JP2009013436A - Spring workpiece and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spring workpiece which can be used even in a corrosive atmosphere because the delayed fracture is controlled, does not need to contain an additional special element and can also reduce a manufacturing cost, and to provide a manufacturing method therefor. <P>SOLUTION: The spring workpiece includes, by mass%, 0.5 to 0.7% C, 0.15 to 0.35% Si, 0.6 to 0.9% Mn, and the balance Fe with unavoidable impurities; and has a structure which is a mixed structure of a bainite phase B and a ferrite phase α. Furthermore, the grain size number of a former austenite grain is 10 or more, and the area of the ferrite phase α existing in the grain boundary faces γs of the former austenite grains occupies 5 to 20% of the area of the whole mixed structure. Then, a base metal is formed by working a base material having the above composition at a working rate of 5% or more; the workpiece is formed by working the base metal into a predetermined shape; the workpiece is heated to and held at an A<SB>3</SB>transformation temperature or higher but a temperature which is higher than the A<SB>3</SB>transformation temperature by 40°C or lower; and the workpiece is subjected to austempering treatment. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a spring processed product suitable for, for example, a hose clamp for automobiles and the like, and a method for manufacturing the spring processed product that hardly causes delayed fracture even in a corrosive atmosphere.

  For example, when connecting hoses used as automobile internal pipes, a flexible hose is placed on the outer periphery of the pipe provided in the device to be connected, and the outer periphery of the hose is further tightened by a hose clamp. Therefore, the fixing method is widely used. This type of hose clamp is composed of a tightening ring that is bent in a substantially C shape, and a pair of knob pieces that intersect from both ends of the tightening ring and rise in the radial direction. When close to each other, the tightening ring expands against the elastic force, and when the pair of tab pieces separate, the tightening ring returns to its original reduced diameter state due to the elastic force and tightens the hose. Yes.

  Like the hose clamp, a spring processed product using the elastic force of a metal material, for example, after processing a base material of a predetermined component into a predetermined shape, the processed product is subjected to various performances (tensile strength required for the spring). And toughness) are manufactured by heat treatment such as quenching and tempering. And, in order to obtain a bainite phase having tenacity and good toughness than tempered martensite, which is a structure in the case of quenching and tempering, isothermal transformation is performed by holding at a temperature of about 300 to 500 ° C. for a certain time from the austenite state. It is widely performed to perform so-called austempering.

  As a spring steel subjected to austempering treatment, for example, in Patent Document 1 below, the weight ratio is C: 0.40 to 0.70%, Si: 0.15 to 0.35%, Mn: 0 .70 to 1.50%, Cr: 0.80 to 1.50%, Al: 0.010 to 0.050%, V: 0.05 to 0.50%, N: 0.010 to 0.030 %, With the balance being Fe and impurity elements heated to 820 to 950 ° C. to austenite, then austempered at a temperature of 320 to 400 ° C., then 330 to 450 ° C. and the austempered steel. A steel for thin plate springs, which is tempered at a temperature higher by 10 ° C. or higher than the processing temperature and has excellent heat settling characteristics, characterized in that the structure is a tempered bainite structure.

  By the way, since the hose clamp fastens and fixes the hose, a constant stress is always applied in that state. When such a hose clamp is used to connect pipes installed near the fuel tank where hydrogen is likely to be generated and corrosive atmosphere, hydrogen ductility reduces the ductility and toughness of the hose clamp. Then, when a certain time has passed, there is a possibility that so-called delayed destruction that suddenly breaks may occur. The cause of this delayed fracture is that hydrogen penetrates into the structure of the hose clamp, causing hydrogen to accumulate at the grain interface of the prior austenite grains, resulting in a lower strength at the grain interface, resulting in inability to withstand external stress. It is thought that it is caused by cracking from the grain interface of the prior austenite grains.

