JP2007284777A - Steel material for ultrasonic shock treatment having excellent fatigue strength and ultrasonic shock treatment method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material for ultrasonic shock treatment having excellent fatigue strength, and to provide an ultrasonic shock treatment method. <P>SOLUTION: Regarding the multi-layer steel material in which a soft layer 6 is formed on the surface 1, the thickness of the soft layer 6 is controlled to 0.1 to <1.0 mm, and also, the Vickers hardness Hvs of the soft layer 6 and the Vickers hardness Hvi of an internal layer 7 other than the soft layer 6 are allowed to satisfy equation. Then, the steel material for ultrasonic shock treatment is subjected to ultrasonic shock treatment using a leading edge tool 5 with a width R of 2.0 to 5.0 mm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel material for ultrasonic shock treatment and an ultrasonic shock treatment method excellent in fatigue strength used in automobiles, home appliances, building structures, ships, bridges, construction machines, various plants, penstocks, and the like. In the present invention, the steel material includes not only a base material but also a part subjected to processing such as welding and forming.

  When steel materials are subjected to processing such as welding, pressing, cutting, and punching, and a repeated load is applied to the member, fatigue cracks are generated in these processed parts due to the stress concentration due to the shape and the presence of tensile residual stress. As a result, the fatigue strength decreases. Therefore, a steel material having excellent fatigue resistance characteristics of these processed parts and a processing method capable of improving the fatigue resistance characteristics of the processed parts are desired.

  In recent years, an ultrasonic impact treatment aimed at improving the fatigue strength of welds and the like has been developed to deal with such fatigue problems, and the fatigue strength is improved by applying the ultrasonic impact treatment to welds and machined holes. A method and an apparatus are disclosed (for example, refer to Patent Documents 1 and 2). Also disclosed is a method for improving fatigue strength by performing ultrasonic impact treatment and quality assurance inspection of a welded portion and a plastic working portion (see, for example, Patent Document 3).

  On the other hand, in order to improve the joint fatigue strength, the present inventor has proposed a multilayer steel sheet in which the front and back layers are decarburized layers and the hardness, thickness and ferrite structure ratio of the front and back layers are defined. (See Patent Document 4). Further, conventionally, an invention has also been disclosed in which fatigue strength is improved by forming a soft layer in advance in a portion corresponding to the weld toe portion before welding (see, for example, Patent Document 5).

  The ultrasonic impact treatment refers to the improvement of the surface shape and the residual by applying plastic deformation by pressing ultrasonic vibrations of several tens of kHz generated from an ultrasonic generator against an object through a tool such as a pin. This is a process for stress relaxation / relocation.

US Pat. No. 6,171,415 US Pat. No. 6,338,765 Japanese Patent Laid-Open No. 2003-113418 JP 2003-239037 A JP-A-56-50797

  However, the conventional techniques described above have the following problems. Among the conventional techniques described above, Patent Document 1 discloses an application technique of ultrasonic shock treatment to a welded part and ultrasonic shock to a drill hole edge opened at a fatigue crack tip for the purpose of repairing a welded structure. The application technology of the treatment is disclosed, and with regard to the application to the welded part, the entire welded part is treated so as to follow the trace of the welding from behind, but the metal material to be treated has not been studied at all. In addition, there is no description regarding the metal material to be processed. Patent Document 2 discloses a device in which a needle-like tool is attached to the head of a transducer that converts ultrasonic energy into vibration, and a method for processing a drill hole by the device. Similar to Document 1, no study has been made on the metal material to be treated, and there is no description on the metal material to be treated. Furthermore, Patent Document 3 discloses a method of ultrasonic impact treatment in which plastic processing, deformation correction, and heat treatment are performed as a pretreatment as a general processing method, but as in Patent Documents 1 and 2 described above, No investigation has been made on the metal material to be treated, and there is no description on the metal material to be treated. For this reason, the techniques described in Patent Documents 1 to 3 have a problem that high fatigue strength cannot be obtained depending on the type of metal material to be processed.

  Furthermore, Patent Document 4 discloses a steel sheet for welded structure excellent in fatigue strength that defines the relationship between the thickness and hardness of the front and back layers, but does not describe any ultrasonic impact treatment. In addition, the steel sheet described in Patent Document 4 has a problem that a high fatigue strength cannot be obtained even when applied to ultrasonic impact treatment because the surface layer has a Vickers hardness of 140 or less.

