JP4523755B2 - Manufacturing method of high fatigue strength fillet welded joint of high strength thin steel sheet - Google Patents

Description

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【表２】

【０１１２】
【表３】

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【表４】

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【表５】

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【表６】

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【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 治具
５ 治具
６ 溶け込み深さ
７ へこみ量[0001]
BACKGROUND OF THE INVENTION
  The present invention is particularly suitable for thin steel sheets applied to the production of undercarriage parts for automobiles.High fatigue strengthFillet weldingFor joint manufacturing methodsFor more details,TensionHigh fatigue strength fillet welded joint of thin steel sheet with a strength of 590 MPa or moreMakingIt is about the method.
[0002]
[Prior art]
  Fatigue has a significant impact on the safety and reliability of welded steel structures.CrackSince it is easy to generate | occur | produce in a welding part, the method of improving the fatigue characteristic of the welding part of a welded steel structure has been examined conventionally.
[0003]
  Traditionally the most fatigued weldCrackIs the weld toe, and the main cause is the tensile residual stress that tends to occur at the weld toe.And weld toe shapeIt is known that the stress concentration is due to.
[0004]
  Therefore, as methods for improving the fatigue characteristics of conventional welded joints, methods for improving the weld toe shape by machining such as TIG tanning welding (decorative welding) or grinding after welding, and improving the weld toe shape by peening, etc. And a method of simultaneously introducing compressive residual stress. If these methods are not applied to all the weld lines, the fatigue strength of the entire joint cannot be ensured, the work load increases, and this is not economically preferable. Recently, many techniques have been disclosed in, for example, the United States of America regarding peening using ultrasonic waves when performing peening, that is, ultrasonic peening techniques (eg, 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 this can be expected to reduce the peening work load. However, in order to improve the fatigue strength of the entire welded joint, there is no change in that peening must be performed over the entire weld line, and the problem that the work process increases and the economic load increases is still undecided. Remains.
[0005]
  On the other hand, recently, the composition of the welding material used for welding is designed so that the transformation temperature of the weld metal is lowered, and the compression of the weld toe is introduced by using the volume expansion associated with transformation during welding.TensionA technique for reducing residual stress and improving fatigue characteristics 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, the weld metal is martensitic transformed at a low temperature region where the transformation start temperature is 170 ° C. to 250 ° C. by welding using the low temperature transformation welding material, and the volume expansion is thereby caused. To cancel the tensile stress of the weld toe at room temperatureTensionTechniques for reducing residual stress or compressive residual stress are disclosed.
[0006]
  This technique using low-temperature transformation expansion of weld metal is a post-welding post-processing technique 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. It is an economically superior method that requires fewer work steps and saves labor costs.
[0007]
  However, the technique using the low temperature transformation expansion of the weld metal disclosed in Patent Document 4 has three main problems.
[0008]
  That is, (1) the low temperature transformation welding material used for welding must add a lot of expensive alloy elements in order to lower the transformation temperature, and the cost of the welding material is high accordingly, (2) the same alloy elements The workability at the time of welding construction deteriorates due to the reason why it is added in a large amount, resulting in deterioration of work efficiency and high work cost. For example, the weld metal becomes a hard structure mainly composed of martensite, and mechanical properties, particularly toughness, are deteriorated.
[0009]
  As described above, there are still problems to be solved in the method using the transformation expansion of the weld metal and the ultrasonic peening method, and the fatigue strength of the high fatigue strength welded joint and the welded joint that solved these problems. An improvement method was strongly desired.
[0010]
[Patent Document 1]
  US 6171415 B1
[Patent Document 2]
  US 6338765 B1
[Patent Document 3]
  US 2002/0014100 A1
[Patent Document 4]
  JP 11-138290 A
[0011]
[Problems to be solved by the invention]
  The technology to improve the fatigue strength by sufficiently lowering the transformation start temperature of the weld metal is particularly advantageous because it does not require a new manufacturing process, but there are problems such as poor workability and high cost. is doing. On the other hand, the method of improving the fatigue strength by peening the conventional joint must be performed on all the weld lines, which leads to an increase in manufacturing cost.
[0012]
  In view of the above-mentioned problems of the prior art, the present invention sufficiently improves the fatigue strength of a welded joint by utilizing transformation expansion under conditions where the transformation temperature of the weld metal is higher than that of the prior art, By limiting to the start part and crater part, which are likely to be disturbed, the amount of expensive alloying elements required for conventional low-temperature transformation is greatly reduced, and the peening range is narrowed, making it more economical than before And excellent weld metal toughness,High strengthHigh fatigue fillet welded joint of thin steel sheetMakingIt aims to provide a method.
[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 method for producing a fillet welded joint using a high-strength thin steel plate having a plate thickness of 1.0 to 4.0 mm and a tensile strength of 590 MPa or more,
  Of the steel sheetIt is shown by the following formula (1)Restraint degree of weld(RF)Is 8000 N / mm · mm or less, the penetration depth of the weld metal in the weld is 1/3 or less of the plate thickness of the steel plate, and the weld metal has a transformation start temperature of 400 to 600 ° C. and a tensile strength. In the case of fillet welding the steel plate so that the weld metal is 590 MPa or more,
  When the transformation start temperature of the weld metal is 475 ° C. to 600 ° C., the degree of restraint(RF)When the transformation start temperature of the weld metal is 400 ° C. to less than 475 ° C., the restraint degree is 4000 N / mm · mm or less.(RF)Fillet welded the steel plate with 8000 N / mm · mm or less,
  Then, further, ultrasonic peening treatment is performed on the weld toe over a range of 10 to 100 mm from the start of the weld bead and the end of the crater of the weld, and the high fatigue strength of the high strength thin steel sheet A method for producing a strength fillet welded joint.
RF (N / mm · mm) = E (N / mm ² ) ・ H (mm) / L (mm) ・ ・ (1)
In the above formula, RF: restraint degree, L: length between both fixed ends when fixing both ends of a test piece having a groove portion at the center, H: plate thickness, E: Young's modulus.
[0015]
  In addition, the said weld metal is the mass%, C: 0.2-0.4%, Si: 0.1-0.8%, Mn: 0.4-2.0%, P: 0.03% Hereinafter, S: 0.02% or less is contained, and if necessary, one or more of Ni, Cr, Mo, Cu, V, Nb, Ti, Ca, B, and Mg are combined in a total amount. It is preferable to contain 0.001-1.0% of the weld metal, the balance being iron and inevitable impurities.
[0016]
  Moreover, the said weld metal is the mass%, C: 0.03-less than 0.2%, Si: 0.1-0.8%, Mn: 1.0-2.0%, P: 0.03 % Or less, S: 0.02% or less, Ni: 2.0-7.5%, if necessary, among Cr, Mo, Cu, V, Nb, Ti, Ca, B and Mg It is also preferable that one or two or more types are contained in a total amount of 0.001 to 1.0%, and the balance is a weld metal made of iron and inevitable impurities.
[0017]
  Also, the aboveUltrasoundPeeningprocessingAs a method, it is preferable to use a method using ultrasonic waves having a frequency in the range of 20 kHz to 60 kHz.
[0018]
  Also, the aboveUltrasoundPeeningprocessing1 or more pins having a diameter in the range of 1.5 mm to 7.0 mm are used for the tip portion that gives an impact to the welded portion, and the hardness of the tip of the pin is Vickers hardness It is preferable to use pins that are 450 or more and 900 or less.
[0019]
The degree of the ultrasonic peening treatment is preferably such that the weld toe is recessed 0.03 mm or more and 1/4 or less of the plate thickness from the steel surface.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
  The present invention is described in detail below.
[0021]
  First, the technical idea of the present invention will be described. The technical idea of the present invention isThere are two major, the first technical idea is, Technology to reduce welding residual stress using transformation expansion of weld metalThe second technical idea isThe present invention relates to an ultrasonic peening process. at firstFirstDescribe the technical idea.
[0022]
  The present invention provides a welding method using a conventional low temperature transformation melt having a martensitic transformation start temperature of 170 to 250 ° C., that is, the transformation of the weld metal in a relatively low temperature region during welding to the weld toe. Compared with the method of reducing the tensile residual stress of the weld toe by generating a compressive stress and maintaining the compressive stress to room temperature, the compressive stress is generated at the weld toe using the transformation expansion of the weld metal. Although it is the same in that it is used, a weld metal having a relatively high transformation start temperature of 400 to 600 ° C. is used.The point is greatly different.