As a measure for suppressing the above-mentioned delayed fracture, for example, in the steel structure, a carbide having a strong bond with hydrogen such as TiC or VC is generated, thereby trapping (capturing or fixing) hydrogen. It is known. For example, in Patent Document 2 below, the activation energy for hydrogen desorption is 25 to 50 kJ / mol, and the hydrogen trap capacity is 0.2 wt. At least one of oxides, carbides and nitrides containing any one or more of V, Mo, Ti, Nb and Zr, and any two or more of these composite precipitates, which are ppm or more A high-strength spring steel excellent in hydrogen fatigue resistance, characterized by having a hydrogen trap site comprising:
JP 7-179936 A JP-A-6-271975

  However, in the case of Patent Document 2, a relatively expensive element such as Ti or V has to be added to the steel as a hydrogen trap, which increases the manufacturing cost.

  On the other hand, in the case of Cited Document 1, various elements are added, the austenitizing temperature is set to 820 to 950 ° C., and the steel structure is made to be a uniform austenite phase before the austempering process to finally form a uniform bainite phase. However, no measures are taken against hydrogen embrittlement, and delayed fracture cannot be suppressed.

  Therefore, an object of the present invention is to provide a spring processed product that can be used even in a corrosive atmosphere with suppressed delayed fracture, and that does not require the addition of a special element, and that can reduce the manufacturing cost, and a manufacturing method thereof. There is to do.

  In order to achieve the above object, the first of the present invention is mass%, C: 0.5-0.7%, Si: 0.15-0.35%, Mn: 0.6-0.9% The balance is Fe and inevitable impurities, the structure is a mixed structure of a bainite phase and a ferrite phase, the grain size number of the prior austenite grains is 10 or more, and the entire mixed structure The spring processed product is characterized in that the area of the ferrite phase existing at the grain interface of the prior austenite grains is 5 to 20% with respect to the area.

  According to the above invention, since the prior austenite grains have a particle size number of 10 or more, the prior austenite grains become finer and the grain interface is refined, so that hydrogen that tends to gather at the grain interface is dispersed throughout the entire structure. In addition, cracks originating from the grain interface are less likely to occur, and delayed fracture that occurs when stress is applied to the spring processed product in a corrosive atmosphere in which hydrogen is generated can be suppressed.

  Furthermore, since the ferrite phase having a small hydrogen storage amount is present at the grain interface of the prior austenite grains in an area ratio of 5 to 20% with respect to the area of the entire mixed structure, the fine austenite grains as described above Hydrogen dispersed by the grain interface becomes difficult to be stored in the tissue, and delayed fracture can be further suppressed.

  In addition, since the ferrite phase having a relatively large ductility is present at the grain interface of the fine prior austenite grains, the grain interface of the prior austenite grains becomes difficult to break, and the delayed fracture can be further suppressed. it can.

  Furthermore, since it is not necessary to add a special element other than Si and Mn, the manufacturing cost of the spring processed product can be reduced.

  According to a second aspect of the present invention, there is provided a spring processed product applied as a hose clamp for an automobile in the first aspect.

  According to the above invention, the hose clamp for connecting automobile pipes is often used in a corrosive atmosphere in which hydrogen is easily generated, and there is a possibility that delayed fracture accompanying hydrogen embrittlement may occur. However, by using the spring processed product of the present invention, delayed fracture can be prevented and safety can be improved.

3rd of this invention is the mass%, contains C: 0.5-0.7%, Si: 0.15-0.35%, Mn: 0.6-0.9%, and the remainder is Fe And after hot-rolling a material comprising a composition composed of inevitable impurities, a spheronization treatment is performed to form a base material with a processing rate of 5% or more, and this base material is processed into a predetermined shape to form a processed product. further, the workpiece, at a ₃ transformation point or higher, and, after heating maintained at 40 ° C. higher temperature or lower than the a ₃ transformation point, the production method of the spring workpiece, characterized in that performing austempering Is to provide.

