). According to this, by using a low-temperature transformation welding material and performing a martensitic transformation and a volume expansion of the weld metal in a low-temperature region having a transformation start temperature of 170 ° C. to 250 ° C., a subsequent tensile contraction caused by heat shrinkage occurs. A technique has been disclosed in which the stress is offset to reduce the residual tensile stress at the weld toe at room temperature or to reduce the residual compressive stress at room temperature.
[0006]
The technology utilizing low-temperature transformation expansion of the weld metal is a post-weld post-treatment technology described above in that the fatigue strength of the joint can be improved simply by changing the component design of the welding material used for welding. This is an economically superior method that requires fewer work processes and saves labor costs.
[0007]
However, the technique using the low-temperature transformation expansion of the weld metal disclosed in Patent Document 4 and the like has three major problems.
[0008]
In other words, (1) low-temperature transformation welding materials used for welding must add a large amount of expensive alloy elements in order to lower the transformation temperature, which increases the cost of welding materials. The workability during welding is deteriorated due to the reason of adding a large amount, and the work efficiency is degraded and the work cost is correspondingly high. (3) Since the volume expansion of the martensitic transformation that starts transformation in the low temperature range is used, The weld metal becomes a hard structure mainly composed of martensite, and mechanical properties, particularly, toughness are deteriorated.
[0009]
As described above, there is still a problem that needs to be solved in the method utilizing the transformation expansion of the weld metal and the ultrasonic peening method, and the fatigue strength of a high fatigue strength welded joint and a welded joint that has solved these problems has been solved. An improvement method was strongly desired.
[0010]
[Patent Document 1]
US $ 6171415 $ B1
[Patent Document 2]
US $ 6338765B1
[Patent Document 3]
US $ 2002/0014100 $ A1
[Patent Document 4]
JP-A-11-138290
[0011]
[Problems to be solved by the invention]
The technology to sufficiently lower the transformation start temperature of the weld metal to improve the fatigue strength has great advantages because it does not require a new manufacturing process, but it still has problems such as poor workability and high cost. are doing. On the other hand, the conventional method of improving the fatigue strength by peening the joint has to be performed for all the welding lines, which leads to an increase in manufacturing cost.
[0012]
In view of the above problems of the prior art, the present invention sufficiently improves the fatigue strength of a welded joint by utilizing the transformation expansion under a condition in which the transformation temperature of the weld metal is higher than in the past, and performs the peening process with a bead shape. By reducing the amount of expensive alloying elements required for low-temperature transformation, and narrowing the peening range. It is an object of the present invention to provide a high fatigue fillet welded joint of a thin steel sheet and a method of improving the fatigue strength of a fillet welded joint, which is excellent in the toughness of a weld metal.
[0013]
[Means for Solving the Problems]
The present invention solves the above technical problem, that is, the gist thereof is as follows.
[0014]
(1) In a fillet welded joint of a thin steel plate having a weld toe, a steel plate having a plate thickness of 1.0 to 4.0 mm and a tensile strength of 590 MPa or more, and a plate having a penetration depth of the steel plate A thickness of not more than 1/3 of the thickness, a temperature at which transformation from austenite to martensite or bainite starts at 475-600 ° C, and a tensile strength of 590 MPa or more. A high fatigue strength corner characterized in that the weld toe is recessed from the surface of the steel material by 0.03 mm or more and 1/4 or less of the plate thickness over a range of at least 10 mm or more from the ends of the start part and the crater part of the weld bead. Meat welded joint.
[0015]
(2) The content of the weld metal in mass% is C: 0.2 to 0.4%, Si: 0.1 to 0.8%, Mn: 0.4 to 2.0%, P: 0.03. %, S: 0.02% or less, with the balance being iron and inevitable impurities, the high fatigue strength fillet welded joint according to (1) above.
[0016]
(3) The weld metal further contains, by mass%, one or more of Ni, Cr, Mo, Cu, V, Nb, Ti, Ca, B and Mg in a total amount of 0.001 to 0.001. The high fatigue strength fillet welded joint according to the above (2), which contains 1.0%.
[0017]
(4) The content of the weld metal is, by mass%, C: 0.03 to less than 0.2%, Si: 0.1 to 0.8%, Mn: 1.0 to 2.0%, P: 0. The high fatigue strength according to the above (1), wherein the high fatigue strength contains not more than 03%, not more than 0.02% of S, and not more than 2.0 to less than 4.0% of Ni, with the balance being iron and unavoidable impurities. Fillet weld joint.
[0018]
(5) The weld metal further contains, in mass%, one or more of Cr, Mo, Cu, V, Nb, Ti, Ca, B and Mg in a total amount of 0.001 to 1. The high-fatigue-strength fillet welded joint according to the above (4), which contains 0%.
[0019]
(6) In a fillet welded joint of a thin steel plate having a weld toe, a steel plate having a thickness of 1.0 to 4.0 mm and a tensile strength of 590 MPa or more, and a plate having a penetration depth of the steel plate 1/3 or less of the thickness, a temperature at which transformation from austenite to martensite or bainite is less than 400 to 475 ° C, and a weld metal characterized by having a tensile strength of 590 MPa or more, and A high fatigue strength corner characterized in that the weld toe is recessed from the surface of the steel material by 0.03 mm or more and 1/4 or less of the plate thickness over a range of at least 10 mm or more from the ends of the start part and the crater part of the weld bead. Meat welded joint.
[0020]
(7) The content of the weld metal in mass% is as follows: C: 0.03 to less than 0.2%, Si: 0.1 to 0.8%, Mn: 1.0 to 2.0%, P: 0. The high fatigue strength corner according to the above (6), wherein the high fatigue strength corner contains 0.3% or less, S: 0.02% or less, Ni: 4.0 to 7.5%, and the balance consists of iron and unavoidable impurities. Meat welded joint.
[0021]
(8) The weld metal further contains, in mass%, one or more of Cr, Mo, Cu, V, Nb, Ti, Ca, B and Mg in a total amount of 0.001 to 1. The high fatigue strength fillet welded joint according to the above (7), which contains 0%.
[0022]
(9) In the method of fillet welding of a steel sheet, a steel sheet having a thickness of 1.0 to 4.0 mm and a tensile strength of 590 MPa or more is used, and the degree of constraint of a welded portion of the steel sheet is 4000 N / mm · mm. Below, and the penetration depth of the weld metal in the welded portion is 1/3 or less of the thickness of the steel sheet, and the transformation start temperature of the weld metal in the welded portion is 475 to 600 ° C, and the tensile strength is 590 MPa. Fatigue strength of a fillet weld characterized by forming the above weld metal and peening the weld toe over a range of at least 10 mm or more from the ends of the start portion and the crater portion of the weld bead of the weld. How to improve.
[0023]
(10) The content of the weld metal in mass% is C: 0.2 to 0.4%, Si: 0.1 to 0.8%, Mn: 0.4 to 2.0%, P: 0.03. % Or less, S: 0.02% or less, the balance being iron and unavoidable impurities, the method for improving the fatigue strength of the fillet weld according to the above (9), wherein
[0024]
(11) The weld metal further contains, by mass%, one or more of Ni, Cr, Mo, Cu, V, Nb, Ti, Ca, B and Mg in a total amount of 0.001 to 0.001. The method for improving fatigue strength of a fillet weld according to the above (10), wherein the content is 1.0%.
[0025]
(12) The content of the weld metal is, by mass%, C: 0.03 to less than 0.2%, Si: 0.1 to 0.8%, Mn: 1.0 to 2.0%, P: 0. The fillet weld according to the above (11), which contains not more than 03%, not more than 0.02% of S, and not more than 2.0 to less than 4.0% of Ni, with the balance being iron and unavoidable impurities. Method of improving fatigue strength.
[0026]
(13) The weld metal further contains, by mass%, one or more of Cr, Mo, Cu, V, Nb, Ti, Ca, B and Mg in a total amount of 0.001 to 1. The method for improving fatigue strength of a fillet weld according to the above (12), wherein the content is 0%.
[0027]
(14) In the method of fillet welding of a steel sheet, a steel sheet having a thickness of 1.0 to 4.0 mm and a tensile strength of 590 MPa or more is used, and the degree of constraint of a welded portion of the steel sheet is 8000 N / mm · mm. Below, and the penetration depth of the weld metal in the weld portion is 1/3 or less of the thickness of the steel plate, and the transformation start temperature of the weld metal in the weld portion is less than 400 to 475 ° C, and the tensile strength is Fatigue of a fillet weld, wherein a weld metal of 590 MPa or more is formed and a weld toe is peened at least 10 mm or more from ends of a start portion and a crater portion of a weld bead of the weld portion. Strength improvement method.