[0023]
  Considering the generation process of residual stress in the weld during welding, when the weld metal is solidified and cooled after welding and reaches its transformation start temperature, the weld metal undergoes volume expansion due to transformation, and the surrounding base metal heat affected zone Compressive stress is generated at the weld toe due to the reaction force.
[0024]
  At this time, when 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.TensionThe residual stress at the weld toe when the stress is generated and cooled to room temperature isTensionIt becomes a stress state. Therefore, the conventional welding technology using low-temperature transformation welding material reduces the temperature difference from the end of 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 thermal shrinkage during this period and shift the residual stress at the weld toe to the compressive stress side at room temperature.
[0025]
  In contrast to thisIn the lightDoes not require the transformation start temperature of the weld metal to be in a low temperature range (250 ° C. or lower) as in the prior art, that is, transformation expansion of the weld metal occurs in a very high temperature range of 400 ° C. to 600 ° C. However, it occurs at the weld toe due to cooling and thermal contraction from the end of the transformation expansion to room temperature.TensionThe purpose is to maintain the compressive stress generated at the weld toe during transformation expansion by suppressing the stress itself, and to transfer the residual stress at the weld toe at room temperature to the compressive stress side.
[0026]
  In more detail,Tomorrow(1) In order to generate a compressive stress at the weld toe using the volume expansion accompanying the transformation between the start of transformation expansion of the weld metal in the high temperature range and the end of transformation expansion, MaterialTensionWhile ensuring the strength above the predetermined value, limit the penetration depth of the weld metal below the predetermined value. thisTherefore, the volume expansion accompanying the transformation expansion of the weld metal is suppressed by the base metal part including the heat-affected zone under and around the weld metal, and the weld metal is subjected to the restraint force of the transformation expansion and the weld toe part. , The residual stress of compression is introduced as a reaction force generated due to the restraint force of the transformation expansion of the weld metal. (2) The transformation stress of the weld metal is terminated by the introduction of the compression stress by the mechanism of (1) above. Then, in the process of cooling and heat shrinking to room temperature, by freely shrinking without constraining the heat shrinking part, at the weld toe due to heat shrinkingTensionSuppresses the generation of stress. If this is possible, the compressive stress introduced by the above (1) is maintained even after cooling to room temperature. For this purpose, first, it is necessary to complete the heat conduction of the welding heat to the base material including the heat-affected zone below by the time the transformation expansion of the weld metal is completed by reducing the plate thickness to a predetermined value or less. This means that the temperature difference in the plate thickness direction is eliminated and the weld toe portion on the surface where the compressive stress is generated is not restrained from the plate back surface. 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 the same, so the front surface is not constrained 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 shrinkage to room temperature as much as possible after the weld metal finishes transformation expansion. This realizes a state where the weld toe portion where the compressive stress is generated is not restrained not only from the back surface but also from the surroundings. Through this process, the compressive stress generated when the weld metal undergoes transformation expansion is caused by subsequent thermal contraction.TensionIt is maintained until cooled to room temperature without changing to stress.
[0027]
  The present inventionFirst inThe technical idea is to maintain the compressive stress accompanying the transformation expansion of the weld metal, which occurs at a relatively high temperature, to room temperature by reducing the temperature difference in the thickness direction and reducing the degree of restraint. .
[0028]
  BookinventionSecond inThe technical idea is to apply ultrasonic peening to the start and crater portions of the weld bead to ensure the fatigue strength of the portions. In addition to the problem that the bead shape is likely to be disturbed, the start portion and the crater portion are portions where stress concentration tends to occur structurally. Therefore, in the present invention, about the weld toe other than the start part and the crater partFirstThe fatigue strength is ensured by using the technical idea, and the fatigue strength is ensured by using ultrasonic peening for the start portion and the crater portion, thereby realizing high fatigue strength as a whole welded joint. Thus, by limiting the ultrasonic peening process to the vicinity of the start portion and the crater portion, it is possible to reduce the load of the ultrasonic peening work as much as possible.
[0029]
  The present invention is characterized in that peening using ultrasonic waves is used as the peening process. When ultrasonic peening is used, the time for peening treatment is shortened accordingly, and the merit is great. The purpose of the present invention is to provide a technique for improving the fatigue strength by a method that is as simple as possible. Therefore, the significance of using ultrasonic peening is great.
[0030]
  The followingBright cornerMeat welding method and construction of welded joint using the sameLimit ofThe reason is described.
[0031]
  (Limitation of base metal plate thickness 1.0 to 4.0 mm)
  The reason why the base material plate thickness is limited based on the present invention will be described.
[0032]
  In the present invention, the thickness of the steel material is reduced in order to transmit the welding heat immediately 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 back surface of the plate occur simultaneously. The thicker the steel plate, the longer it takes to transfer the welding heat, and even if the weld metal finishes transformation, the welding heat is not transmitted to the back surface. Is restrained by the steel material below it,TensionStress is generated.
[0033]
  In the present invention, a welding material having a transformation start temperature nearly 200 ° C. higher than the transformation temperature of the conventional low temperature transformation melt is used, and the transformation start temperature of the weld metal is as high as 600 ° C. to 400 ° C., and the transformation expansion is completed. High temperature. ThatTherefore, in order to suppress the thermal contraction of the weld metal caused by cooling from such a high temperature region to room temperature and to keep the compressive residual stress at the weld toe generated by the transformation expansion of the weld metal to room temperature. At least when the transformation expansion of the weld metal ends, heat must be transferred to the back surface.
[0034]
  In addition, in order to control the transformation start temperature at a low cost, when welding using a component-based welding material in which expensive alloy elements are reduced and C is added to a higher level, the amount of C in the weld metal becomes higher. In particular, solidification cracking is likely to occur during butt solidification when the steel plate is thick.
[0035]
  Butt solidification, which causes solidification cracking, occurs when the steel plate thickness increases and the heat capacity of the steel itself increases, so that welding heat is easily transmitted in the width direction of the weld bead.. ThatFor this reason, when welding using a high-C component-based welding material, it is necessary to reduce the thickness of the steel plate from the viewpoint of preventing solidification cracking of the weld metal.
[0036]
  Furthermore, it is advantageous that the thickness of the steel material is thin from the viewpoint of reducing the degree of restraint described later. The restraint that the weld metal undergoes thermal shrinkage is restrained not only from the back side of the steel material but also from the entire structure of the welded joint. To reduce this restraint, it is meaningful to reduce the steel plate thickness. is there.
[0037]
  When the steel plate thickness exceeds 4.0 mm, the transfer of welding heat to the back surface becomes slow, and the welding heat is not transmitted to the back surface at the end of the transformation of the weld metal. The weld metal is restrained from the bottom of the steel material andTensionSince stress is generated, the fatigue strength of the welded joint decreases. Also, when welding using high-C component welding materials, there is a danger of solidification cracking of the weld metal.SteepGet higher. Furthermore, the degree of restraint determined by the relationship with the structure of the welded joint becomes high, and a sufficiently low degree of restraint cannot be obtained.
[0038]
  On the other hand, when the steel plate thickness is reduced to less than 1.0 mm, the penetration depth of the weld metal described later is relative to the plate thickness.Control by valueNot only is it difficult to limit, but the base metal portion directly below the weld metal acting as a reaction force during transformation expansion of the weld metal is reduced, and it is difficult to introduce compressive stress to the weld toe. Therefore, in the present invention, the upper limit of the steel thickness is 4.0 mm, and the lower limit is 1.0 mm.