  Furthermore, Patent Document 5 discloses a method for improving the fatigue strength of a welded portion in which a soft layer having a lower strength than the base material is formed on the surface, but nothing is described about ultrasonic impact treatment. . Moreover, since the steel plate shown in the Example of this patent document 5 has a surface layer thickness as thick as 1.0 mm or more, there exists a problem that high fatigue strength cannot be obtained as it is welded.

  This invention is made | formed in view of each problem mentioned above, Comprising: It aims at providing the steel material for ultrasonic impact processing and the ultrasonic impact processing method which were excellent in fatigue strength.

  The steel material for ultrasonic impact treatment with excellent fatigue strength according to the present invention is a multi-layer steel material having a soft layer formed on the surface thereof, the Vickers hardness Hvs of the soft layer and the Vickers hardness of the inner layer excluding the soft layer. The thickness Hvi satisfies Formula 1 below, and the thickness of the soft layer is 0.1 mm or more and less than 1.0 mm.

  The soft layer may have a lower C content (% by mass) than the inner layer.

  Further, the soft layer is, for example, mass%, C: 0.0005 to 0.25%, Si: 0.01 to 2.00%, Mn: 0.01 to 3.00%, S: 0.00. It may contain 050% or less and Al: 0.001 to 0.1%, and the balance may be composed of Fe and inevitable impurities, and the inner layer may be, for example, C: 0.003 to 0.00. 30%, Si: 0.01 to 2.00%, Mn: 0.01 to 3.00%, S: 0.050% or less and Al: 0.001 to 0.1%, the balance being Fe And a composition comprising inevitable impurities.

  In that case, the soft layer and the inner layer are further in mass%, P: 0.02 to 0.20%, Ti: 0.005 to 1.0%, B: 0.0001 to 0.01%, Ni: 0.1-5.0%, Cu: 0.1-2.0%, Mo: 0.1-4.0%, Nb: 0.005-1.0% and V: 0.005- One or more elements selected from the group consisting of 2.0% may be contained.

  The ultrasonic impact treatment method with excellent fatigue strength according to the present invention is characterized in that the ultrasonic impact treatment is performed on the above-described steel for ultrasonic impact treatment using a tip tool having a width of 2.0 to 5.0 mm. And

  According to the present invention, since the soft layer with the appropriate thickness and hardness is formed on the surface, the compressive residual stress due to the ultrasonic impact treatment is maintained largely in the soft layer of the surface layer as well as the inner layer. The fatigue strength can be remarkably improved, and a high fatigue strength can be obtained regardless of the type of the member to be processed.

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a diagram showing the surface shape and the distribution of residual stress in the thickness direction when the steel for ultrasonic shock treatment of the present invention is subjected to ultrasonic shock treatment. In FIG. 1, the compressive residual stress is shown on the right side of the drawing and the tensile residual stress is shown on the left side of the drawing from the y axis existing on the extension of the center line of the tip tool 5, and the same applies to the following drawings. To do.

  The present inventor has studied a method for further improving the fatigue characteristics of a steel material subjected to ultrasonic impact treatment. Specifically, first, using a homogeneous steel material having no difference in hardness distribution between the surface layer and the inner layer, the distribution in the thickness direction of the residual stress in the portion subjected to ultrasonic impact treatment was examined. FIG. 2 is a diagram showing the surface shape and the distribution of residual stress in the thickness direction when a homogeneous steel material with uniform hardness is subjected to ultrasonic impact treatment. As a result of the examination, as shown in FIG. 2, it was found that the compressive residual stress 3 generated in the steel material by the ultrasonic impact treatment takes a maximum value 1 mm or more inside the surface 1. Further, as a result of examining the relationship between the compressive residual stress distribution and the fatigue strength, the position 4 where the compressive residual stress 3 shows the maximum value is closer to the surface 1, that is, the position 4 where the compressive residual stress 3 becomes maximum and the surface 1 It was found that the shorter the distance L, the greater the degree of improvement in fatigue strength. Furthermore, as a result of studying by changing the width (diameter) R of the tip tool 5, if the width (diameter) R of the tip tool 5a is reduced, the radius of curvature of the tip tool 5a is inevitably reduced, and the steel material processing portion Since the curvature radius of 2 is also small, it has been found that stress concentration is large and the effect of improving fatigue strength is reduced.