[0028]
(15) The content of the weld metal is, by mass%, C: 0.03 to less than 0.2%, Si: 0.1 to 0.8%, Mn: 1.0 to 2.0%, P: 0. The fillet weld according to the above (14), which contains not more than 03%, not more than 0.02% of S, and not more than 4.0 to 7.5% of Ni, with the balance being iron and unavoidable impurities. Method of improving fatigue strength.
[0029]
(16) {1 or more of Cr, Mo, Cu, V, Nb, Ti, Ca, B and Mg in a total amount of 0.001 to 1. The method for improving fatigue strength of a fillet weld according to the above (15), wherein the content is 0%.
[0030]
(17) The corner according to any of (9) to (15) or (16), wherein a method using ultrasonic waves having a frequency in a range of 20 kHz to 60 kHz is used as the peening method. A method for improving the fatigue strength of meat welds.
[0031]
(18) At the time of peening, one or a plurality of pins having a diameter in the range of 1.5 mm to 7.0 mm are used at a tip portion for applying an impact to the welded portion, and the hardness of the pin tip is The method for improving the fatigue strength of a fillet weld according to the above (17), wherein a pin having a Vickers hardness of 450 or more and 900 or less is used.
[0032]
(19) The high fatigue strength corner according to (1) to (7) or (8), which is produced by using the method for improving fatigue strength of a fillet weld according to (9) to (17) or (18). Meat welded joint.
[0033]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
[0034]
First, the technical concept of the present invention will be described. There are two major technical ideas of the present invention. The first technical idea relates to a technology for reducing the residual welding stress by utilizing the transformation expansion of a weld metal, and the second technical idea relates to a peening process. is there. First, the first technical concept will be described.
[0035]
The present invention provides a conventional welding method using a low-temperature transformation material, that is, a compression stress is generated at a weld toe by utilizing transformation expansion of a weld metal in a low-temperature region during welding, and the compression stress is maintained at room temperature. In comparison with the method of reducing the tensile residual stress at the weld toe by making the weld toe generate compression stress at the weld toe using the transformation expansion of the weld metal, the transformation start temperature is the same. The point that is higher than before is greatly different.
[0036]
Considering the process of generating residual stress in the weld during welding, when the weld metal solidifies and cools after welding and reaches its transformation start temperature, the weld metal expands in volume due to the transformation, and the surrounding base metal heat affected zone Compressive stress is generated at the weld toe due to the reaction force.
[0037]
At this time, if the transformation start temperature of the weld metal is high, the volume expansion due to the transformation of the weld metal occurs at a high temperature, so that tensile stress is applied to the weld toe due to thermal contraction in the cooling process after the transformation expansion. The generated residual stress at the weld toe at the time when it is generated and cooled to room temperature is in a tensile stress state. Therefore, the welding technique using the conventional low-temperature transformation welding material reduces the temperature difference from the end of the transformation expansion of the weld metal to room temperature by setting the transformation start temperature of the weld metal as low as possible (250 ° C. or less). The technical idea is to reduce the amount of cooling and heat shrinkage during this period and to shift the residual stress at the weld toe to the compressive stress side at room temperature.
[0038]
On the other hand, in the present invention, it is not necessary to set the transformation start temperature of the weld metal to a low temperature range (250 ° C. or lower) as in the prior art, that is, in a very high temperature range of 400 ° C. to 600 ° C. Even if the metal undergoes transformation expansion, it suppresses the tensile stress itself generated at the weld toe due to cooling and thermal contraction from the end of the transformation expansion to room temperature. The purpose of the present invention is to maintain the compressive stress and to transfer the residual stress at the weld toe at room temperature to the compressive stress side.
[0039]
More specifically, the present invention (1) generates a compressive stress at the weld toe using the volume expansion accompanying the transformation between the start of the transformation expansion and the end of the transformation expansion of the weld metal in a high temperature range. In order to ensure that the tensile strength of the weld metal and the base metal is equal to or higher than a predetermined value, and that the penetration depth of the weld metal is limited to a predetermined value or less, the volume expansion accompanying the transformation expansion of the weld metal is reduced below the weld metal. And the base material including the heat-affected zone around it, which is generated due to the restraining force of the transformation expansion of the weld metal and at the weld toe due to the restraining force of the transformation expansion of the weld metal. (2) The compressive stress is introduced by the above-mentioned mechanism (1) to complete the transformation expansion of the weld metal, and thereafter, during the cooling and heat shrinking process to room temperature, the heat shrinkage occurs. Constrain parts And without by free shrink, suppressing the occurrence of tensile stress in the weld toe due to heat shrinkage. If this is possible, the compressive stress introduced by the above (1) is maintained even when cooled to room temperature. For this purpose, first, it is necessary to reduce the thickness of the plate to a predetermined value or less to complete the heat conduction of the welding heat to the base material including the heat affected zone at the lower portion by the end of the transformation expansion of the weld metal. This means that the temperature difference in the thickness direction is eliminated and the weld toe on the surface where the compressive stress is generated is not restrained from the back surface of the plate. If the temperature of the front and back surfaces of the plate is the same, the heat shrinkage of the front and back surfaces is also the same, so that the front surface is not restrained from the back surface. Next, it is necessary to reduce the degree of restraint of the welded portion to a predetermined value or less, and to generate heat contraction to room temperature, which occurs after the transformation expansion of the weld metal, as freely as possible. This realizes a state in which the weld toe where the compressive stress is generated is not restricted not only from the back surface of the plate but also from the surroundings. Through such a process, the compressive stress generated when the weld metal undergoes the transformation expansion is maintained until it is cooled to room temperature without being changed into a tensile stress due to the subsequent thermal contraction.
[0040]
The first technical idea of the present invention is to maintain the compressive stress accompanying the transformation expansion of the weld metal generated at a relatively high temperature to room temperature by reducing the temperature difference in the thickness direction and reducing the degree of constraint. Let's make it happen.
[0041]
A second technical idea of the present invention is to perform a peening process on a start portion and a crater portion of a weld bead to secure the fatigue strength of that portion. The start portion and the crater portion are portions where the bead shape is likely to be disordered and also where stress is easily concentrated structurally. Therefore, in the present invention, the fatigue strength is secured for the weld toe portion other than the start portion and the crater portion by using the first technical concept, and the fatigue strength is secured for the start portion and the crater portion by using the peening process. This realizes high fatigue strength of the entire welded joint. Thus, by limiting the peening process to the vicinity of the start portion and the crater portion, it is possible to reduce the load of the peening work as much as possible.
[0042]
The present invention is characterized in that peening using particularly ultrasonic waves is used as the peening process. When ultrasonic peening is used, the time for peening is shortened accordingly, and the merit is correspondingly great. Since the purpose of the present invention is to provide a technique for improving fatigue strength by a method as simple as possible, the use of ultrasonic peening is significant.
[0043]
Hereinafter, the fillet welding method of the present invention, the configuration of a welded joint using the same, and the reasons for limiting the structure will be described.
[0044]
(Limited to base material thickness of 1.0 to 4.0 mm)
The reason for limiting the thickness of the base material will be described.
[0045]
In the present invention, the thickness of the steel material is reduced so that the welding heat is immediately transmitted to the back surface of the material to be joined during welding. This is because, after the welding heat reaches the back surface, the weld metal is not restrained from the back surface of the steel material, and the heat shrinkage of the weld metal and the heat shrinkage of the plate back surface occur simultaneously. The greater the thickness of the steel material, the longer it takes to transfer the welding heat, and even if the transformation of the weld metal is completed, the welding heat will not be transmitted to the back surface. Is restrained by the steel material underneath, and tensile stress is generated at the weld toe.
[0046]
In the present invention, a welding material having a transformation start temperature near 200 ° C. higher than the transformation temperature of the conventional low-temperature transformation molten material is used, the transformation start temperature of the weld metal is as high as 600 ° C. to 400 ° C., and the transformation expansion ends. Since the temperature is high, thermal contraction of the weld metal caused by cooling from such a high temperature range to room temperature is suppressed, and the compressive residual stress at the weld toe generated by the transformation expansion of the weld metal is kept at room temperature. Therefore, heat must be transmitted to the back surface at least at the time when the transformation expansion of the weld metal ends.
[0047]
Also, in order to control the transformation start temperature at low cost, when welding is performed using a component-type welding material in which expensive alloying elements are reduced and C is added to be higher, the amount of C in the weld metal increases. In particular, solidification cracking is likely to occur during butt solidification when the thickness of the steel material is large.
[0048]
Butt solidification, which causes this solidification cracking, occurs when the thickness of the steel material increases, the heat capacity of the steel material itself increases, and the welding heat is easily transmitted in the width direction of the weld bead. When welding is performed using the above welding material, it is necessary to reduce the thickness of the steel sheet also from the viewpoint of preventing solidification cracking of the weld metal.