[0039]
  (Restriction of 4000N / mm · mm or less or 8000N / mm · mm or less)
  Main departureBright and meltedThe reason for limiting the degree of restraint of the joint will be described.
[0040]
  Conventionally, the degree of restraint of a welded joint is a parameter that is generally used to evaluate cracks that occur in a weld during welding. In the present invention, however, the cooling and heat shrinkage processes after the transformation expansion of the weld metal is completed. It was adopted as an index that quantitatively shows how strongly the weld metal is constrained from its surroundings.
[0041]
  In general, the degree of restraint (RF) is the welding line direction required to shrink the weld groove by the unit length.MukaiPer unit lengthGet inFrom the relationship between the length (L) between the two fixed ends, the plate thickness (H), and the Young's modulus (E) when both ends of the test piece that is defined as a load and has a groove portion at the center are fixed When the groove width is sufficiently small with respect to the distance (L) between the fixed ends, the following equation (1) is given.
    RF (N / mm · mm) = E (N / mm²) · H (mm) / L (mm) (1)
The unit of restraint (RF) is conventionally expressed as N / mm · mm.
[0042]
  From the relationship of equation (1), the degree of restraint (RF) is the thickness of the steel material during welding (H)Or by increasing the distance (L) between the fixed ends determined by the welded joint structure. As a method of adjusting the degree of restraint (RF) in actual welding construction, the method of changing the distance of the fixed end by devising the restraining jig, or the design of the welding member, the thickness H of the steel plate is changed. A method to make it possible is considered.
[0043]
  Main departureIn the lightIn the process of cooling and heat shrinkage from the end of transformation expansion of the weld metal to room temperature, the heat shrinkage of the weld metal is close to free shrinkage, and at the weld toeTensionIn order to suppress the occurrence of stress, the upper limit of the steel plate thickness is restricted to 4 mm or less as described above, and the upper limit of the degree of restraint (RF) is used as an index of the restraint state from the surroundings when the weld metal is thermally contracted. It regulates like this.
[0044]
  As described above, in order to maintain the residual stress at the weld toe at room temperature on the compression side with the aim of improving the fatigue strength of the welded joint, (1) transformation from the start of transformation expansion of the weld metal At the time of volume expansion until the end of expansion, the stress generated by restraining the expansion and the reaction force generated in the surrounding base metal heat-affected zone are secured, and a compressive stress is generated at the weld toe ( 2) At the time of thermal contraction from the end of transformation expansion of the weld metal to room temperature, the restraint from the periphery of the weld metal is reduced to allow free contraction, and at the weld toe.TensionIt is necessary to suppress the generation of stress. Among these, the upper limit regulation of the degree of restraint is the weld toe portion in the thermal contraction of the weld metal of (2) above.TensionResidual stress at the weld toe at room temperature shifts to the compression side by reducing the upper limit of the degree of restraint under the same conditions that have the effect of suppressing the generation of stress and the transformation start temperature of the weld metal is the same. The fatigue strength of the joint is improved.
[0045]
  However, since the transformation start temperature of the weld metal increases, the stress generated in the weld metal during the transformation expansion of the weld metal (1) and the reaction force generated in the surrounding base metal heat-affected zone decrease. The compressive stress generated at the weld toe decreases, the transformation end temperature increases, and the temperature difference from room temperature increases, so that at the weld toe due to the thermal contraction of the weld metal of (2) above,TensionStress also increases. As a result, in order to bring the residual stress at room temperature to the compression side by the action of (2) above, it is necessary to further reduce the degree of restraint as the transformation start temperature of the weld metal increases.
[0046]
  Main departureIn the lightAs will be described later, two types of transformation starting temperatures of 475 to 600 ° C. and two different types of transformation starting from 400 ° C. to less than 475 ° C. are obtained by welding using welding materials of two types of components. In order to perform welding under temperature conditions, the upper limit value of the degree of restraint is defined as follows according to these transformation start temperatures.
[0047]
  In other words, this departureIn the lightIs400 ° C to 600 ° CAmong the transformation start temperatures, the weld metal transformation start temperature is higher.4In the case of 75 to 600 ° C., the upper limit value of the restraint degree is set lower to 4000 N / mm · mm., StrangeState start temperature is 400 ° C to 475 ° CLess thanIn this case, the upper limit value of the constraint degree is set to 8000 N / mm · mm. Even if the upper limit of any transformation start temperature is exceeded, it occurs at the weld toe due to thermal contraction after the transformation expansion of the weld metalTensionThe effect of reducing the stress becomes insufficient, making it difficult to set the residual stress at room temperature to the compression side, and the fatigue strength of the welded joint cannot be sufficiently improved.
[0048]
  (Welded metal and steelTension(Strength of 590 MPa or more)
  As a premise of the present invention, welding metal and steelTensionThe reason why the strength is limited will be described. In the base technology of the present invention, welding is performed under conditions where the transformation start temperature of the weld metal is considerably higher than before, so the weld metal and its surroundings in the volume expansion process from the transformation expansion start temperature to the transformation expansion end temperature of the weld metal. Of the base metal heat affected zoneTensionThe strength is considered to be considerably lower than before. Also, in welding using conventional low-temperature transformation welding materials, the weld metal is an alloy component.ManyBecause it is a component system with high hardenability and uses volume expansion due to martensitic transformation, due to the hard structure of martensite during transformation expansion of the weld metal, sufficient strength of the weld metal is secured during transformation expansion. be able to. However, this departureMysteriousIn welding using high-temperature transformation welding material, compared to the case of low-temperature transformation welding material, weld metal is a component system with less alloy components and low hardenability, compared with the case of using low-temperature transformation welding material in the prior art. The strength of the weld metal during transformation expansion is low.
[0049]
  Main departureIn the lightAs described above, by limiting the conditions of the plate thickness and the degree of restraint, the weld toe at the weld toe that occurs at the time of thermal contraction from the end of transformation expansion of the weld metal to room temperature.TensionIt is possible to reduce stressTheIn addition to this, in order to set the residual stress at the weld toe at room temperature to the compressive stress side, sufficient compression is applied to the weld toe using the volume expansion from the start of transformation expansion to the end of transformation expansion of the weld metal. In order to generate stress, it is necessary to secure the stress generated by restraining the expansion of the weld metal and the reaction force generated in the surrounding base metal heat affected zone.Thefor that reasonTheWeld metal and steel equivalent to theseTensionStrength must be secured. For example, if the weld metal in the temperature range during the transformation expansion of the weld metalTensionWhen the strength is 0, the weld metal is plastically deformed at the time of transformation expansion of the weld metal, and the transformation expansion is merely changed to plastic strain, and the compressive stress at the weld toe is still zero. Then, even if the thermal contraction of the weld metal is suppressed until it is cooled to room temperature, and this state is maintained, the weld toe cannot be made a compressive residual stress.
[0050]
  Based on the above,In the lightIn order to secure the minimum stress generated in the weld metal and the reaction force of the surrounding base metal heat affected zone to generate sufficient compressive stress at the weld toe using volume expansion due to transformation of the weld metal Of weld metal and steelTensionThe strength was 590 MPa or more.
[0051]
  In addition, this departureIn the lightOf steel and weld metalTensionThere is no need to specify the upper limit of strength.TensionStrength is also limited. However, steel and weld metalTensionstrengthHigh degreeIn this case, since it is necessary to add a considerable amount of alloying elements to the steel material and the weld metal, from the viewpoint of improving the toughness of the weld and reducing the manufacturing cost, it is preferable that the steel material and the weld metal areTensionIt is desirable that the upper limit value of strength be 980 MPa.
[0052]
  (The regulation that the penetration depth of the weld metal is 1/3 or less of the plate thickness of the steel plate)
  Main departureBright and meltedThe reason for limiting the penetration depth of the metal contact will be described.
[0053]