  Therefore, as a result of earnestly examining the means for solving these conflicting phenomena, the present inventor, in order to increase the compressive residual stress 3 near the surface 1, so that the surface layer of the steel material undergoes sufficient plastic deformation, It came to the conclusion that it is effective to make this part softer than the inner layer, that is, to form the soft layer 6 having a lower hardness than the inner layer 7 on the surface of the steel as shown in FIG. As a result, in the distribution of the compressive residual stress 3 after the ultrasonic impact treatment, a compressive residual stress larger than that obtained when the homogeneous steel material shown in FIG. 2 is subjected to the ultrasonic impact treatment is obtained in the vicinity of the surface 1 of the steel material. I found. The inventor has also found that by increasing the Vickers hardness of the soft layer 6 to be greater than 140, the compressive residual stress when subjected to plastic deformation can be increased and the fatigue strength can be increased. . Furthermore, the present inventor has also studied the thickness of the soft layer 6 and found that it is necessary to improve the fatigue strength that the thickness of the soft layer 6 is 0.1 mm or more and less than 1.0 mm. It was.

  Based on the above examination results, in the steel for ultrasonic shock treatment of the present invention, in the multilayer steel material having the soft layer 6 formed on the surface, the thickness of the soft layer 6 is set to 0.1 mm or more and less than 1.0 mm. The Vickers hardness Hvs of the soft layer 6 and the Vickers hardness Hvi of the inner layer 7 excluding the soft layer 6 satisfy the following formula 2.

  Hereinafter, the reason for the numerical limitation in the steel material for ultrasonic shock treatment of the present invention will be described.

Vickers hardness: 140 <Hvs <Hvi
When the Vickers hardness Hvs of the soft layer 6 is 140 or less, the surface layer (soft layer 6) is easily plastically deformed by ultrasonic impact treatment, but the compressive residual stress does not occur above the yield stress level. The strength improvement effect is not so great. On the other hand, when the Vickers hardness Hvs of the soft layer 6 exceeds 140, the absolute value of the yield stress and further the compressive residual stress increases, and the effect of improving the fatigue strength increases. In addition, the upper limit of the Vickers hardness Hvs of the soft layer 6 should just be a range which does not exceed the Vickers hardness Hvi of the inner layer 7, and does not need to prescribe | regulate in particular. In the present invention, the Vickers hardness of the soft layer 6 is determined based on the hardness test measurement method defined in JIS G0558 “Decarburization layer depth measurement method for steel”. The Vickers hardness of the inner layer 7 is defined as the average value of the hardness of the inner layer 7 obtained by the same method as that of the soft layer 6 described above.

Soft layer thickness: 0.1 mm or more and less than 1.0 mm When the thickness of the soft layer 6 is 1.0 mm or more, when the ultrasonic impact treatment is performed using the tip tool 5 having a small width (diameter) R, Since the position 4 of the maximum value of the compressive residual stress is included in the soft layer 6, the compressive residual stress including the maximum value is reduced as a whole, and the effect of improving the fatigue strength is extremely reduced. When the thickness of the soft layer 6 is 1.0 mm or more, the strength of the entire thickness is reduced when the plate thickness is small, so that the mechanical characteristics necessary for the structural material cannot be obtained, and the ultrasonic shock is applied. When the treatment is performed, a sufficient improvement in fatigue strength may not be obtained. On the other hand, if the thickness of the soft layer 6 is smaller than 0.1 mm, the thickness at which the compressive residual stress at the yield stress level is distributed is also reduced, and the soft layer 6 is stretched to the periphery by plastic deformation, and thus becomes thinner. The effect of improving fatigue strength is extremely small. Therefore, the thickness of the soft layer 6 shall be 0.1 mm or more and less than 1.0 mm.

  In the present invention, the thickness of the soft layer 6 is measured by a microscope, a hardness test, or a carbon concentration measurement method described in JIS G0558 “Decarburization layer depth measurement method of steel”. It is defined as the average value of the decarburized layer depth measured based on When the steel for ultrasonic shock treatment of the present invention is manufactured by multi-layer casting, the thickness of the soft layer 6 is the depth of the decarburized layer measured based on a measurement method using a microscope or a measurement method using a hardness test. It is defined as the average value.