[0049]
Further, it is more advantageous that the thickness of the steel material is thinner from the viewpoint of lowering the degree of constraint described later. The constraint on the thermal contraction of the weld metal is not only from the back of the steel but also from the entire structure of the welded joint.To reduce this constraint, it is meaningful to reduce the thickness of the steel plate. is there.
[0050]
When the thickness of the steel material exceeds 4.0 mm, the transfer of the welding heat to the back surface is delayed, and the welding heat is not transferred to the back surface at the end of the transformation of the weld metal. The weld metal is restrained from below the steel material and tensile stress is generated at the weld toe, so that the fatigue strength of the welded joint is reduced. In addition, when welding is performed using a high-C component-based welding material, the risk of solidification cracking of the weld metal increases. Further, the degree of constraint determined by the relationship with the structure of the welded joint increases, and a sufficiently low degree of constraint cannot be obtained.
[0051]
On the other hand, when the thickness of the steel material is reduced to less than 1.0 mm, not only is it difficult to limit the penetration value of the weld metal described later relative to the thickness, but also acts as a reaction force during transformation expansion of the weld metal. The base material immediately below the weld metal is reduced, making it difficult to introduce compressive stress to the weld toe. Therefore, in the present invention, the upper limit of the thickness of the steel material is 4.0 mm, and the lower limit is 1.0 mm.
[0052]
(Regulation of 4000 N / mm-mm or less or 8000 N / mm-mm or less)
The reason for limiting the degree of constraint of the welded joint will be described.
[0053]
Conventionally, the degree of constraint of a welded joint is a parameter generally used to evaluate cracks generated in a weld during welding. It was employed as an index that quantitatively represents how strongly the weld metal was constrained from its surroundings.
[0054]
In general, the degree of constraint (R_F) Is defined as the load per unit length in the welding line direction required to reduce the weld groove by the unit length, and both fixed ends when the both ends of the test piece with the groove formed in the center are fixed From the relationship between the length (L), the plate thickness (H), and the Young's modulus (E), if the groove width is sufficiently smaller than the distance (L) between the fixed ends, the following ( It is given by the expression 1).
R_F(N / mm · mm) = E (N / mm²) ・ H (mm) / L (mm) ・ ・ ・ (1)
Note that the constraint degree (R_FThe unit of () is conventionally expressed as N / mm · mm.
[0055]
From the relationship of equation (1), the constraint degree (R_F) Can be reduced by reducing the thickness (H) of the steel material at the time of welding or increasing the distance (L) between the fixed ends determined by the welded joint structure. The degree of constraint (R_FAs a method of adjusting the thickness of the steel plate, a method of changing the distance of the fixed end by devising the restraining jig or a method of changing the thickness H of the steel plate by devising the design of the welding member can be considered.
[0056]
In the present invention, in the process of cooling and heat shrinking from the end of transformation expansion of the weld metal to room temperature, to make the heat shrinkage of the weld metal close to free shrinkage, and to suppress the occurrence of tensile stress at the weld toe, As described above, the upper limit of the thickness of the steel material is restricted to 4 mm or less, and the degree of constraint (R_F) Is regulated as follows.
[0057]
As described above, in order to improve the fatigue strength of the welded joint and to maintain the residual stress of the weld toe at room temperature on the compression side, (1) transformation from the start of transformation expansion of the weld metal At the time of volume expansion up to the end of expansion, the stress generated by restraining the expansion and the reaction force generated in the surrounding heat affected zone of the base metal are secured, and a compressive stress is generated at the weld toe, and ▲ 2) At the time of thermal contraction from the end of transformation expansion of the weld metal to room temperature, it is necessary to reduce the restraint from the periphery of the weld metal and allow it to shrink freely to suppress the generation of tensile stress at the weld toe. . Among these, the upper limit regulation of the degree of constraint has the effect of suppressing the occurrence of tensile stress at the weld toe in the heat shrinkage of the weld metal in the above (2), and under the same condition where the transformation start temperature of the weld metal is the same, By lowering the upper limit of the degree of constraint, the residual stress at the weld toe at room temperature shifts to the compression side, and the fatigue strength of the welded joint is improved.
[0058]
However, as the transformation start temperature of the weld metal increases, the stress generated in the weld metal during the transformation expansion of the weld metal in (1) above and the reaction force generated in the heat affected zone around the base metal decrease. Since the compressive stress generated at the weld toe decreases, and the temperature at which the transformation ends increases, and the temperature difference from room temperature increases, the tension at the weld toe due to the thermal shrinkage of the weld metal in (2) above. Stress also increases. As a result, in order to reduce the residual stress at room temperature to the compression side by the effect of (2), it is necessary to further reduce the degree of constraint as the transformation start temperature of the weld metal increases.
[0059]
In the present invention, as will be described later, in practice, by using two types of component-type welding materials, welding is performed, so that the transformation start temperature of the weld metal is 475 to 600 ° C and two different types of 400 to 475 ° C. In order to perform welding under the transformation start temperature condition, the upper limit value of the degree of constraint is defined as follows according to these transformation start temperatures.
[0060]
That is, in the present invention, when the transformation start temperature of the weld metal, which is the higher of the two transformation start temperatures, is 475 to 600 ° C., the upper limit of the degree of constraint is set to 4000 N / mm · mm. When the other transformation start temperature is 400 ° C. to 475 ° C., the upper limit of the degree of constraint is set to 8000 N / mm · mm. When the upper limit of any transformation start temperature is exceeded, the effect of reducing the tensile stress generated at the weld toe due to the thermal contraction after the transformation expansion of the weld metal becomes insufficient and compresses the residual stress at room temperature. Side, it is difficult to improve the fatigue strength of the welded joint.
[0061]
(Provision of tensile strength of weld metal and steel material of 590MPa or more)
The reason for limiting the tensile strength of the weld metal and steel will be described.
In the present invention, since welding is performed under conditions where the transformation start temperature of the weld metal is considerably higher than before, the heat of the weld metal and its surrounding base metal during the volume expansion process from the transformation expansion start temperature to the transformation expansion end temperature of the weld metal is increased. It is considered that the tensile strength of the affected part is considerably lower than before. Also, in the conventional welding using low-temperature transformation welding material, since the weld metal is a component system having a lot of alloy components and high quenchability and utilizing volume expansion due to martensitic transformation, during transformation expansion of the weld metal, Due to the hard structure of martensite, the strength of the weld metal during transformation expansion can be sufficiently ensured. However, in the welding using the high-temperature transformation welding material of the present invention, compared with the case of the low-temperature transformation welding material, the weld metal is a component system having a small alloying component and low quenchability. The strength of the weld metal at the time of transformation expansion is lower than that in the case where it is present.
[0062]
In the present invention, as described above, the tensile stress at the weld toe generated at the time of thermal contraction from the end of the transformation expansion of the weld metal to room temperature by limiting the conditions of the plate thickness and the degree of constraint is reduced. However, in order to reduce the residual stress at the weld toe at room temperature to the compressive stress side, in addition to this, the weld metal is utilized by utilizing the volume expansion from the start of transformation expansion to the end of transformation expansion. In order to generate sufficient compressive stress at the end, it is necessary to secure the stress generated by restraining the expansion of the weld metal and the reaction force generated in the surrounding heat-affected zone of the base metal. The tensile strength of the weld metal and steel material corresponding to the above must be ensured. For example, if the tensile strength of the weld metal in the temperature range during the transformation expansion of the weld metal becomes 0, the weld metal undergoes plastic deformation during the transformation expansion of the weld metal, and the transformation expansion simply changes to plastic strain. Therefore, the compressive stress at the weld toe remains zero, and if the weld toe is thereafter cooled to room temperature, the thermal shrinkage of the weld metal is suppressed. It cannot be a compressive residual stress.
[0063]
Based on the above, in the present invention, the minimum stress generated in the weld metal to generate a sufficient compressive stress at the weld toe using the volume expansion due to the transformation of the weld metal and the heat of the surrounding base metal are generated. In order to secure the reaction force of the affected part, the tensile strength of the weld metal and the steel was set to 590 MPa or more.
[0064]
In the present invention, it is not necessary to particularly define the upper limit of the tensile strength of the steel material and the weld metal, and particularly, the tensile strength of the weld metal is restricted by the lower limit of the transformation start temperature. However, when the tensile strength of the steel material and the weld metal is increased, it is necessary to add a considerable amount of alloying elements to the steel material and the weld metal. It is desirable that the upper limit of the tensile strength of the steel material and the weld metal be 980 MPa.
[0065]
(The penetration depth of the weld metal shall be less than 1/3 of the thickness of the steel sheet)
The reason why the penetration depth of the weld metal is limited will be described.