  If the penetration depth of the weld metal is excessively large, the reaction force of the steel material including the heat-affected zone below the weld metal cannot be sufficiently obtained during transformation expansion of the weld metal, and the compressive residual stress at the weld toe becomes small. Fatigue strength is not improved sufficiently. For example, as shown in FIG. 1, when the penetration depth of the weld metal W is large, the unmelted portion indicated by A is reduced during transformation expansion of the weld metal, so that the expansion of the weld metal can hardly be restricted. Plastic deformation occurs, the weld metal expands almost freely, and no compressive residual stress is generated at the weld toe. On the other hand, if a method is used in which welding is performed while maintaining a high degree of restraint by restraining the weld joint structure or restraint without relying on restraint of the unmelted portion of A, welding is performed during transformation expansion of the weld metal. Although the toe part is in a compressive stress state, it becomes a weld toe part due to thermal contraction due to cooling to room temperature after the transformation of the weld metal is completed.TensionSince stress is generated and the compressive stress at the time of transformation expansion is offset, this is not an effective method.
[0054]
  In welding under the condition that the steel plate thickness is relatively thick, the problem of lowering the base metal restraint under the weld metal due to the penetration depth of the weld metal is eliminated.. However, The reason mentioned earlierIn the present invention,To ensure the thermal conductivity of the base metal under the weld metalTo steelLimit board thickness to 4mm or less. thisBecause of such thin plate thicknessIs meltedThe penetration depth of the metal must be limitedMeltReduced restraint of base metal including heat affected zone under metal contactAnd meltCan generate sufficient compressive residual stress at the contact end.Absent.As a result, the fatigue strength of the welded joint cannot be improved.
[0055]
  Main departureIn the lightDefines the penetration depth of the weld metal to 1/3 of the steel plate thickness in order to ensure sufficient restraint of the lower unmelted portion during transformation expansion of the weld metal as described above. Here, the penetration depth is the maximum penetration depth, which is the largest of the weld metals.Show valueThe steel plate thickness is the plate thickness before welding.
[0056]
  (Regulation of transformation start temperature of weld metal of 475 to 600 ° C or less than 400 to 475 ° C)
MeltingThe reason for limiting the range of the transformation start temperature of the metal contact will be described. Main departureClearlyThe transformation start temperature of the weld metal in theAccompanying the stateFatigue strength improvement technology of welded joints using volume expansionIs largelistenDifferentThe transformation expansion of the weld metal under the condition that the transformation start temperature of the weld metal is 200 ° C. or more higher than the conventional temperature is utilized. Main departureIn the lightIs not a transformation under conditions where the transformation start temperature is low as in the prior art because the transformation start temperature of the weld metal is very high, but it uses volume expansion due to bainite transformation or ferrite pearlite transformation in addition to martensite.The for that reason,Weld metal welded jointIs obedientIt becomes a bainite transformation or a ferrite pearlite main structure whose hardness is lower than the conventional hard structure mainly composed of martensite, and a weld metal having high toughness is obtained. In addition, this departureIn the lightCompared with welding using conventional low temperature transformation welding materials, the transformation start temperature of weld metal is very high. ThatTherefore, it is possible to reduce the amount of expensive alloy components added to lower the welding metal transformation start temperature in the welding material.FromThe manufacturing cost of the welding material can be reduced compared to the conventional case.
[0057]
  However, since the strength of weld metal and base metal generally decreases with increasing temperature,MysteriousThus, when welding is performed under conditions where the transformation start temperature of the weld metal is high, the strength decreases accordingly.. ThatFor this reason, the stress generated by restraining the expansion of the weld metal during transformation expansion and the reaction force generated in the base material including the heat-affected zone around it are reduced.WeirdCompressive stress generated at the weld toe during state expansion decreasesTo do. at the same timeBecause the temperature difference between the transformation end temperature and room temperature increases, the weld toe at the weld toe that occurs due to the heat shrinkage of the weld metal due to cooling between the temperatures.TensionIncreased stressTo do.As a result, it is difficult to improve the fatigue strength of the weld joint by setting the residual stress at the weld toe at room temperature to the compression side. Therefore, this departureIn the lightAs described above, by prescribing the transformation start temperature of the weld metal according to the level of restraint in welding, it is possible to freely contract at the time of thermal contraction after the transformation of the weld metal andTensionSuppresses increase in stress.
[0058]
  Main departureIn the lightClassifies the transformation start temperature condition of the weld metal in welding into two different transformation start temperature levels of 475 to 600 ° C. where the transformation start temperature is high and 400 to 475 ° C. which are lower than that.
[0059]
  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. Therefore, the weld metal of the welded joint has a bainite transformation or a ferrite pearlite main structure and is harder. Resulting in a welded joint with lower toughness and better toughnessThe for that reasonIn addition, it is possible to further reduce the amount of the expensive alloy component added to lower the transformation start temperature in the welding material, and to further reduce the manufacturing cost of the welded joint. In addition, the fatigue strength of the welded joint is sufficiently ensured by introducing compressive stress to the weld toe at a transformation start temperature of the weld metal of 475 ° C. to 600 ° C. so that the residual stress at room temperature and the compressive stress side. For this purpose, as described above, the degree of restraint needs to be regulated to 4000 N / mm · mm or less. However, even when welding under such a low constraint condition, if the transformation start temperature of the weld metal exceeds 600 ° C., it becomes difficult to make the residual stress at the weld toe part compressive stress side, and the fatigue strength of the welded joint 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 is lower than 475 ° C., the lower limit value of the transformation start temperature of the weld metal can improve the fatigue strength of the welded joint. 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 manufacturing cost.
[0060]
  When welding is performed under conditions where 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 main structure as compared with the welding conditions where the transformation start temperature of the weld metal is high. However, the toughness of the weld is slightly reduced.To do. for that reason,The amount of expensive alloy components added to reduce the transformation start temperature of the weld metal in the welding material is also increased, and the manufacturing cost of the welded joint is slightly increased.. HoweverEven if welding is performed under high restraint conditions with a restraint degree of 8000 N / mm · mm or less, the residual stress at the weld toe can be brought to the compressive stress side, and the fatigue strength of the welded joint can be sufficiently secured. It is. Therefore, it is particularly effective in welding when it is difficult to weld under construction conditions in which the degree of restraint is sufficiently reduced due to the structure of the welded joint, and the degree of freedom of the welding construction conditions can be improved.
[0061]
  Under these welding conditions, the degree of restraint is relatively high, and the influence of heat shrinkage after the end of 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 as low as less than 475 ° C., Residual stress at the weld toe is reduced by constraining the shrinkage during the heat shrinkage process after the transformation expansion of the weld metal.TensionIt shifts to the stress side and sufficient fatigue strength cannot be improved.. ThatTherefore, the upper limit of the transformation start temperature of the weld metal is set to less than 475 ° C. On the other hand, the lower limit of the transformation start temperature of the weld metal can improve the fatigue strength of the welded joint even when the transformation start temperature is lower than 400 ° C. Therefore, the lower limit of the transformation start temperature of the weld metal was set to 400 ° C. from the viewpoint of economy and manufacturing cost.
[0062]
  Next, in the present invention, from the steel material surface of the weld toepreferableDentTo quantityDescribe.
[0063]