  The soft layer 6 in the steel for ultrasonic shock treatment of the present invention configured as described above can be easily formed, for example, by making the C content of the surface layer of the steel material lower than the C content of the inner layer. it can. If the C content is low, a structure with increased ductility is obtained, so that an effect of reducing the progress rate of fatigue cracks can be obtained, which is extremely effective for improving fatigue strength.

In addition to the method for reducing the C content of the surface layer, examples of the method for reducing the Vickers hardness of the surface layer of the steel material compared to the inner layer include surface softening by heating, clad formation, and multilayer casting method. As a result of the study by the present inventors, it has been concluded that the decarburization treatment is particularly effective. Although the method of this decarburization treatment is not particularly limited, for example, in an atmosphere containing one or more gases of water vapor, carbon dioxide gas, oxygen gas and ammonia gas that are decarburized, A method of generating a decarburized ferrite layer on the surface layer of the steel material can be applied by holding for a certain period of time under a temperature condition of 550 ° C. or more and A _c1 transformation point or less. At that time, the treatment atmosphere may contain components that do not contribute to the oxidation / reduction reaction, such as argon, which is an inert gas, and quasi-inert nitrogen gas, in addition to the decarburizing gas described above. Moreover, as long as the carbon potential lower than the carbon concentration in the steel material can be maintained, it may contain hydrogen gas and carbon monoxide gas which are reducible.

  Next, the preferable range of the steel component in the ultrasonic shock-treated steel material of the present invention will be described. In the following description, mass% in the steel composition is simply expressed as%.

C: Soft layer 0.0005 to 0.25%, inner layer 0.003 to 0.30%
C is an element necessary for ensuring the strength of the steel material. However, if the C content of the inner layer 7 increases, specifically, if the C content of the inner layer 7 exceeds 0.30%, weldability may be impaired. Moreover, although it is desirable to reduce C content of the inner layer 7, when it will be less than 0.003%, the intensity | strength of the whole steel materials may not be ensured. Therefore, the C content of the inner layer 7 is preferably 0.003 to 0.30%. On the other hand, if the C content exceeds 0.25%, the ductility in the ferrite structure may not be secured, and if the C content is less than 0.0005%, the strength of the entire steel material May not be secured. Therefore, the C content of the soft layer 6 is preferably 0.0005 to 0.25%.

  In addition, since C content of the soft layer 6 is changing continuously, C content of the soft layer 6 in this invention is defined as C content in the center part of the soft layer 6. FIG. The C content is measured by the method described in Appendix 3 “Method for measuring carbon concentration by electron microanalysis” of “Method for measuring depth of steel decarburized layer” defined in JIS G0558. The carbon concentration at the center of the soft layer 6 in the carbon concentration transition curve from the surface of the soft layer 6 to the inner layer 7 thus obtained is defined as the C content of the soft layer 6.

  Next, elements other than C will be described. The preferred range of the element content shown below is common to the soft layer 6 and the inner layer 7.

Si: 0.01 to 2.00%
Si is an element useful for ensuring strength, but when the Si content is less than 0.01%, the effect may not be obtained. On the other hand, if the Si content exceeds 2.00%, weldability may be impaired. Therefore, the Si content in the soft layer 6 and the inner layer 7 is preferably 0.01 to 2.00%.

Mn: 0.01 to 3.00%
Mn is useful as an element that can improve strength at low cost. However, when the Mn content is less than 0.01%, sufficient strength may not be ensured. On the other hand, if the Mn content exceeds 3.00%, weldability may be impaired. Therefore, the Mn content in the soft layer 6 and the inner layer 7 is preferably 0.01 to 3.00%.

S: 0.050% or less S is an impurity inevitably contained in the steel material in the steelmaking process. When the S content is too much, specifically, when the S content exceeds 0.050%, welding is performed. And toughness may be impaired. Therefore, the S content in the soft layer 6 and the inner layer 7 is preferably regulated to 0.050% or less.