[0066]
If the penetration depth of the weld metal is excessively large, the reaction force of the steel material, including the heat affected zone below it, cannot be obtained sufficiently during the transformation expansion of the weld metal, and the compressive residual stress at the weld toe becomes small. Fatigue strength does not improve sufficiently. For example, when the penetration depth of the weld metal W is large as shown in FIG. 1, the unmelted portion indicated by A decreases during the transformation expansion of the weld metal, so that the expansion of the weld metal can hardly be restrained. Due to plastic deformation, the weld metal expands almost freely, and no compressive residual stress is generated at the weld toe. On the other hand, if the welding method is used while maintaining the degree of restraint high by restraining the structure of the welded joint or restraint without relying on the restraint of the unmelted portion of A, the weld metal is deformed during transformation expansion of the weld metal. Although the toe is in a compressive stress state, tensile stress occurs at the weld toe due to thermal contraction due to cooling to room temperature after the transformation of the weld metal is completed, which results in offsetting the compressive stress during the transformation expansion. It is not an effective method.
[0067]
In the welding under the condition that the steel plate thickness is relatively thick, the problem of the lowering of the base metal restraint at the lower portion of the weld metal due to the penetration depth of the weld metal disappears. In order to secure the thermal conductivity of the base metal, the steel plate thickness is limited to 4 mm or less. In such a thin plate, if the penetration depth of the weld metal is not limited, the heat affected zone under the weld metal must be As a result, the compressive residual stress at the weld toe cannot be sufficiently generated, and as a result, the fatigue strength of the welded joint cannot be improved.
[0068]
In the present invention, the penetration depth of the weld metal is set to 1/3 of the steel plate thickness in order to sufficiently secure the lower unmelted portion during the transformation expansion of the weld metal as described above. Here, the penetration depth indicates the maximum penetration depth of the weld metal having the largest penetration depth, and the steel sheet thickness is the thickness before welding.
[0069]
(Specification of transformation start temperature of weld metal of 475 to 600 ° C or 400 to less than 475 ° C)
The reason for limiting the range of the transformation start temperature of the weld metal will be described.
The transformation start temperature of the weld metal in the present invention is significantly different from the conventional technique for improving the fatigue strength of a welded joint using volume expansion accompanying transformation of the weld metal, and the transformation start temperature of the weld metal is lower than in the past. It utilizes the transformation expansion of the weld metal under conditions higher than 200 ° C. In the present invention, since the transformation start temperature of the weld metal is very high, instead of the transformation under the condition that the transformation start temperature is low as in the past, a method utilizing volume expansion due to bainite transformation or ferrite pearlite transformation besides martensite The weld metal of the welded joint has a bainite transformation or a ferrite-pearlite-based structure having a lower hardness than a conventional hard structure mainly composed of martensite, and a weld metal having high toughness can be obtained. In addition, in the present invention, since the transformation start temperature of the weld metal is much higher than the conventional low-temperature transformation welding using the welding material, the high cost required to lower the transformation start temperature of the weld metal in the welding material. Since the addition amount of the alloy component can be reduced, the manufacturing cost of the welding material can be reduced as compared with the related art.
[0070]
However, in general, the strength of the weld metal or the base metal decreases as the temperature increases, so when welding is performed under the condition that the transformation start temperature of the weld metal is high as in the present invention, the strength decreases accordingly. Therefore, the compressive stress generated at the weld toe during the transformation expansion is reduced because the stress generated by the transformation expansion of the weld metal due to its restraint and the reaction force generated in the base metal including the heat affected zone around it are reduced. As the temperature difference between the transformation end temperature and room temperature increases, the tensile stress at the weld toe caused by the heat shrinkage of the weld metal due to cooling between the temperatures increases, and as a result, at room temperature It is difficult to reduce the residual stress at the weld toe to the compression side and to improve the fatigue strength of the welded joint. Accordingly, in the present invention, as described above, by defining the transformation start temperature of the weld metal in accordance with the level of the degree of constraint in welding, the weld metal can be freely shrunk at the time of thermal contraction after the transformation of the weld metal is completed, and the welding stop is performed. It suppresses an increase in tensile stress at the end.
[0071]
In the present invention, the transformation start temperature conditions of the weld metal in welding are classified into two different transformation start temperature levels of 475 to 600 ° C. having a high transformation start temperature and 400 ° C. to less than 475 ° C. which are lower than the above. .
[0072]
When welding is performed under the condition that the transformation start temperature of the weld metal is 475 ° C. to 600 ° C., the transformation of the weld metal starts at a higher temperature. Thus, a welded joint having low toughness and excellent toughness can be obtained, the amount of expensive alloy component added to lower the transformation start temperature in the welding material can be further reduced, and the production cost of the welded joint can be further reduced. In addition, the fatigue strength of the welded joint is sufficiently ensured by introducing a compressive stress to the weld toe under the condition that the transformation start temperature of the weld metal is 475 ° C. to 600 ° C. to make the residual stress at room temperature and the compressive stress side. For this purpose, as described above, it is necessary to regulate the degree of constraint to 4000 N / mm · mm or less. However, even when welding is performed under such low constraint conditions, when the transformation start temperature of the weld metal exceeds 600 ° C., it is difficult to reduce the residual stress at the weld toe to the compressive stress side, and the fatigue strength of the welded joint is reduced. Therefore, the upper limit of the transformation start temperature of the weld metal was set to 600 ° C. On the other hand, when the transformation start temperature of the weld metal is lower than 475 ° C., the effect of improving the fatigue strength of the welded joint can be obtained. Therefore, the lower limit value of the transformation start temperature of the weld metal was set to 475 ° C. from the viewpoints of economy and production cost because the production cost and the toughness of the welded portion decreased.
[0073]
When welding is performed under the condition that the transformation start temperature of the weld metal is 400 ° C. to less than 475 ° C., the weld metal of the welded joint has a bainite transformation or a ferrite-pearlite-based structure as compared with the welding conditions in which the transformation start temperature of the weld metal is high. However, the hardness increases slightly, the toughness of the weld decreases slightly, and the amount of expensive alloy components added to lower the transformation start temperature of the weld metal in the welding material also increases, resulting in the manufacturing cost of welded joints. However, even when welding is performed under a high constraint of 8000 N / mm · mm or less, the residual stress at the weld toe can be reduced to the compressive stress side, and the fatigue strength of the welded joint can be sufficiently increased. It is possible to secure. Therefore, the present invention is particularly effective in welding where it is difficult to perform welding under a condition in which the degree of constraint is sufficiently reduced due to the structure of the welded joint, and the degree of freedom in the welding condition can be improved.
[0074]
Under these welding conditions, the degree of restraint is relatively high, and the effect of heat shrinkage after the transformation expansion of the weld metal tends to be relatively large. Therefore, unless the upper limit of the transformation start temperature of the weld metal is regulated to be lower than 475 ° C., Since the contracted portion is constrained during the heat shrinkage process after the transformation expansion of the weld metal, the residual stress at the weld toe shifts to the tensile stress side, and sufficient fatigue strength cannot be obtained. The upper limit of the transformation start temperature of the metal is set to less than 475 ° C. On the other hand, the lower limit of the transformation start temperature of the weld metal is such that even when the transformation start temperature is lower than 400 ° C., the effect of improving the fatigue strength of the welded joint can be obtained, Since the toughness of the part is reduced, the lower limit of the transformation start temperature of the weld metal is set to 400 ° C. from the viewpoint of economy and production cost.
[0075]
Next, the reason why the dent amount of the weld toe from the steel material surface is limited will be described.
[0076]
In the present invention, the improvement of the fatigue strength of the welded joint is achieved by using both transformation expansion of the weld metal and peening treatment. The method of improving the fatigue strength by using the transformation expansion of the weld metal can secure the effect of improving the fatigue strength by limiting the transformation start temperature of the weld metal or limiting the components of the weld metal. , It is not always clear whether or not a fatigue strength improving effect is obtained by the treatment. Therefore, the present inventors focused on the amount of dent at the weld toe after the peening treatment, and investigated the amount of dent at which the effect of improving fatigue strength was obtained. The lower limit of the dent amount of 0.03 mm is set to a value of the dent amount smaller than the lower limit because the effect of the peening treatment is small and the fatigue strength is not improved. The value of ピ ー of the upper limit of the plate thickness is set because the peening treatment in which the dent amount is larger than this causes an increase in local stress due to a decrease in the plate thickness, which is not preferable from the viewpoint of improving the fatigue strength of the joint. Next, the reason why the range of the area where the peening process is performed on the toe or the area where the dent of the toe is present will be described.