  In the present invention, the fatigue strength of the welded joint is improved by using both the transformation expansion of the weld metal and the ultrasonic peening treatment. The method for improving the fatigue strength by using the transformation expansion of the weld metal can ensure the fatigue strength improvement effect by limiting the transformation start temperature of the weld metal or by limiting the components of the weld metal. When a peening process is performed, it is not always clear whether the process has an effect of improving fatigue strength. Accordingly, the present inventors have investigated the amount of dents that can provide an effect of improving fatigue strength, focusing on the amount of dents at the weld toe after ultrasonic peening.as a result,Dented amountButBelow 0.03mmWhenThe effect of ultrasonic peening treatment is small and fatigue strength does not improveIn some cases, it is not preferable, and the amount of dent is1/4 of the plate thicknessAbove, Causing an increase in local stress due to a decrease in plate thickness, which is undesirable from the viewpoint of improving fatigue strength as a jointIt turns out that there may be cases. Therefore, in the present invention, the degree of the ultrasonic peening treatment is preferably such that the weld toe is recessed 0.03 mm or more and 1/4 or less of the plate thickness from the steel surface.
[0064]
Next, the reason why the range of the region where the ultrasonic peening process is performed on the toe portion or the region where the indentation of the toe portion exists is limited will be described.
[0065]
  In the present invention, the purpose of performing the ultrasonic peening treatment is to ensure the fatigue strength of the start portion and the crater portion of the weld bead where the bead shape becomes defective. For this reason, it is necessary to perform an ultrasonic peening process or to cover an area where the dent of the toe portion is present to cause a bead shape defect. In particular, the parts that are problematic in terms of fatigue characteristics are the ends of the beads. Therefore, the ultrasonic peening process to the toe part or the dent to the toe part must cover at least the both end parts. And in the range of 10 mm from both ends where the bead shape is bad, the ultrasonic peening process must be carried out to ensure the dent. In the present invention, the area where ultrasonic peening is applied to the toe or the dent to the toe is the end of the start part and the crater part.Et 1If it is over 0mmLower limitThe reason for this is as described above. MaTheUpper limit of ultrasonic peening area or indentation areaaboutIn addition, the ultrasonic peening process itself increases the manufacturing cost, and in the present invention, the fatigue strength is improved by the low temperature transformation weld metal.ThisThe upper limit of 100mmThe
[0066]
  (Rules for weld metal components)
  The present inventionMeltingPreferred components for metal contactRegulationExplain why.
[0067]
  The present inventionMeltingAs an embodiment of the metal-contacting component system, depending on two different transformation start temperature levels of 475 to 600 ° C. where the transformation start temperature is relatively high and 400 ° C. to less than 475 ° C., which are lower than the following two, A variety of component systems are used.
[0068]
  As a component system of a 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 lowers the transformation start temperature mainly by adding Ni was used. In addition, 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 mainly by adding Ni (hereinafter referred to as a Ni-based component) is used. It was.
[0069]
  Among these, C-based weld metal has a small additive amount of expensive alloy 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 characteristics. This is advantageous from the viewpoint of economy when manufacturing a welded joint. On the other hand, Ni-based weld metal is disadvantageous from the viewpoint of economics of welded joints because it adds a relatively large amount of expensive Ni alloy elements, but the effect of Ni is further increased under the same conditions for the transformation start temperature of the weld metal. Can improve the toughness, and is effective when manufacturing a welded joint that requires a high toughness level as well as fatigue characteristics. The selection of the component system of these weld metals and the welding material that realizes them is selected based on the respective characteristics.
[0070]
  (Composition rules for C weld metal)
  The reasons for limiting the components of the C-based weld metal and the content thereof will be described. C is a quenching element and is an effective element from both the viewpoint of improving the strength of the weld metal and reducing the transformation temperature. The lower limit of the C content is 0.2%. If the addition amount is less than this, the transformation start temperature of the C-based weld metal cannot be adjusted within the range of 475 to 600 ° C., and the strength of the weld metal is ensured. This value was set because it would cause problems. On the other hand, the higher the C content, the higher the risk of causing solidification cracks in the weld metal during butt solidification, particularly when the steel plate thickness is thick, so the upper limit of the C content is set to 0.4%.
[0071]
  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 is increased due to air mixing during welding. The lower limit of the Si content is less than 0.1%, so that the deoxidation 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, Si exceeds 0.8%WhenIn this case, the upper limit of the content is set to 0.8% because it causes toughness deterioration.
[0072]
  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 metalClearlyResidual weld toeTensionThis is important from the viewpoint of ensuring yield strength at the time of transformation expansion of the weld metal, which is a stress reduction mechanism, and generating sufficient compressive stress at the weld toe.
[0073]
  The lower limit of the Mn content is 0.4% as the 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 amount of Mn added is adjusted as a complementary component of C. However, if the amount added is excessively large, the manufacturing cost of the welding material increases, which is not preferable from the viewpoint of economy. Therefore, the upper limit of the amount of Mn added is set to 2.0%.
[0074]
  P and S are inevitable impurity elements.In the lightWhen these elements are present in large amounts in the weld metal, the toughness deteriorates, so the upper limits of the P and S contents were 0.03% and 0.02%, respectively.
[0075]
  The above is the departureClearlyIt is a basic component of the C-based weld metal in this, and the fatigue strength of the weld metal can be sufficiently obtained by these component rules, but in order to further improve the strength and toughness of the weld metal, according to their required characteristics, 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 content necessary for improving the strength and toughness of the weld metal, and the upper limit thereof excessively increases the content of alloy elements.SakoIn order to increase the manufacturing cost of the welded joint, the upper limit was made 1.0%.
[0076]
  (Component rules for Ni weld metal)
  The reason for limiting the components of the Ni-based weld metal and the content thereof will be described.
[0077]
  C is a quenching element, and is an effective element from the viewpoint of improving the strength of the weld metal and reducing the transformation temperature. However, in the Ni-based component, the transformation start temperature of the weld metal is realized mainly by addition of Ni. In order to complement the effect of lowering the transformation temperature of the Ni weld metal and to obtain sufficient strength, the lower limit is defined as 0.03%. On the other hand, excessive addition of C causes toughness deterioration of the weld metal, so the upper limit of its content was made less than 0.2%.
[0078]
  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 is increased due to air mixing during welding. The lower limit of the Si content is that if the Si content is less than 0.1%, the deoxidation effect is reduced and the oxygen level in the weld metal becomes too high, causing the deterioration of the mechanical properties of the weld metal, particularly toughness. Therefore, the lower limit of the content is set to 0.1%. On the other hand, excessive addition of Si also causes toughness deterioration, so the upper limit of its content was made 0.8%.
[0079]
  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 metalClearlyResidual weld toeTensionThis is important from the viewpoint of ensuring yield strength at the time of transformation expansion of the weld metal, which is a stress reduction mechanism, and generating sufficient compressive stress at the weld toe.
[0080]
  The lower limit of the Mn content is 1.0% as the 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 amount of Mn added is adjusted as a complementary component of Ni. However, if the amount added is excessively increased, the toughness of the weld metal is deteriorated, so the upper limit is set to 2.0%. It was.
[0081]
  P and S are inevitable impurity elements.In the lightWhen these elements are present in large amounts in the weld metal, the toughness deteriorates, so the upper limits of the P and S contents were set to 0.03% and 0.02%, respectively.
[0082]
  Ni is a metal element having an austenite structure (face-centered structure), which further stabilizes the austenite state of the weld metal at high temperatures and delays transformation to ferrite (body-centered structure) at low temperatures. It is an element that lowers the temperature. In addition, even if Ni is added in the same amount, Ni does not increase the risk of solidification cracking of the weld metal compared to C. Therefore, Ni is an element effective for further lowering the transformation temperature while maintaining the toughness of the weld metal. It is.
[0083]
  Main departureClearlyHowever, when the transformation start temperature of the Ni-based weld metal is adjusted to the range of 475 to 600 ° C., the fatigue strength of the welded joint is improved as in the case of the C-based weld metal even if the C addition amount is reduced.Is achievedAnd improved toughness compared to C-based weld metalMakebe able to. Therefore, the lower limit of the Ni content is set to 2.0% in order to improve the fatigue strength of the welded joint. On the other hand, the upper limit of the Ni content is less than 4.0% in order to sufficiently maintain the economical efficiency, toughness and weldability of the welded joint.