  According to the above invention, a processed product made of a base material having a processing rate of 5% or more and a refined ferrite phase is heated and held under the above conditions, so that the austenite phase does not become a uniform austenite phase. Since the ferrite phase is left in addition to the phase and the austempering treatment is performed in this state, a spring processed product having a mixed structure of the bainite phase and the ferrite phase can be manufactured. At this time, since the grain interface of the ferrite phase of the base material before the heat treatment is refined in advance by setting the processing rate to 5% or more as described above, the spring having the grain size number of the prior austenite grains of 10 or more A processed product can be obtained. In addition, since the ferrite phase has been refined in advance, the austenite phase also becomes fine when heated and held at the above temperature, and as a result, the ferrite phase tends to precipitate along the interface of the austenite phase, and the area of the entire mixed structure On the other hand, a spring processed product in which the area of the ferrite phase existing at the grain interface of the prior austenite grains is 5 to 20% can be obtained. In this way, by setting the processing rate of the base material to 5% or more in advance and performing the austempering treatment under the heat treatment conditions, a spring processed product having the above composition can be manufactured. Since there is no need to add a special element other than Si and Mn, the manufacturing cost of the spring processed product can be reduced.

  According to the spring processed product of the present invention, the grain number of the prior austenite grains is 10 or more, the grain interface of the prior austenite grains is refined, hydrogen that tends to gather at the grain interface can be dispersed throughout the structure, Cracks at the grain interface are less likely to occur, and delayed fracture under a corrosive atmosphere can be suppressed. Furthermore, since the ferrite phase having a small hydrogen storage amount is present in the above-described proportion at the grain interface of the prior austenite grains, it becomes difficult for hydrogen to be stored in the structure, and delayed fracture is further suppressed.

  On the other hand, according to the method for manufacturing a spring processed product of the present invention, a processed product made of a base material with a processing rate of 5% is heated and held under the above conditions, so that it becomes an austenitic phase without forming a uniform austenitic phase. Since the austemper treatment is performed with the ferrite phase remaining, a spring processed product having the above composition can be manufactured.

  The spring processed product of the present invention can be used, for example, in an automobile hose clamp 10 shown in FIG. In addition to the hose clamp 10, it can be used for a clip for a car, a push nut, and the like, and is not particularly limited.

  As shown in FIG. 1, the hose clamp 10 protrudes radially outward from the substantially C-shaped tightening ring 11 and one end and the other end of the tightening ring 11, and is disposed opposite to each other. The pair of knob pieces 13 and 13 and the connection piece 15 for connecting the pair of knob pieces 13 and 13 to each other. In the free state, the clamping ring 11 has a pair of knob pieces 13 and 13 which are reduced in diameter in the circumferential direction, and both the knob pieces 13 and 13 are opposed to each other against the elastic force of the clamping ring 11. When close to each other, the diameter is expanded. Therefore, the hose clamp 10 is arranged in an expanded state on the outer periphery of the hose H attached to the outer periphery of the connection portion of one pipe P and the other pipe (not shown) connected thereto, and one knob piece 13 By removing the connecting piece 15 extending from the other knob piece 13, the fastening ring 11 is reduced in diameter, the hose H is fastened and fixed, and both pipes are connected.

  And in the spring processed product of this invention, the composition is the mass% (Hereinafter,% of a composition component means the mass%), C: 0.5-0.7%, Si: 0.15- It contains 0.35%, Mn: 0.6-0.9%, and the balance consists of Fe and inevitable impurities.

  C is necessary to obtain an appropriate strength as a spring processed product. If C is less than 0.5%, the desired hardness cannot be obtained, which is not preferable. On the other hand, if C exceeds 0.7%, coarse carbides are easily generated and the toughness is reduced. It is not preferable.

  Si acts as a deoxidizer when steelmaking the material before forming the spring processed product, and has an effect of strengthening by dissolving in ferrite. If the Si content is less than 0.15%, the above-described effect cannot be obtained, and therefore, it is not preferable. On the other hand, if the Si content exceeds 0.35%, the crystal grains become coarse and the toughness decreases, which is not preferable.

  Mn acts as a deoxidizer like Si, and has an effect of removing S harmful to steel and further improving hardenability. If Mn is less than 0.6%, the above effect cannot be obtained, which is not preferable. On the other hand, if Mn exceeds 0.9%, the toughness decreases, which is not preferable.