[0077]
In the present invention, the purpose of performing the peening treatment is to ensure the fatigue strength of the start portion and the crater portion of the weld bead where the bead shape is defective. Therefore, it is necessary to perform the peening process or to cover the area where the bead shape is defective in the area where the dent of the toe exists. In particular, portions that are problematic in terms of fatigue characteristics are both end portions of the bead. Therefore, the peening process or the dent to the toe must cover at least both ends. Then, in the case where the bead shape is poor and the range of 10 mm from both ends is peened, dents must be ensured by performing peening treatment surely. In the present invention, the reason why the area where the toe is subjected to the peening process or the dent to the toe is limited to at least 10 mm or more from the ends of the start part and the crater part is as described above. Further, in the present invention, no particular upper limit is set for the peening region or the dent region. This is because the effect of further improving the fatigue strength is not necessarily obtained by setting the upper limit. However, the peening treatment itself causes an increase in manufacturing cost, and in the present invention, the fatigue strength is improved by the low-temperature transformation weld metal. Therefore, it is preferable that the upper limit is set to 100 mm.
[0078]
(Provisions for the components of the weld metal)
The reason for limiting the components of the weld metal will be described.
[0079]
As an embodiment of the component system of the weld metal of the present invention, the above-mentioned transformation onset temperature is relatively high at 475 to 600 ° C, and lower than 400 ° C to less than 475 ° C according to two different transformation onset temperature levels, The following two types of component systems are used.
[0080]
As the component system of the weld metal having a relatively high transformation start temperature of 475 to 600 ° C., a component system that lowers the transformation start temperature of the weld metal mainly by adding a relatively large amount of C (hereinafter referred to as C system). A component system (hereinafter, referred to as Ni system) that mainly lowers the transformation start temperature by adding Ni was used. Further, as a component system of a weld metal having a relatively low transformation start temperature of 400 ° C. to less than 475 ° C., a component system that lowers the transformation start temperature by adding Ni mainly (hereinafter, referred to as a Ni-based component) is used. Was.
[0081]
Among these, the C-based weld metal has a small amount of expensive alloying elements, so that the manufacturing cost of the welding material for obtaining the weld metal can be reduced and the toughness of the weld metal is slightly inferior but excellent in fatigue properties. This is advantageous from the viewpoint of economy when manufacturing a welded joint. On the other hand, a Ni-based weld metal is disadvantageous from the viewpoint of the economics of a welded joint because a relatively large amount of expensive Ni alloy element is added. Can be used to improve toughness, which is effective when manufacturing welded joints that require a high toughness level as well as fatigue properties. The selection of the constituent systems of these weld metals and the welding materials for realizing them are selected based on their respective characteristics.
[0082]
(Specification of components of C-based weld metal)
The components of the C-based weld metal and the reasons for limiting the content will be described.
C is a quenching element and is an effective element from the viewpoints of both improving the strength of the weld metal and reducing the transformation temperature. If the lower limit of 0.2% of the C content is less than this, not only the transformation start temperature of the C-based weld metal cannot be adjusted within the range of 475 to 600 ° C, but also the strength of the weld metal is secured. This value is set because there is a problem in performing the operation. On the other hand, when the content of C increases, the risk of solidification cracking in the weld metal during butt solidification increases, especially when the steel plate thickness is large. Therefore, the upper limit of the amount of C added is set to 0.4%.
[0083]
Si is mainly added as a deoxidizing element, and has an effect of lowering the oxygen level even when the oxygen concentration of the weld metal increases due to mixing of air during welding. If the lower limit of the Si content is less than 0.1%, the deoxidizing effect is insufficient and oxygen in the weld metal cannot be sufficiently reduced, and the mechanical properties of the weld metal, particularly toughness, are deteriorated. The lower limit of the amount was 0.1%. On the other hand, even when Si is added in an amount exceeding 0.8%, the toughness is deteriorated, so the upper limit of the content is set to 0.8%.
[0084]
Mn is a quenching element and has the effect of improving the strength of the weld metal and lowering its transformation temperature. Ensuring the strength of the weld metal is important because it secures the yield strength during the transformation expansion of the weld metal, which is the mechanism for reducing the residual tensile stress at the weld toe in the present invention, and generates sufficient compressive stress at the weld toe. It becomes.
[0085]
The lower limit of the Mn content was set to 0.4% as the minimum addition amount in view of securing the strength of the weld metal. From the viewpoint of lowering the transformation temperature of the weld metal, the addition amount of Mn is adjusted as a complementary component of C. However, if the addition amount is excessively large, the production cost of the welding material increases, which is not preferable from the viewpoint of economy. Therefore, the upper limit of the added amount of Mn is set to 2.0%.
[0086]
P and S are unavoidable impurity elements. In the present invention, if these elements are present in a large amount in the weld metal, the toughness is deteriorated. Therefore, the upper limits of the contents of P and S are set to 0.03% and 0.1%, respectively. 02%.
[0087]
The above are the basic components of the C-based weld metal in the present invention, and the fatigue strength of the weld metal can be sufficiently obtained by specifying these components. However, in order to further improve the strength and toughness of the weld metal, the required Depending on the properties, one or more of Ni, Cr, Mo, Cu, V, Nb, Ti, Ca, B and Mg may be contained in a total amount of 0.001 to 1.0%. . The lower limit of the total value of this content is the minimum necessary content to improve the strength and toughness of the weld metal, and the upper limit is the production of welded joints by excessively increasing the content of alloying elements. Since the cost is increased, the upper limit is set to 1.0%.
[0088]
(Specification of components of Ni-based weld metal)
The components of the Ni-based weld metal and the reasons for limiting the content will be described.
[0089]
C is a quenching element, which is an effective element from the viewpoint of improving the strength of the weld metal and reducing the transformation temperature. In the case of Ni-based components, the transformation start temperature of the weld metal is realized mainly by adding Ni. In order to complement the effect of lowering the transformation temperature of the weld metal of Ni and obtain sufficient strength, the lower limit is specified as 0.03% as the minimum content. On the other hand, excessive addition of C causes deterioration of the toughness of the weld metal, so the upper limit of the content is set to less than 0.2%.
[0090]
Si is mainly added as a deoxidizing element, and has an effect of lowering the oxygen level even when the oxygen concentration of the weld metal increases due to mixing of air during welding. The lower limit of the Si content is that when the Si content is less than 0.1%, the deoxidizing effect is reduced, the oxygen level in the weld metal becomes too high, and the mechanical properties of the weld metal, particularly, the toughness may deteriorate. Therefore, the lower limit of the content is set to 0.1%. On the other hand, excessive addition of Si also causes toughness degradation, so the upper limit of the content was set to 0.8%.
[0091]
Mn is a quenching element and has the effect of improving the strength of the weld metal and lowering its transformation temperature. Ensuring the strength of the weld metal is important because it secures the yield strength during the transformation expansion of the weld metal, which is the mechanism for reducing the residual tensile stress at the weld toe in the present invention, and generates sufficient compressive stress at the weld toe. It becomes.
[0092]
The lower limit of the Mn content was set to 1.0% as a minimum addition amount from the viewpoint of securing the strength of the weld metal. From the viewpoint of lowering the transformation temperature of the weld metal, the addition amount of Mn is adjusted as a complementary component of Ni. However, if the addition amount is excessively large, the toughness of the weld metal is deteriorated. And
[0093]
P and S are unavoidable impurity elements. In the present invention, if these elements are present in a large amount in the weld metal, the toughness of the weld metal deteriorates. Therefore, the upper limits of the contents of P and S are set to 0.03% and 0.02%, respectively. %.
[0094]
Ni is a metal element having an austenitic structure (face-centered structure), which further stabilizes the austenitic state of the weld metal in a high-temperature region and delays the transformation to ferrite (body-centered structure) in a low-temperature region. It is an element that lowers the temperature. Further, Ni does not increase the risk of solidification cracking of the weld metal as compared with C even if the same content is added, and thus is an effective element for further lowering the transformation temperature while maintaining the toughness of the weld metal. It is.
[0095]
In the present invention, when the transformation start temperature of the Ni-based weld metal is adjusted to a range of 475 to 600 ° C., the fatigue strength of the welded joint can be improved similarly to the C-based weld metal even if the amount of C added is reduced. In addition, the toughness can be further improved as compared with the C-based weld metal. The lower limit of the Ni content is set to 2.0% for improving the fatigue strength of the welded joint. On the other hand, the upper limit of the Ni content is set to less than 4.0% in order to sufficiently maintain the economy, toughness and weldability of the welded joint.