[0084]
  Main departureClearlyHowever, when the transformation start temperature of the Ni-based weld metal is adjusted to a range of less than 400 to 475 ° C., the C-based weld metal tends to cause a problem of solidification cracking of the weld metal due to an increase in the C content. Further, by setting the Ni content to 4.0 to 7.5%, the transformation start temperature of the weld metal can be lowered and suppressed to 400 to less than 475 ° C. while suppressing solidification cracking. Further, unlike C, Ni does not necessarily deteriorate toughness even if the addition amount is slightly increased, and in this case as well, toughness equivalent to or higher than that of C-based weld metal can be secured. The lower limit of the Ni content is set to 4.0% in order to improve the fatigue strength of the welded joint. On the other hand, if the upper limit of Ni content exceeds 7.5%, the economics of welded joints deteriorates and the weldability such as toughness and weld solidification cracks may deteriorate. Was defined as 7.5%.
[0085]
  The above is the departureClearlyIt is a basic component of Ni-based weld metal, and the fatigue strength of the weld metal can be sufficiently obtained by the provisions of these components. Furthermore, in order to further improve the strength and toughness of the weld metal, according to the 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 content is the minimum content necessary 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 alloy elements. In order to increase the cost, the upper limit was made 1.0%.
[0086]
  As aboveClearlyThe components of the C-based and Ni-based weld metals and the reasons for limiting the content thereof have been described, but the adjustment of the weld metal component content can be performed by using a welding wire, a combination of a welding wire and a filling flux, or This can be realized by designing the components of each welding material in consideration of the component yield in the weld metal when welding using either the core wire of the welding rod or the coating flux.
[0087]
  Next, the reason why the peening process is limited to the peening process using ultrasonic waves in the present invention will be described.
[0088]
  Here, ultrasonic peening means a frequency having a frequency in the range of 20 kHz to 60 kHz. The greatest merit of using ultrasonic waves is that a sufficiently large impact force can be applied even if the weight of the pin at the peening tip is small, and as a result, a sufficient peening effect can be achieved with a small work time. The principle of improving the fatigue strength by performing the peening process is to improve the shape of the portion there and to apply a compressive residual stress. For this purpose, plastic strain must be introduced into the peening portion. This is because no stress remains within the elastic strain range. In order to introduce plastic strain, it is necessary to apply an impact stress that exceeds the yield strength of the material. If this is to be achieved with static stress, a stress greater than the yield stress is applied to the weld. It is necessary to increase the work load by increasing the size of the apparatus. On the other hand, when ultrasonic waves are used, it can be seen that the stress applied to the peening portion is sufficiently large even if the mass of the pin is about 10 g, for example.
[0089]
  This principle will be briefly described.
[0090]
  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 pin speed, V, is
    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 peening process portion. If this speed change occurs in 1/10 time within one frequency, ie 1 / 330,000 seconds, the time change in speed, ie acceleration, A is
    A = dV / dt = 2 × 330000 = 660000 m / s²
It becomes. The impact force F can be obtained by multiplying the acceleration by the weight of the pin 10 g = 1/100 kg,
    F = 660000 × 1/100 = 6600N
It becomes. Stress S is the cross-sectional area of the pin, 1.5 × 1.5 × 3.14 = 7.1 mm²Divided by
    S = 6600 / 7.1 = 930 N / mm²= 930 MPa
It becomes. It should be noted that this stress is a value when the weight of the pin is only 10 g. In the case of actual ultrasonic peening, it is considered that the time interval at which the speed inversion occurs is shorter than the setting of the above calculation, and therefore, it is considered that a larger impact stress is generated.
[0091]
  As described above, the peening processSenseIt can be seen that the method using ultrasonic waves has the advantage that the mass of the pin is small and the weight of the device can be reduced accordingly.
[0092]
  Next, the preferred frequency of ultrasonic peeningRegulationExplain why.
[0093]
  When the frequency is lower than 20 kHz, the lower limit of 20 kHz falls within a frequency range that can be heard by humans, that is, an audible frequency range, 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 in which the work environment deteriorates deviates from the intent of the present invention. Further, as can be seen from the above calculation of the impact stress, the higher the ultrasonic frequency, the higher the impact stress and the more advantageous. The lower limit of 20 kHz was set as a frequency at which a sufficient peening effect can be obtained with a simple apparatus, and as a value that does not deteriorate the working environment. The lower limit of 20 kHz is preferably 23 kHz or more from the viewpoint of obtaining higher impact stress. If the upper limit of 60 kHz is a frequency higher than this, it is difficult to obtain ultrasonic waves with a simple device with the current technology, and although it is inaudible to the human ear, there is a problem in health care management. It was set.
[0094]
  Next, the reason why the preferable hardness of the pin is limited will be described.
[0095]
  In the present invention, the strength of the steel material and the weld metal is limited. This is intended to effectively change the transformation expansion of the weld metal into a compression elastic strain. However, with respect to the start and crater portions of the weld bead, it is necessary to ensure fatigue strength by peening treatment or the like due to the 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,TensionWhen the 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 or reduce the residual stress by peening, plastic strain must be introduced into the peening portion. For that purpose, it is necessary to make the pin 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 was set to this value because there is a material harder than this, but the cost of the pin itself is increased and the peening effect is not significantly increased.
[0096]
  Next, the reason for limiting the preferred diameter of the pin will be described.
[0097]
  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 larger impact stress, the pin may be made thinner, for example, like a needle. In this case, however, there is a risk that the pin will break or buckle, and unnecessary thin shapes are rather negative. The lower limit of 1.5 mm was set as a value that can sufficiently withstand the peening treatment without buckling or breaking of the pin. Conversely, the upper limit of 7 mm of the pin diameter is set to this value because the cross-sectional area of the pin is too large at a diameter exceeding this, and the impact stress may not be a predetermined value although it is sufficient as the impact force. .
[0098]
【Example】
  Examples of the present invention are shown below.
[0099]
  2 and 3 are schematic views of the fatigue test method used in this example. The actual degree of restraint of the member during welding can be determined by numerical calculation such as the finite element method, or by measuring the change in groove width at that time by applying a load to the groove before welding. . However, with such a method, the degree of restraint cannot always be controlled arbitrarily, and there is also a problem that the test cost becomes enormous. In view of the problems of these test methods, in this embodiment, a fatigue test method devised to arbitrarily determine the degree of restraint when producing a fatigue test piece by welding as shown in FIGS. It is. Here, two types of joints shown in FIGS. 2 and 3 were prepared as shapes of fillet weld joints. In any of the joints, the fatigue test piece was fixed in advance with jigs 4 and 5 before welding. This is to keep the degree of restraint of the weld joint constant. Next, fillet welding was performed in this state to produce a fatigue test piece. In addition, fillet welding of a fatigue test piece uses a wire and CO.₂The welding conditions were such that the current 125A and voltage 17V were constant, and the heat input during welding was adjusted by changing the welding speed. Usually, the welded joint used for the fatigue test piece is deleted by machining so that the start and crater parts 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 part and the crater part. Therefore, it is the joint shape of FIGS. 2 and 3 that can reproduce the fatigue behavior of such a structure. 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 obvious that both the joints A and B have a structural stress concentration portion in addition to the condition that the bead is easily disturbed in the start portion 1 and the crater portion 2. In addition to the start portion 1 and the crater portion 2, the corner portion 3 is a structural stress concentration portion.
[0100]
  Moreover, the restraint degree (RF) in welding at the time of fatigue test piece preparation changes the distance (L of FIG. 2, 3) between the test piece being fixed with the jigs 4 and 5 below (L). 1) The degree of constraint calculated using the equation was arbitrarily set.
    RF (N / mm · mm) = E (N / mm²) · H (mm) / L (mm) (1)
However, RF: Restraint degree, E: Young's modulus, H: Test material plate thickness, L: Distance between fixed ends (L)
[0101]