Al: 0.001 to 0.1%
Al is a deoxidizing element, but if its content is less than 0.001%, the effect may not be sufficiently obtained. On the other hand, if the Al content exceeds 0.1%, the amount of inclusions in the steel becomes excessive and the toughness may be lowered. Therefore, the Al content in the soft layer 6 and the inner layer 7 is preferably 0.001 to 0.1%.

  The soft layer 6 and the inner layer 7 in the ultrasonic shock-treated steel material of the present invention include one or two selected from the group consisting of P, Ti, B, Ni, Cu, Mo, Nb and V in addition to the above components. It may contain more than seed elements.

  P, Ti, B, Ni, Cu, Mo, Nb and V are all elements that improve fatigue strength by a solid solution strengthening mechanism or a precipitation strengthening mechanism, and these elements are synergistic components in this respect. However, the P content is less than 0.02%, the Ti content is less than 0.005%, the B content is less than 0.0001%, the Ni content is less than 0.1%, and the Cu content is 0.1%. If the Mo content is less than 0.1%, the Nb content is less than 0.005%, and the V content is less than 0.005%, the above-described effects cannot be obtained. On the other hand, when these elements are added excessively, specifically, the P content exceeds 0.20%, the Ti content exceeds 1.0%, or the B content exceeds 0.01%. Or whether the Ni content exceeds 5.0%, the Cu content exceeds 2.0%, the Mo content exceeds 4.0%, or the Nb content exceeds 1.0%, Or when V content exceeds 2.0%, a material will deteriorate. Therefore, when these elements are added to the soft layer 6 and the inner layer 7, P: 0.02 to 0.20%, Ti: 0.005 to 1.0%, B: 0.0001 to 0.01, respectively. %, Ni: 0.1-5.0%, Cu: 0.1-2.0%, Mo: 0.1-4.0%, Nb: 0.005-1.0% and V: 0.00. 005 to 2.0%. Note that P is an element inevitably contained in the steel material in a normal steelmaking process, but it is desirable that the P content be 0.02% or more in order to further improve the fatigue strength.

  Components other than the above-described elements in the soft layer 6 and the inner layer 7 of the ultrasonic shock-treated steel material of the present invention, that is, the balance, are Fe and inevitable impurities. Moreover, in the ultrasonic impact-treated steel material of the present invention, N has no particular effect on the fatigue strength, so the component range is not particularly limited.

  Furthermore, the shape of the ultrasonic shock-treated steel material of the present invention is not particularly limited, and plate-shaped, rod-shaped, tubular and shaped materials, and complicated-shaped members are applied to steel materials in which fatigue failure is a problem. Is possible.

  Next, an ultrasonic impact treatment method using the ultrasonic impact treated steel material of the present invention will be described. The inventor examined the width (diameter) R of the tip tool in the ultrasonic impact treatment. FIG. 3 is a diagram showing the shape of the surface and the distribution of residual stress in the thickness direction when the ultrasonic impact-treated steel material of the present invention is subjected to ultrasonic impact treatment using a narrow tip tool. These are figures which show the shape of the surface at the time of performing an ultrasonic impact process using a wide tip tool, and distribution in the thickness direction of residual stress. As a result, as shown in FIG. 3, when the width (diameter) R of the tip tool 5a is smaller than 2.0 mm, the position 4 of the maximum value of the compressive residual stress exists in the soft layer 6, and the surface 1 When the distance L from the portion is 1.0 mm or more, the compressive residual stress becomes small. On the other hand, when the width (diameter) R of the tip tool 5b is larger than 5.0 mm as shown in FIG. It has been found that sufficient plastic deformation may not be obtained with (soft layer 6), and similarly sufficient compressive residual stress cannot be obtained. Therefore, in the ultrasonic impact treatment method of the present invention, the width (diameter) R of the tip tool used is set to 2.0 to 5.0 mm.

  In the ultrasonic impact treatment method of the present invention, it is preferable that the applied ultrasonic wave is 20 to 32 kHz, the pin amplitude is 25 to 35 μm, and the plastic deformation amount (deformation depth) is 20 to 30 μm.