  In addition, the said composition is equivalent to S50C-S70C which is what is called carbon steel material for mechanical structures (prescribed in JIS G 4051), and reduction of manufacturing cost can be aimed at by using such an existing steel material. It is like that.

  And the spring processed product of the present invention having the above composition has a mixed structure of a bainite phase and a ferrite phase, and the grain size number of the prior austenite grains is 10 or more, and the entire mixed structure Is characterized in that the area of the ferrite phase existing at the grain interface of the prior austenite grains is 5 to 20%. As a result, the grain interface of the prior austenite grains is refined to disperse hydrogen throughout the structure, and cracking of the grain interface is less likely to occur, and moreover, hydrogen is stored by the ferrite phase present at the above ratio at the grain interface. This makes it difficult to prevent delayed destruction.

  The structure will be described with reference to the drawings. As shown in FIGS. 4A to 4C, the spring processed product has a bainite phase B as a parent phase, and a ferrite phase in the bainite phase B. α exists and has a mixed structure composed of a bainite phase B and a ferrite phase α. This is observed by etching the tissue surface with a predetermined etching solution. The ferrite phase α is a portion surrounded by black circles (for convenience and different from the actual structure), and the bainite phase B is the other portion.

  Furthermore, the grain interface of the prior austenite grains can be observed by etching the same texture plane as that shown in FIGS. 4A to 4C by an etching method different from the above. That is, as shown in FIGS. 5A to 5C, the grain interface γs of the prior austenite grains exists on the texture planes corresponding to FIGS. 4A to 4C, respectively (FIG. 4 ( a) and FIG. 5 (a) are C: 0.61%, FIG. 4 (b) and FIG. 5 (b) are C: 0.56%, and FIG. 4 (c) and FIG. 5 (c) are C. : 0.56%). The grain interface of prior austenite grains refers to the grain interface of the remaining austenite phase when the austenite phase is isothermally transformed into a bainite phase by austempering. In the present invention, the grain size number of the prior austenite grains is 10 or more, and the grain interface of the prior austenite grains is more than that having a grain size number of less than 10 shown in FIGS. 7 (a) to (c). γs is fine. The grain size number here is defined by JIS G 0551 “Austenite grain size test method for steel”, and the larger the number, the finer, and the smaller the number, the coarser. When the particle size number of the prior austenite grains is less than 10, the prior austenite grains are too large to disperse hydrogen throughout the structure and are coarse grains. Since stress easily propagates and cracks are likely to occur at the grain interface γs, it is difficult to suppress delayed fracture, and therefore it is inappropriate in the present invention.

In the spring processed product of the present invention, as shown in the conceptual diagram of the structure state of FIG. 2, the area of the ferrite phase α present at the grain interface γs of the prior austenite grains is the bainite phase B and the ferrite phase α. It is 5 to 20% with respect to the area of the whole mixed structure. That is, the total area of the ferrite phase alpha and S _alpha, the total area of the bainite phase B and S _B, if the total area of such other spheroidal cementite was S _c, so to satisfy the following expression (i) ing.

_{S α / (S α + S} B + S c) × 100 = 5~20% ··· (i)
In addition, the ferrite phase α is only counted especially at the grain interface γs of the prior austenite grains, and not precipitated at the parent phase bainite phase B. That is, it is based on counting the ferrite phase α present on the grain interface γs of the prior austenite grains, and preferably present over three or more grain boundaries γs of the prior austenite grains, and spheroidized cementite (Fig. The ferrite phase α larger than (not shown) is counted. For example, the ferrite phase α ′ (indicated by an imaginary line in the figure) existing at the grain interface γs of four prior austenite grains may be counted. In addition, when the area of the ferrite phase α is less than 5% with respect to the total area of the mixed structure, the ferrite storage phase α cannot be expected to reduce the hydrogen storage amount, and the suppression of delayed fracture becomes insufficient. In the present invention, if it exceeds 20%, a ferrite phase α that is soft and has a large ductility is excessively present in the entire mixed structure, and the strength of the spring processed product is reduced. Is also unsuitable for the present invention.