[0096]
In the present invention, in the case where the transformation start temperature of the Ni-based weld metal is adjusted to a range of 400 to less than 475 ° C., in the C-based weld metal, a problem of solidification cracking of the weld metal due to an increase in the C content is likely to occur. However, by setting the Ni content to 4.0 to 7.5%, the transformation start temperature of the weld metal can be lowered and the temperature can be adjusted to 400 to less than 475 ° C. while suppressing solidification cracking. Also, unlike Ni, the toughness does not necessarily occur even if the addition amount is slightly increased, so that even in this case, the toughness equal to or higher than that of the C-based weld metal can be secured. The lower limit of the Ni content was set to 4.0% in order to improve the fatigue strength of the welded joint. On the other hand, if the upper limit of the Ni content exceeds 7.5%, there is a possibility that the weldability such as toughness and weld solidification cracking may deteriorate as well as the economical efficiency of the welded joint. Was defined as 7.5%.
[0097]
The above are the basic components of the Ni-based weld metal according to the present invention, and the fatigue strength of the weld metal can be sufficiently obtained by defining these components, but in order to further improve the strength and toughness of the weld metal, it is necessary to use those components. Depending on required characteristics, one or more of Cr, Mo, Cu, V, Nb, Ti, Ca, B and Mg may be contained in a total amount of 0.001 to 1.0%. The lower limit of the total value of this content is the minimum necessary content to improve the strength and toughness of the weld metal, and the upper limit is the production of welded joints by excessively increasing the content of alloying elements. Since the cost is increased, the upper limit is set to 1.0%.
[0098]
As described above, the components of the C-based and Ni-based weld metals and the reasons for limiting the content thereof have been described. Adjustment of the component content of the weld metal is performed by welding, a welding wire, a combination of a welding wire and a filling flux, Alternatively, it can be realized by designing the components of the respective welding materials in consideration of the component yield in the weld metal when welding is performed using any one of the core wire of the welding rod and the coating flux.
[0099]
Next, the reason why the peening process is limited to the peening process using ultrasonic waves will be described.
[0100]
Here, the ultrasonic peening has a frequency within a range of 20 kHz to 60 kHz. The greatest advantage of using ultrasonic waves is that a sufficiently large impact force can be applied even if the weight of the pin at the tip of the peening is small, and as a result, a sufficient peening effect can be obtained in a short working time. The principle of improving the fatigue strength by performing peening treatment is to improve the shape of that portion and to apply compressive residual stress. For that purpose, plastic strain must be introduced into the peened portion. This is because no stress remains within the range of elastic strain. In order to introduce plastic strain, it is necessary to apply an impact stress greater than the yield strength of the material, but if this is to be achieved with static stress, apply a stress greater than the yield stress to the weld. This necessitates an increase in the size of the device, which increases the work load. On the other hand, when using ultrasonic waves, it can be seen that the stress applied to the peened portion is sufficiently large even if the pin mass is, for example, about 10 g.
[0101]
This principle will be briefly described.
[0102]
It is assumed that the frequency is 33 kHz, the mass of the pin is 10 g, the range in which the pin vibrates is 0.03 mm, and the diameter of the tip of the pin is 3 mm. At this time, the speed of the pin, V,
V = 0.03 × 33000 = 1000 mm / s = 1 m / s
It is. Assuming that the pin changes the speed from +1 m / s to -1 m / s once every 1/33000 seconds, the change occurs at the moment when the pin hits the peened portion. Assuming that this speed change occurs within 1/10 time within one frequency, that is, 1/330000 seconds, the time change of speed, that is, acceleration, A is
A = dV / dt = 2 × 330000 = 660000 m / s²
Becomes The impact force F is obtained by applying a pin weight of 10 g = 1/100 kg to the acceleration,
F = 660000 × 1/100 = 6600N
Becomes The stress S is calculated as follows: the cross-sectional area of the pin, 1.5 × 1.5 × 3.14 = 7.1 mm²Can be calculated by dividing by
S = 6600 / 7.1 = 930 N / mm2 = 930 MPa
Becomes It should be noted that this stress is the value when the weight of the pin is only 10 g. In the case of actual ultrasonic peening, the time interval at which the speed reversal occurs is considered to be shorter than the setting in the above calculation, and thus it is considered that a larger impact stress is generated.
[0103]
As described above, among the peening processes, the method using ultrasonic waves in particular has the advantage that the mass of the pin is small and the weight of the device can be reduced accordingly.
[0104]
Next, the reason why the frequency of the ultrasonic peening is limited will be described.
[0105]
If the lower limit of 20 kHz is a frequency lower than this, it falls within the range of human audible frequencies, that is, the range of audible frequencies, which is not preferable from the viewpoint of peening work. Since the intent of the present invention is to provide a simple method for improving fatigue strength, a method that degrades the working environment deviates from the intent of the present invention. Further, as can be seen from the above-described calculation of the impact stress, the higher the frequency of the ultrasonic wave, the higher the impact stress and the more advantageous it becomes. The lower limit of 20 kHz is set as a frequency at which a sufficient peening effect can be obtained with a simple device, and as a value that does not deteriorate the working environment. The lower limit of 20 kHz is preferably set to 23 kHz or more from the viewpoint of obtaining a higher impact stress. If the upper limit of 60 kHz is a frequency higher than this, it is difficult to obtain an ultrasonic wave with a simple device using current technology, and a problem in health management occurs although it cannot be heard by human ears. It was set.
[0106]
Next, the reason why the hardness of the pin is limited will be described.
[0107]
In the present invention, the strength of the steel material and the weld metal is limited. This aims at effectively changing the transformation expansion of the weld metal into compressive elastic strain. However, it is necessary to secure the fatigue strength of the start portion and crater portion of the weld bead by peening or the like due to deterioration of the bead shape. On the other hand, regarding the strength, the start portion and the crater portion also have a predetermined strength. For example, when the tensile strength is 780 MPa, the hardness is about 280 Hv. At 980 MPa, the hardness is close to 350 Hv. In order to improve the shape of the portion and reduce the residual stress by peening, it is necessary to introduce a plastic strain into the peened portion. For that purpose, the hardness of the pin needs to be harder than steel and weld metal. Therefore, in the present invention, the lower limit of the hardness of the pin is set to 450 Hv. The upper limit of 900 Hv is set because the cost of the pin itself is increased and the peening effect is not significantly increased although there is a harder material than this.
[0108]
Next, the reason why the diameter of the pin is limited will be described.
[0109]
As can be seen from the above-described calculation example of impact stress, the final impact stress can be obtained by dividing the impact force by the pin cross-sectional area, and the impact stress tends to increase as the cross-sectional area decreases. In order to obtain a higher impact stress, the pin may be made thinner, for example, like a needle. In this case, however, there is a risk that the pin may break or buckle, and the unnecessary thin shape is rather negative. The lower limit of 1.5 mm is set as a value that can sufficiently withstand the peening process without causing the pin to buckle or break. Conversely, if the diameter of the pin is too large, the upper limit of 7 mm is too large. If the diameter is larger than this, the cross-sectional area of the pin is too large, and although the impact force is sufficient, the impact stress may not reach a predetermined value. It was set.
[0110]
【Example】
Hereinafter, examples of the present invention will be described.
[0111]
2 and 3 are schematic diagrams of the fatigue test method used in this example. The degree of constraint of the members during actual welding may be determined by numerical calculation such as the finite element method or by applying a load to the groove before welding and measuring the change in the groove width at that time. . However, with such a method, the degree of constraint cannot always be arbitrarily controlled, and there is also a problem that the test cost is enormous. In view of the problems of these test methods, in the present embodiment, as shown in FIGS. 2 and 3, when a fatigue test piece is produced by welding, a fatigue test method designed to arbitrarily determine the degree of constraint is set. It is. Here, two types of joints shown in FIGS. 2 and 3 were prepared as the shape of the fillet weld joint. In each of the joints, a fatigue test piece was fixed in advance with jigs 4 and 5 before welding. This is to keep the degree of constraint of the weld joint constant. Next, fillet welding was performed in this state to produce a fatigue test piece. The fillet welding of the fatigue test piece was performed using a wire and CO₂The welding was performed under the conditions of a constant current of 125 A and a voltage of 17 V, and the heat input during welding was adjusted by changing the welding speed. Usually, the welded joint used for the fatigue test piece is machined so that the start portion and the crater portion of the weld bead do not remain on the test piece. However, depending on the actual structure, there are cases where it is impossible or technically and economically difficult to delete the start portion and the crater portion. Therefore, the joint shape shown in FIGS. 2 and 3 is such that the fatigue behavior of such a structure can be reproduced. The joint in FIG. 2 (hereinafter referred to as joint A) has a U-shaped weld bead, and the joint in FIG. 3 (hereinafter referred to as joint B) has a V-shape. It is clear that the start portion 1 and the crater portion 2 of both the joints A and B are structural stress concentration portions in addition to the condition that the beads are easily disturbed. In addition to the start part 1 and the crater part 2, the corner part 3 is a structural stress concentration.