  Fatigue test pieces prepared by welding were prepared with or without ultrasonic peening treatment at the start and craters, and fatigued by applying a fatigue load in the direction of the arrows shown in FIGS. A test was conducted. The 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 the stress ratio is 0.1 and the load is between 111-1111 N and 5 million. It means that it does not break even when it is repeatedly loaded, and breaks at a number of repetitions less than 5 million times in a stress range exceeding that. The fatigue fracture is determined by attaching a strain gauge to the start part, crater part and corner part of the test piece, and when the strain gauge reading is reduced by 20% from the initial value during the fatigue test. It is regarded as having done. Moreover, since the strain gauge was affixed to the test piece after welding, the influence of welding residual stress is not included. Moreover, the penetration depth of the weld metal of the fatigue test piece was obtained by actually measuring the penetration depth 6 shown in FIG. 4 by taking a cross-sectional macro test piece from the test piece after the fatigue test was completed. Similarly, as for the amount of dent at the time of ultrasonic peening treatment, a macro test piece was collected after the end of the fatigue test, and the amount of dent 7 shown in FIG. 5 was measured.
[0102]
  Table 1 shows the composition of the weld metal, the transformation start temperature, the specimens taken from the weld metal parts of a plurality of fatigue test pieces prepared under the same welding conditions,TensionIntensity and 0 ° C Charpy absorbed energy are shown. The transformation start temperature of the weld metal was measured using a four master test, and the 0 ° C. Charpy absorbed energy was determined by carrying out an all-depot test under welding conditions of 270A-30V-25 cm / min according to JIS Z3111. However, thisAkiraWeld metal No. corresponding to the specified C-based weld metal. For 1 and 2, the C content is high and the possibility of hot cracking is high. Therefore, in order to prevent this, an all-depot test was conducted under the welding conditions when preparing fatigue test pieces, that is, 125A-17V-40 cm / min. It was.
[0103]
  In Table 1, the weld metal symbols A, B, E and F areIn the lightSpecified welding metal transformation start temperature: satisfies the range of 475 to 600 ° C., of which welding metals A and B areIn the lightSpecified C: Corresponds to 0.2 to 0.4% C-based weld metal, and weld metals E and F areIn the lightSpecified Ni: Corresponds to Ni-based weld metal of 2.0 to less than 4.0% (C: less than 0.03 to 0.2%). Weld metals H and I areIn the lightSpecified welding metal transformation start temperature: It satisfies the range of 400 to less than 475 ° C.In the lightSpecified Ni: 4.0 to 7.5% (C: 0.03 to less than 0.2%) Ni-based weld metal. Weld metals C, D and G areIn the lightThis is outside the specified transformation start temperature range of the weld metal. When the mechanical properties of weld metals A, B, E, F, H, and I are compared, all are at the same level.TensionHas strength, but the main parts of weld metals E, F, H and IIn the lightThe specified Ni-based weld metal has a Charpy absorbed energy at 0 ° C of more than 100 J.In the lightSpecified C-type weld metal (vE0: 70-7It was higher than 5J).
[0104]
  Table 2 shows the weld metal symbols shown in Table 1 and the conditions for producing the fatigue test pieces. The joint shapes A and B are the 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. Further, the degree of restraint is a value calculated by equation (1) using the plate thickness and L. In addition, this departureMysteriousFor out-of-range conditions, out-of-range items are also listed in Table 2.
[0105]
  The test pieces in Table 2 were subjected to ultrasonic peening treatment under various conditions, and fatigue test pieces were prepared and subjected to fatigue tests. Tables 3 and 4 show the fatigue test results of the joint A shown in FIG. 2, and Tables 5 and 6 show the results of the joint B shown in FIG. Since different plate thicknesses are included for each joint, the fatigue test results are shown separately when the plate thickness is 2 mm or more and 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 ultrasonic peening in each table | surface has shown the range which implemented the ultrasonic peening from the both ends of the start part of a weld bead and a crater part.
[0106]
  Test No. shown in Table 3. 2 for the conditions other than ultrasonic peeningMysteriousAlthough it is within the range, since the ultrasonic peening treatment is not performed, fatigue occurs from the start portion and the crater portion, and the fatigue limit of 5 million times is 11.5 kN. Test No. 3 is a case where the ultrasonic peening range is too narrow and fatigue of the start portion and the crater portion cannot be sufficiently prevented. Test No. 4 and 8 show that the transformation start temperature and strength of the weld metalMysteriousUltrasonic peening out of rangeDidAlthough the fatigue strength of the start part and the crater part has been improved, the occurrence of fatigue from the corner part cannot be prevented. Test No. No. 5 is a test no. No. 4 is a case where the ultrasonic peening process is omitted, and the fatigue strength is the lowest in Table 3. Test No. 6 is the ultrasonic peening condition, steel and weld metal areMysteriousAlthough it is within the range, the degree of restraint under welding conditions is high (the weld metal E of test No. 6 has a transformation start temperature of 530 ° C. from Table 2,MysteriousIn order to be within the range, the degree of restraint must be 4000 N / mm · mm or less), and the case where the residual stress reduction is insufficient and the fatigue strength of the corner is insufficient. Test No. 7 has a pin diameter of 1 mm and is too thin.IsThis is a case where the ultrasonic peening treatment is insufficient. On the other hand, test No. which is within the scope of the present invention. No. 1 has a fatigue strength of 20.3 kN, which is approximately twice that of other joints.
[0107]
  Table 4 is an example in which the joint shape is the same as in Table 3 but the plate thickness is less than 2 mm. Test No. 12 is the strength of steel materialMysteriousOut of range, the transformation start temperature of the weld metal isMysteriousThis is the case where the welding residual stress could not be sufficiently reduced because the reaction force from the steel material was inadequate. Test No. 13 is a case where the ultrasonic peening process is excessively performed, the dent amount becomes 1/3 of the plate thickness, the plate thickness here is reduced, the local stress is increased, and the fatigue strength is improved. This is the case. On the other hand, even with the same steel material and weld metal, the test No. No. 11 is the test No. 11 because the ultrasonic peening treatment conditions are within the scope of the present invention. It is about twice as high as 12 and 13.
[0108]
  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. In No. 21, the weld metal is G in Table 2, and the transformation start temperature isMysteriousIt is the case that is out of range, fatigue from the corner of Fig. 3CrackIs generated, and the fatigue strength is insufficient. Test No. No. 22 is a test number. This corresponds to the case where the ultrasonic peening process is not performed at 21 and the fatigue strength of the start portion and the crater portion is lower than that of the corner portion, and in Table 5, the fatigue strength is the lowest. Test No. No. 23 had a narrow ultrasonic peening range of 5 mm, and the fatigue strength of the start part and crater part was insufficient. Test No. 24 is a test No. 24. 23, the ultrasonic peening range is within the range of the present invention, and the fatigue strength exceeds 20 kN. Test No. 25 has the highest joint restraint of 8400 N / mm · mm, which is the highest of the conditions in Table 2, and the transformation start temperature of the weld metal is 430 ° C.MysteriousDespite being within the range, the residual stress is not sufficiently reduced, so fatigue starts from the corner.CrackThis is the case. In order to reduce the residual stress with such a high degree of restraint, it is necessary to use a conventional low temperature transformation welding material as disclosed in Patent Document 4. Test No. No. 26 is a case where the residual stress could not be sufficiently reduced because the steel material strength was as low as 240 MPa and the steel material reaction force was small. Test No. 27 is a case where the hardness of the pin is too low to sufficiently perform the ultrasonic peening process, and as a result, the dent amount becomes insufficient. Test No. No. 28 has a pin diameter of 10 mm, which is outside the preferred range of the present invention. Similarly to 27, this is a case where the ultrasonic peening effect is insufficient. Test No. 29 is condition No. 2 in Table 2. As shown in FIG. 13, the plate thickness is as thick as 5 mm, and a temperature difference still remains in the plate thickness direction even in the temperature region where the weld metal is transformed. This is the case. For these tests, test no. No. 24 is within the scope of the present invention. In Table 5, the fatigue strength is more than 20 kN, and the effect of improving fatigue strength is clear.
[0109]
  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 has a plate thickness of 0.6 mm.MysteriousIn this case, the depth of penetration exceeds the range of 1/3 of the plate thickness, resulting in insufficient residual stress reduction. Test No. 33 is the thicknessMysteriousAlthough it is within the range, the welding condition for the plate thickness is not appropriate, and the penetration becomes half the plate thickness, resulting in insufficient residual stress reduction. Test No. 33, test no. No. 32 is suitable for selecting the welding conditions even with the same thickness, and the penetration depth is 1/3 or less of the thickness.MysteriousThe fatigue strength is 9.5 kN, which is about twice that of the other two comparative examples.
[0110]
[Table 1]