  Next, the manufacturing method of the spring processed product of this invention is demonstrated. The spring processed product having the above-described composition is obtained, for example, by the method for manufacturing a spring processed product of the present invention.

In manufacturing the spring processed product, it contains C: 0.5 to 0.7%, Si: 0.15 to 0.35%, Mn: 0.6 to 0.9% in mass%, and the balance is A material composed of Fe and inevitable impurities is used. In addition, about each composition ratio, since it is as above-mentioned, it abbreviate | omits. After hot-rolling a material having such a composition component, spheroidizing treatment (spheroidizing annealing: treatment for spheroidizing cementite in the pearlite phase) is performed to form a base material with a processing rate of 5% or more. To do. Here, the terms working ratio referred, the cross-sectional area of the pre-rolling stock and A _0, if the cross-sectional area of the base material after rolling was A _1, are those derived by the following equation (ii).

Processing rate (%) = (A ₀ −A ₁ ) / A ₀ × 100 (ii)
If the material processing rate is less than 5%, the ferrite phase becomes coarse and the desired structure cannot be obtained by the heat treatment described later, which is not suitable in the present invention.

Then, the base material having a processing rate of 5% is processed into a predetermined shape to form a processed product. In this embodiment, it forms in the shape of the hose clamp 10 shown in FIG. Then, the workpiece, at A ₃ transformation point or higher, and, after heating maintained at 40 ° C. higher temperature or lower than the A ₃ transformation point, so that the performing austempering. FIG. 3 shows the essential part of the equilibrium diagram of the Fe—C system. In the present invention, C is in the range of 0.5 to 0.7%, and further, the A ₃ transformation point (FIG. among 3, a _3-wire reference) or more and, at 40 ° C. higher temperature in the region T than a ₃ transformation point, so as to heat hold workpiece with the form of a hose clamp.

  By heating and holding in this region T, the ferrite phase of the processed product having the shape of the hose clamp 10 does not become a uniform austenite phase γ, but leaves the ferrite phase α in addition to the austenite phase γ. be able to.

In FIG. 3, the region T is in the austenitized region, but in reality, not all of the ferrite phase of the processed product becomes a uniform austenite phase. That is, the equilibrium diagram in the Fe-C system in FIG. 3 shows the state of the structure at each temperature when the rate of heating and cooling is infinitely small (equilibrium state), and the ferrite phase α + austenite phase γ from state is workpiece heating, even beyond the transformation point shown in a _3-wire shown in Figure 3, hardly immediately transformation such as ferrite phase α becomes homogeneous austenite phase γ disappeared occurs, a It is known that it transforms to a uniform austenite phase γ at a temperature higher than ₃ lines (assuming that the A ₃ line in the equilibrium state is Ae ₃ and the A ₃ line at the time of heating is Ac ₁ , Ac ₃ > Ae ₃ ). Therefore, to apply the austempering, when a uniform austenite phase γ steel, it is common to heat held in 50-60 ° C. higher temperature than A _3-wire. Also, the A _3-wire, even the addition amount of various components contained in the processed products, may be shifted.

In the present invention, the temperature condition is selected so that the ferrite phase α + austenite phase γ are mixed, and the processed product is heated and held under this temperature condition, whereby the ferrite phase of the processed product is uniformly austenite phase γ. In this case, the austenite phase γ and the ferrite phase α are allowed to remain. In the above heating and holding conditions, when the heating retention temperature than A ₃ transformation point is low, the amount of the ferrite phase α of the workpiece is too large, the strength of the spring workpiece is reduced, in the present invention is inadequate, whereas, than 40 ° C. higher temperature than a ₃ transformation point, when the heating retention temperature is high, the workpiece becomes a homogeneous austenite phase γ does not remain the ferrite phase alpha, the present invention Then it is inappropriate.

  Further, in this embodiment, the time for holding the processed product in the region T is, for example, 15 to 30 minutes.

  And the spring processed goods which have the mixed structure of the bainite phase B and the ferrite phase (alpha) can be manufactured by performing an austemper process about the processed goods heated and hold | maintained in the said area | region T. FIG. For example, the austempering treatment can be performed on the processed product by holding the processed product in an atmosphere of 300 to 400 ° C. for 1 to 30 minutes.