[0112]
In addition, the degree of constraint (R_F) Changes the distance (L in FIGS. 2 and 3) while the test piece is fixed by the jigs 4 and 5 to arbitrarily set the degree of constraint calculated using the following equation (1). did.
R_F(N / mm · mm) = E (N / mm2) · H (mm) / L (mm) · · · (1)
Where R_F: Constraint degree, E: Young's modulus, H: Test material thickness, L: Distance between fixed ends (L)
[0113]
For the fatigue test specimens prepared by welding, those with and without the peening treatment at the start and crater parts were prepared, and the fatigue test was performed by applying a fatigue load in the direction of the arrows shown in FIGS. went. Fatigue strength indicates a load that does not break even when a load of 5 million times is applied. For example, a fatigue strength of 1000 N means that a stress ratio of 0.1 and a load of 111 to 1111 N are 5 million. It does not break even if it is repeatedly loaded, and it means that it breaks with less than 5 million repetitions in a stress range exceeding it. The fatigue rupture was determined by attaching strain gauges to the start, crater, and corner of the test piece. It is considered to have been done. In addition, since the strain gauge was attached to the test piece after welding, the influence of welding residual stress was not included. The penetration depth of the weld metal in the fatigue test piece was measured by measuring a penetration depth 6 shown in FIG. 4 by taking a cross-sectional macro test piece from the test piece after finishing the fatigue test. Similarly, for the amount of dent when peening was performed, a macro test piece was sampled after the end of the fatigue test, and the amount of dent 7 shown in FIG. 5 was actually measured.
[0114]
Table 1 shows the composition of the weld metal, the transformation starting temperature, the tensile strength, and the Charpy absorbed energy at 0 ° C., which were measured by collecting test specimens from the weld metal parts of a plurality of fatigue test specimens prepared under the same welding conditions. The transformation start temperature of the weld metal was measured using a Formaster test, and the Charpy absorbed energy at 0 ° C. was determined by performing an all-depot test under welding conditions of 270A-30V-25 cm / min according to JIS Z3111. However, the weld metal No. shown in Table 1 corresponding to the C-based weld metal specified by the present invention. Regarding 1 and 2, since the C content is high and the possibility of high-temperature cracking is high, an all-deposit test was conducted under the welding conditions at the time of producing the fatigue test piece, that is, 125A-17V-40cm / min, in order to prevent this. Was.
[0115]
In Table 1, the weld metal symbols A, B, E and F satisfy the transformation start temperature of the weld metal defined in the present invention: 475 to 600 ° C. C: 0.2 to 0.4% of C-based weld metal specified in the present invention, and the weld metals E and F are Ni: 2.0 to less than 4.0% specified in the present invention (C: 0 to less than 4.0%). (Less than 0.3% to less than 0.2%). The weld metals H and I satisfy the transformation start temperature of the weld metal specified in the present invention: 400 to less than 475 ° C., and the Ni specified in the present invention is 4.0 to 7.5% (C : 0.03 to less than 0.2%). Further, the weld metals C, D, and G fall outside the transformation start temperature range of the weld metal specified in the present invention. When the respective mechanical properties of the weld metals A, B, E, F, H and I are compared, they all have the same level of tensile strength, but the Ni-based welds of the weld metals E, F, H and I according to the present invention are specified. The Charpy absorbed energy at 0 ° C. of the metal was higher than 100 J, which was higher than that of the C-type weld metal of the present invention A and B (vE0: 70 to 75 J).
[0116]
Table 2 shows the weld metal symbols shown in Table 1 and the conditions for producing the fatigue test pieces. Joint shapes A and B are joints A and B shown in FIGS. 2 and 3, and the penetration depth is a result of measuring a cross-sectional macro taken from a test piece. The constraint is a value calculated by the equation (1) using the plate thickness and L. In addition, with respect to the conditions outside the range of the present invention, the items outside the range are also listed in Table 2.
[0117]
Peening treatment was performed on the test pieces in Table 2 under various conditions, and fatigue test pieces were prepared and subjected to a fatigue test. Tables 3 and 4 show the results of the fatigue test of the joint A whose joint shape is shown in FIG. 2, and Tables 5 and 6 show the results of the joint B of FIG. Since different plate thicknesses were included in each joint, the results of the fatigue test were shown for cases where the plate thickness was 2 mm or more and cases where the plate thickness was less than 2 mm. Tables 3 and 5 show cases where the plate thickness is 2 mm or more, and Tables 4 and 6 show cases where the plate thickness is less than 2 mm. In addition, the range of the peening in each table shows the range where the peening was performed from both ends of the start portion and the crater portion of the weld bead.
[0118]
Test No. shown in Table 3 No. 2, although conditions other than the peening treatment were within the scope of the present invention, since the peening treatment was not performed, fatigue occurred from the start part and the crater part, and the fatigue limit of 5 million times was 11.5 kN. Is the case. Test No. No. 3 is a case where the peening range was too narrow, and the occurrence of fatigue in the start portion and the crater portion was not sufficiently prevented. Test No. Nos. 4 and 8 show that the transformation start temperature and strength of the weld metal were out of the range of the present invention, and the peening was performed and the fatigue strength of the start part and the crater part was improved, but the generation of fatigue from the corner part could not be prevented. It is. Test No. 5 is the test No. 5. 4 shows the case where the peening treatment was omitted, and the case where the fatigue strength was the lowest in Table 3. Test No. No. 6 is peening condition, steel material and weld metal are within the range of the present invention, but the degree of restraint under welding condition is high (Because weld metal E of test No. 6 has a transformation start temperature of 530 ° C. from Table 2) In order to fall within the scope of the present invention, the degree of constraint needs to be 4000 N / mm · mm or less), where the residual stress is insufficiently reduced and the fatigue strength at the corners is insufficient. Test No. No. 7 is a case where the pin diameter was too thin as 1 mm, the pin was broken during the peening, and the peening treatment was insufficient. On the other hand, the test Nos. No. 1 has a fatigue strength of 20.3 kN, which is about twice that of other joints.
[0119]
Table 4 is an example in which the joint shape is the same as Table 3, but the plate thickness is less than 2 mm. Test No. No. 12, the steel material strength was out of the range of the present invention example, and although the transformation start temperature of the weld metal was within the range of the present invention example, the reaction force from the steel material was insufficient, so that the welding residual stress could not be sufficiently reduced. Is the case. Test No. No. 13 shows a case where the peening treatment was performed excessively, and the dent amount was reduced to 1/3 of the plate thickness, the plate thickness was reduced, the local stress was increased, and the fatigue strength was not improved. Is the case. On the other hand, even with the same steel material and weld metal, Test No. In No. 11, the fatigue strength of Test No. 11 was determined because the peening condition was within the range of the present invention. It is about twice as high as 12 and 13.
[0120]
Table 5 shows the fatigue test results when the joint shape is B in FIG. 3 and the plate thickness is 2 mm. Test No. 21 is a case where the weld metal is G in Table 2 and the transformation start temperature was out of the range of the present invention example. A fatigue crack was generated from the corner portion in FIG. 3 and the fatigue strength was insufficient. Is the case. Test No. Test No. 22 is Test No. In the case where the peening process was not performed at 21, the fatigue strength of the start portion and the crater portion was lower than that of the corner portion, and corresponds to the case where the fatigue strength is lowest in Table 5. Test No. In No. 23, the peening range was as narrow as 5 mm, and the fatigue strength of the start portion and the crater portion was insufficient. Test No. 24 is the test No. 24. 23 is a case where the peening range is within the range of the present invention, and a case where the fatigue strength exceeds 20 kN. Test No. In the case of No. 25, the joint restraint degree was 8400 N / mm · mm, which is the highest among the conditions in Table 2, and the transformation start temperature of the weld metal was 430 ° C., which was in the range of the present invention, but the residual stress reduction was insufficient. Therefore, this is the case where a fatigue crack occurs from the corner. In order to reduce the residual stress at such a high degree of constraint, it is necessary to use a low-temperature transformation welding material according to the related art as disclosed in Patent Document 4. Test No. No. 26 is the case where the steel material strength was as low as 240 MPa and the steel material reaction force was small, so that the residual stress could not be sufficiently reduced. Test No. No. 27 is a case where the hardness of the pin is too low to perform the peening process sufficiently, resulting in an insufficient dent amount. Test No. In Test No. 28, the diameter of the pin was 10 mm, which is out of the range of the present invention. Similarly to the case 27, the peening effect is insufficient. Test No. No. 29 is the condition No. in Table 2. As shown in FIG. 13, the plate thickness was as thick as 5 mm, and a temperature difference still remained in the plate thickness direction even in the temperature region where the weld metal was transformed, so that the constraint from the back surface of the plate could not be removed, and the residual stress reduction effect was poor. It is enough. For these tests, Test No. 24 is within the range of the present invention, and in Table 6, only the case where the fatigue strength exceeds 20 kN, the effect of improving the fatigue strength is apparent.