[0111]
[Table 2]

[0112]
[Table 3]

[0113]
[Table 4]

[0114]
[Table 5]

[0115]
[Table 6]

[0116]
【The invention's effect】
  As described above, according to the present invention, the fatigue strength of the welded joint can be significantly improved as compared with the conventional joint. Therefore, the present invention is an invention with extremely great industrial value.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram of a lap joint portion.
FIG. 2 is a diagram illustrating the shape of a test piece of a joint A and the direction of fatigue load.
FIG. 3 is a diagram illustrating the shape of a test piece of a joint B and a fatigue load loading direction.
FIG. 4 is a conceptual diagram illustrating the penetration depth.
FIG. 5 is a diagram for explaining a dent amount at a weld toe.
[Explanation of symbols]
  1 Start
  2 Crater
  3 Corner
  4 Jig
  5 Jig
  6 penetration depth
  7 dent amount

Claims (1)

In the method for producing a fillet welded joint using a high-strength thin steel plate having a plate thickness of 1.0 to 4.0 mm and a tensile strength of 590 MPa or more,
The restraint degree (RF) of the welded part represented by the following formula (1) of the steel sheet is 8000 N / mm · mm or less, and the penetration depth of the weld metal in the welded part is 1/3 or less of the plate thickness of the steel sheet. , When the fillet weld of the steel sheet so that the weld metal has a transformation start temperature of 400 to 600 ° C. and a tensile strength of 590 MPa or more,
When the transformation start temperature of the weld metal is 475 ° C. to 600 ° C., the degree of restraint (RF) is 4000 N / mm · mm or less, and when the transformation start temperature of the weld metal is 400 ° C. to less than 475 ° C. The steel plate is fillet welded at a degree (RF) of 8000 N / mm · mm or less,
Then, further, ultrasonic peening treatment is performed on the weld toe over a range of 10 to 100 mm from the start of the weld bead and the end of the crater of the weld, and the high fatigue strength of the high strength thin steel sheet A method for producing a strength fillet welded joint.
RF (N / mm · mm) = E (N / mm ² ) · H (mm) / L (mm) ·· (1)
In the above formula, RF: restraint degree, L: length between both fixed ends when fixing both ends of a test piece having a groove portion at the center, H: plate thickness, E: Young's modulus.

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