  As described above, according to the method for manufacturing a spring processed product of the present invention, a processed product made of a base material in which the processing rate is 5% or more and the ferrite phase is refined is heated and held under the above conditions. As a result, the austenite phase γ is left in addition to the austenite phase γ without forming a uniform austenite phase γ, and the austempering treatment is performed in that state. A spring processed product such as the hose clamp 10 having a mixed structure of the bainite phase B and the ferrite phase α shown in (c) can be manufactured.

  At this time, the grain interface number of the prior austenite grain is 10 because the grain interface of the ferrite phase of the base material before the heat treatment is refined in advance by setting the processing rate of the base material to 5% or more as described above. More than the number of spring processed products can be obtained. In addition, since the ferrite phase has been refined in advance, the austenite phase γ when heated and held under the above temperature condition also becomes fine, and as a result, the ferrite phase α is likely to precipitate along the interface of the austenite phase γ, A spring processed product in which the area of the ferrite phase α existing at the grain interface γs of the prior austenite grains is 5 to 20% with respect to the area of the entire mixed structure can be obtained.

  In this way, by setting the processing rate of the base material to 5% or more in advance and performing the austempering treatment under the heat treatment conditions, a spring processed product having the above composition can be manufactured. Since there is no need to add a special element other than Si and Mn, the manufacturing cost of the spring processed product can be reduced.

  And the spring processed goods of this invention manufactured with said manufacturing method show | play the following effects. That is, by setting the grain size number of the prior austenite grains to 10 or more, the prior austenite grains become finer and the grain interface γs is refined, so that hydrogen that tends to gather at the grain interface can be dispersed throughout the entire structure. At the same time, cracks originating from the grain interface γs are less likely to occur, and delayed fracture that occurs when stress is applied to the spring processed product in a corrosive atmosphere in which hydrogen is generated can be suppressed.

  Further, since the ferrite phase α having a small hydrogen occlusion amount is present at the grain interface of the prior austenite grains in an area ratio of 5 to 20% with respect to the total area of the mixed structure, the fine prior austenite grains as described above are present. Hydrogen dispersed by the grain interface γs becomes difficult to be stored in the structure, and delayed fracture can be further suppressed.

  Further, since the ferrite phase having a relatively large ductility is present in the fine austenite grain interface γs at the above-mentioned ratio, the grain interface γs of the prior austenite grain becomes difficult to break, and the delayed fracture is further suppressed. be able to. Furthermore, when an impact force is applied from the outside, the impact force propagates along the grain interface γs of the prior austenite grains. The soft and ductile ferrite phase α softens the impact force and relaxes the impact force. Therefore, the strength of the spring processed product can be increased.

  Further, since the ferrite phase α present at the grain interface γs of the prior austenite grains has a property that only 0.02% of C is dissolved, more C is diffused in the bainite phase B which is the base material. It is supposed to be. Therefore, the decrease in hardness caused by the presence of the ferrite phase α is almost offset by improving the hardness of the bainite phase by dissolving C in the bainite phase B. The hardness can be maintained.

  In this embodiment, the spring processed product is applied as a hose clamp 10 for an automobile (see FIG. 1). By the way, the hose clamp 10 for connecting pipes for automobiles is often used in a corrosive atmosphere in which hydrogen is likely to be generated, and delayed fracture due to hydrogen embrittlement is likely to occur. By using the spring processed product according to the form, safety can be improved.

  The hydrogen embrittlement resistance, the particle size number of the prior austenite grains, and the ferrite phase ratio of various spring processed products were evaluated.