  Hereinafter, the effects of the present invention will be specifically described with reference to Examples and Comparative Examples. In this example, each treatment shown in the following Table 1 was performed, and hot-rolled steel sheets having the mechanical properties and thickness shown in Table 3 below were manufactured with the steel compositions shown in Table 2 below. That is, no. A, No. B and No. In the process of D, the decarburization process of the homogeneous steel plate was performed on each condition shown in the following Table 1. At that time, the decarburizing atmosphere gas contains carbon dioxide gas: 14% by volume, carbon monoxide gas: 2% by volume, water vapor: 2% by volume, hydrogen gas: 2% by volume, and the balance is composed of nitrogen gas. . No. In the process of C, after inserting a wire made of an iron alloy containing Si and Mn into the molten steel, a multi-layer slab is manufactured by adjusting the drawing speed, and the thickness is 3.2 mm by hot rolling. The steel plate was manufactured. The balance in the steel composition shown in Table 2 below is Fe and impurities. In addition, the Vickers hardness of the soft layer and the inner layer shown in Table 3 below was determined by the method described in “Method for measuring depth of decarburized layer of steel” defined in JIS G0558, assuming that the load is 0.98N. Value. Furthermore, the underline in the following Table 3 indicates that it is outside the scope of the present invention.

  FIG. 5 is a view showing a fatigue test piece in the example of the present invention. Next, after steel plates of these examples and comparative examples were TIG welded to produce the lap fillet welded joint shown in FIG. 5, the tip tool having the width shown in Table 3 above was used, and it was in the center of the test piece. Ultrasonic impact treatment was performed on the weld toe. This ultrasonic impact treatment was one pass. And the fatigue test was done about the sample piece after ultrasonic impact treatment. As a fatigue test method, a double swing load control fatigue test using a plane bending mechanism with a stress ratio (= minimum load / maximum load) of −1 was adopted and performed in room temperature and in the atmosphere. As a result, the life that makes it difficult to control the load was defined as the rupture life, and the surface stress amplitude at which the rupture life was 2 million times was compared as the fatigue strength. The results are also shown in Table 3 above.

  As shown in Table 2 and Table 3 above, Example No. 1-No. The steel plate No. 6 is a thick steel plate having a thickness of 2.3 mm that does not contain a special additive element, and is an example in which the thickness of the soft layer and the Vickers hardness Hvs are within the scope of the present invention. In contrast, Comparative Example No. The steel plate No. 27 is a steel plate having a thickness of 2.3 mm that does not contain any special additive element, but is an example in which the soft layer has a Vickers hardness Hvs of less than 140. As a result of the fatigue test, Example No. 1-No. The steel plate of No. 6 is Comparative Example No. Compared with 27 steel plates, a fatigue strength improvement effect of 30% or more was recognized, and it was confirmed that the steel plate for ultrasonic shock treatment of the present invention was effective in improving fatigue strength.

  No. 7-No. The steel plate No. 10 is an example having the same plate thickness of 2.3 mm and containing the additive element defined in claim 4 of the present invention. 1-No. A fatigue strength improvement of about 10% was observed compared with the steel plate No. 6. Furthermore, no. 11-No. Steel plate No. 24 is an example relating to a steel material having a plate thickness of 3.2 mm. 11-No. Steel plate No. 16 is a steel material produced by multi-layer casting, No. 17-No. The steel plate of 24 is an Example regarding the steel materials produced by decarburization. And Example No. 11-No. The steel plate No. 24 had the same C content in the soft layer and the inner layer, but Si and Mn were added to the inner layer, so that the inner layer exhibited a Vickers hardness greater than that of the soft layer. In addition, Example No. 11-No. Steel plate No. 16 is a steel material by multi-layer casting having the same plate thickness, and the soft layer has a Vickers hardness outside the scope of the present invention. Compared with 29 steel plates, a fatigue strength improvement effect of about 40% was observed. Moreover, it is a steel material by multi-layer casting with the same plate thickness, and the comparative example No. having a large surface layer thickness. 30 and no. Compared with 31 steel plates, a fatigue strength improvement effect of about 50% was recognized. Thereby, it was confirmed that the ultrasonic impact-treated steel sheet of the present invention is effective in improving fatigue strength.