[0121]
Table 6 is an example when the joint shape is B in FIG. 3 and the plate thickness is less than 2 mm. Test No. No. 31 is a case where the plate thickness is 0.6 mm, which is out of the range of the present invention, and as a result, the penetration depth exceeds 1/3 of the plate thickness, and the residual stress reduction is insufficient. Test No. No. 33 is a case where the plate thickness was within the range of the present invention example, but the welding conditions for the plate thickness were not appropriate, and the penetration was reduced to half of the plate thickness, resulting in insufficient reduction of residual stress. Test No. For Test No. 33, No. 32 shows the case where the welding conditions were properly selected and the penetration depth was 1/3 or less of the plate thickness even with the same plate thickness, and the fatigue strength was 9.5 kN, which was about twice that of the other two comparative examples. Shows the strength.
[0122]
[Table 1]

[0123]
[Table 2]

[0124]
[Table 3]

[0125]
[Table 4]

[0126]
[Table 5]

[0127]
[Table 6]

[0128]
【The invention's effect】
As described above, according to the present invention, the fatigue strength of a welded joint can be significantly improved as compared with a conventional joint. Therefore, the present invention is an invention having extremely large industrial value.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram of a lap joint portion.
FIG. 2 is a diagram illustrating a test piece shape and a fatigue load application direction of a joint A.
FIG. 3 is a diagram illustrating the shape of a test piece of a joint B and a direction in which a fatigue load is applied.
FIG. 4 is a conceptual diagram illustrating a penetration depth.
FIG. 5 is a diagram illustrating the amount of dent at the weld toe.
[Explanation of symbols]
1 Start section
2 Crater section
3 corner
4 Jig
5 jig
6 penetration depth
7 dent amount

Claims (19)

In a fillet welded joint of a thin steel sheet having a weld toe, a steel sheet having a thickness of 1.0 to 4.0 mm and a tensile strength of 590 MPa or more, and a penetration depth of 1 mm of the thickness of the steel sheet / 3 or less, the temperature at which transformation from austenite to martensite or bainite starts is 475-600 ° C., and the tensile strength is 590 MPa or more. A high fatigue strength fillet weld joint characterized in that the weld toe is recessed from the surface of the steel material by 0.03 mm or more and 1/4 or less of the plate thickness over a range of at least 10 mm or more from the ends of the crater portion and the crater portion. The weld metal is, in mass%, C: 0.2 to 0.4%, Si: 0.1 to 0.8%, Mn: 0.4 to 2.0%, P: 0.03% or less, The high-fatigue-strength fillet welded joint according to claim 1, wherein S contains 0.02% or less, and the balance consists of iron and inevitable impurities. The weld metal further contains, by mass%, one or more of Ni, Cr, Mo, Cu, V, Nb, Ti, Ca, B, and Mg in a total amount of 0.001 to 1.0. The high fatigue strength fillet welded joint according to claim 2, wherein C: 0.03 to less than 0.2%, Si: 0.1 to 0.8%, Mn: 1.0 to 2.0%, P: 0.03% or less by mass% of the weld metal. , S: 0.02% or less, Ni: 2.0 to less than 4.0%, the balance being iron and unavoidable impurities, the high fatigue strength fillet welded joint according to claim 1, characterized in that: . The weld metal further contains, by mass%, one or more of Cr, Mo, Cu, V, Nb, Ti, Ca, B, and Mg in a total amount of 0.001 to 1.0%. The high-fatigue-strength fillet welded joint according to claim 4, characterized in that: In a fillet welded joint of a thin steel sheet having a weld toe, a steel sheet having a thickness of 1.0 to 4.0 mm and a tensile strength of 590 MPa or more, and a penetration depth of 1 mm of the thickness of the steel sheet / 3 or less, a temperature at which transformation from austenite to martensite or bainite is started is 400 to less than 475 ° C., and a tensile strength is 590 MPa or more. High fatigue strength fillet welding characterized in that the weld toe is recessed from the surface of the steel material by 0.03 mm or more and 1/4 or less of the sheet thickness over a range of at least 10 mm or more from the ends of the start part and the crater part. Fittings. C: 0.03 to less than 0.2%, Si: 0.1 to 0.8%, Mn: 1.0 to 2.0%, P: 0.03% or less by mass% of the weld metal. The high-fatigue-strength fillet welded joint according to claim 6, characterized in that it contains 0.02% or less of S and Ni and 4.0 to 7.5% of Ni and the balance consists of iron and unavoidable impurities. The weld metal further contains, by mass%, one or more of Cr, Mo, Cu, V, Nb, Ti, Ca, B, and Mg in a total amount of 0.001 to 1.0%. The high fatigue strength fillet welded joint according to claim 7, wherein In the method of fillet welding of a steel sheet, a steel sheet having a thickness of 1.0 to 4.0 mm and a tensile strength of 590 MPa or more is used, and a degree of constraint of a welded portion of the steel sheet is 4000 N / mm · mm or less, and Welding in which the penetration depth of the weld metal in the welded portion is 1/3 or less of the thickness of the steel sheet, the transformation start temperature of the weld metal is 475 to 600 ° C, and the tensile strength is 590 MPa or more in the welded portion. A method for improving the fatigue strength of a fillet weld, comprising forming a metal and peening a weld toe over at least 10 mm or more from an end of a start portion and a crater portion of a weld bead of the weld. The weld metal is, in mass%, C: 0.2 to 0.4%, Si: 0.1 to 0.8%, Mn: 0.4 to 2.0%, P: 0.03% or less, 10. The method for improving fatigue strength of a fillet weld according to claim 9, wherein the content of S is 0.02% or less, and the balance consists of iron and inevitable impurities. The weld metal further contains, by mass%, one or more of Ni, Cr, Mo, Cu, V, Nb, Ti, Ca, B, and Mg in a total amount of 0.001 to 1.0. The method for improving the fatigue strength of a fillet weld according to claim 10, wherein C: 0.03 to less than 0.2%, Si: 0.1 to 0.8%, Mn: 1.0 to 2.0%, P: 0.03% or less by mass% of the weld metal. , S: 0.02% or less, Ni: 2.0 to less than 4.0%, the balance being iron and unavoidable impurities, the improvement of the fatigue strength of the fillet weld according to claim 11, Method. The weld metal further contains, by mass%, one or more of Cr, Mo, Cu, V, Nb, Ti, Ca, B, and Mg in a total amount of 0.001 to 1.0%. The method for improving the fatigue strength of a fillet weld according to claim 12, wherein In the method for fillet welding of a steel sheet, a steel sheet having a thickness of 1.0 to 4.0 mm and a tensile strength of 590 MPa or more is used, and a degree of constraint of a welded portion of the steel sheet is 8000 N / mm · mm or less, and The depth of penetration of the weld metal in the weld portion is 1/3 or less of the thickness of the steel sheet, and the transformation start temperature of the weld metal in the weld portion is less than 400 to 475 ° C, and the tensile strength is 590 MPa or more. A method for improving fatigue strength of a fillet weld, comprising forming a weld metal and peening a weld toe over at least 10 mm or more from the ends of a start portion and a crater portion of a weld bead of the weld. . C: 0.03 to less than 0.2%, Si: 0.1 to 0.8%, Mn: 1.0 to 2.0%, P: 0.03% or less by mass% of the weld metal. 15. The improvement of the fatigue strength of the fillet weld according to claim 14, wherein the content of S is 0.02% or less and the content of Ni is 4.0 to 7.5%, with the balance being iron and unavoidable impurities. Method. The weld metal further contains, by mass%, one or more of Cr, Mo, Cu, V, Nb, Ti, Ca, B, and Mg in a total amount of 0.001 to 1.0%. The method for improving the fatigue strength of a fillet weld according to claim 15, wherein: The method for improving the fatigue strength of a fillet weld according to any one of claims 9 to 15 or 16, wherein a method using ultrasonic waves having a frequency in the range of 20 kHz to 60 kHz is used as the peening method. . At the time of peening, one or a plurality of pins having a diameter in the range of 1.5 mm to 7.0 mm are used at the tip portion that applies an impact to the welded portion, and the hardness of the pin tip is Vickers hardness. The method for improving the fatigue strength of a fillet weld according to claim 17, wherein a pin having a value of 450 or more and 900 or less is used. The high-fatigue-strength fillet welded joint according to any one of claims 1 to 7 or 8, which is produced using the method for improving the fatigue strength of a fillet weld according to claim 9 to 17.

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