  No. which is an example of steel material produced by decarburization. 17-No. The steel plate No. 24 has the same thickness and is produced by decarburization, but the soft layer has a Vickers hardness Hvs outside the scope of the present invention. Compared to 28 steel plates, a fatigue strength improvement effect of 40% or more was recognized. In addition, Comparative Example No. in which the thickness of the soft layer is thinner than the range of the present invention. Compared to 32 steel plates, a fatigue strength improvement effect of 40% or more was recognized. Furthermore, No. 2 which is an embodiment of the invention according to claim 2 of the present invention. 17-No. The steel plate No. 24 has a C content of the soft layer lower than that of the inner layer, and has an effect of delaying the fatigue crack growth rate with a structure with improved ductility. . 11-No. Compared to 16 steel plates, the fatigue strength was about 20% higher.

  Example No. 25 and no. In the steel plate 26, the steel plate itself is within the scope of the present invention, but the width of the tip tool used in the ultrasonic impact treatment is out of the range defined in the invention according to claim 5 of the present invention. For this reason, Comparative Example No. Compared with the steel plate of No. 29, an effect of improving the fatigue strength by 30% or more is recognized, but the plate thickness is the same and Example No. 17-No. Compared with 24 steel plates, the fatigue strength was about 20% lower.

  From the results described above, it was found that the steel material for ultrasonic impact treatment and the ultrasonic impact treatment method of the present invention are extremely effective in improving the fatigue strength of the steel material.

It is a figure which shows the shape of the surface at the time of ultrasonic impact treatment of the steel material for ultrasonic impact treatment of this invention, and distribution in the thickness direction of a residual stress. It is a figure which shows the distribution in the thickness direction of the surface shape at the time of carrying out ultrasonic impact treatment of the homogeneous steel material with uniform hardness. It is a figure which shows the surface shape at the time of carrying out the ultrasonic impact process of the ultrasonic impact-treated steel material of this invention using the narrow tool with a narrow width | variety, and distribution in the thickness direction of a residual stress. It is a figure which shows the shape of the surface at the time of carrying out the ultrasonic impact process of the ultrasonic impact-treated steel material of this invention using a wide tip tool, and distribution in the thickness direction of a residual stress. It is a figure which shows the fatigue test piece in the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel material surface 2 Ultrasonic impact process part 3 Compressive residual stress 3a Tensile residual stress 4 Position where compression residual stress becomes the maximum 5,5a, 5b Tip tool 6 Soft layer 7 Inner layer L Surface 1 and position where compression residual stress becomes the maximum Distance from 4 R Width (diameter) of tip tool 5, 5a

Claims (5)

A multilayer steel material having a soft layer formed on the surface,
The Vickers hardness Hvs of the soft layer and the Vickers hardness Hvi of the inner layer excluding the soft layer satisfy the following formula,
A steel material for ultrasonic impact treatment excellent in fatigue strength, wherein the soft layer has a thickness of 0.1 mm or more and less than 1.0 mm.

  The steel material for ultrasonic impact treatment according to claim 1, wherein the soft layer has a lower C content (mass%) than the inner layer, and is excellent in fatigue strength. The soft layer is, in mass%, C: 0.0005 to 0.25%, Si: 0.01 to 2.00%, Mn: 0.01 to 3.00%, S: 0.050% or less and Al: 0.001 to 0.1% is contained, the balance has a composition consisting of Fe and inevitable impurities,
The inner layer is composed of C: 0.003 to 0.30%, Si: 0.01 to 2.00%, Mn: 0.01 to 3.00%, S: 0.050% or less, and Al: 0.001. The steel material for ultrasonic impact treatment with excellent fatigue strength according to claim 1 or 2, wherein the steel material contains ~ 0.1%, and the balance is composed of Fe and inevitable impurities.   The soft layer and the inner layer are further in mass%, P: 0.02 to 0.20%, Ti: 0.005 to 1.0%, B: 0.0001 to 0.01%, Ni: 0. 0.1-5.0%, Cu: 0.1-2.0%, Mo: 0.1-4.0%, Nb: 0.005-1.0% and V: 0.005-2.0 The steel material for ultrasonic impact treatment with excellent fatigue strength according to claim 3, comprising one or more elements selected from the group consisting of%.   An ultrasonic impact excellent in fatigue strength, wherein the steel material according to any one of claims 1 to 4 is subjected to an ultrasonic impact treatment using a tip tool having a width of 2.0 to 5.0 mm. Processing method.

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