(1) Preparation of test piece (Example 1)
A base material containing, by mass%, C: 0.61%, Si: 0.20%, Mn: 0.65%, the balance being Fe and inevitable impurities, and processing rate% , Orth by cut into 70 mm × 15 mm × 2 mm strip to form a workpiece, which after 30 minutes at _{a 3} transformation point + 40 ℃ as austenitizing temperature, holding for 30 minutes in an atmosphere of 330 ° C. A temper treatment was performed to produce Example 1.
(Example 2)
% By mass, C: 0.56%, Si: 0.20%, Mn: 0.67%, the same as in Example 1 except that a material consisting of Fe and inevitable impurities is used as the balance. Example 2 was manufactured under conditions.
(Example 3)
Example 3 was produced under the same conditions as in Example 2 assuming variation in test results.
(Comparative Example 1)
Using the same materials as in Example 2 (C: 0.56%), except that the austenitizing temperature was A ₃ transformation point + 50 ℃ is under the same conditions as those in Example 1 to prepare Comparative Example 1.
(Comparative Example 2)
Using the same materials as in Example 1 (C: 0.61%), except that the austenitizing temperature was A ₃ transformation point + 60 ℃ is under the same conditions as those in Example 1 to prepare Comparative Example 1.
(Comparative Example 3)
Except that the austenitizing temperature was A ₃ transformation point + 80 ° C. in a similar condition as in Comparative Example 2 was prepared in Comparative Example 3.

(2) Hydrogen embrittlement characteristic evaluation test Each of the above Examples and Comparative Examples is tested by a three-point bending test method. That is, the strip-shaped examples and comparative examples are attached to a jig of a bending load tester and supported between two points at both ends in the longitudinal direction on the bottom surface side of each example and comparative example (between fulcrums). The distance is set to 60 mm), and the presser is pushed in from the middle in the longitudinal direction of each of the examples and comparative examples, and a predetermined bending stress (1300 MPa, 1400 MPa, 1500 MPa) is applied.

  In a state in which the bending stress is applied, that is, in a state where each example and comparative example are set in a jig, each example and comparative example together with the jig has a hydrochloric acid (HCl) 36% in order to make a corrosive atmosphere. Immerse in the solution for 2 minutes. Then, it is washed with water and dried by leaving it at room temperature for 24 hours. That is, the steps of immersion in hydrochloric acid → washing with water → drying were repeated, and this was defined as one cycle, and the number of cycles at which each of the examples and comparative examples was broken with the above bending stresses was measured. The result is shown in FIG.

(3) Measurement of particle size number of prior austenite grains After embedding each example and comparative example broken by the test of (2) above in a thermosetting resin and polishing the cut surface to obtain a smooth surface, The surface is corroded by being immersed in a caustic solution containing a picric acid solution, and a grain interface of prior austenite grains appears in the structure. And the surface was observed with the metal microscope of 400 magnifications, while the prior austenite grain was counted, and the structure | tissue was image | photographed. FIGS. 5A to 5C show the structures of Examples 1 to 3, and FIGS. 7A to 7C show the structures of Comparative Examples 1 to 3, respectively. And the quantity of the former austenite grain was counted, and the particle size number was computed in accordance with JIS G 0551 "Austenite grain size test method of steel".

(4) Measurement of the ratio of ferrite phase After calculating the particle size numbers of the prior austenite grains in (2) above, the surface is further corroded by immersing them in a nitral corrosive solution, and the bainite phase and the ferrite phase To appear. FIGS. 4A to 4C show the structures of Examples 1 to 3, and FIGS. 6A to 6C show the structures of Comparative Examples 1 to 3, respectively. Then, the surface is observed with a 400 × magnification metal microscope, and in comparison with the grain interface of the prior austenite grains observed in (2) above, the grain is present at the grain interface of the prior austenite grains and at least three former austenite grains. Only the ferrite phase α that straddles the grain boundaries of the grains and that is larger than the spheroidized cementite was counted. Further, the ratio of the total area of the ferrite phase α to the entire visual field was calculated with the entire visual field at 400 magnification of the metal microscope as 100%.

  The above (2) hydrogen embrittlement characteristic evaluation test results, (3) the particle size number measurement results of prior austenite grains, and (4) the ferrite phase ratio measurement results are summarized in Table 1 below.

(5) Summary As shown in Table 1, in Examples 1 to 3, the grain size number of the prior austenite grains is 10 or more, and the ferrite phase is in a ratio of 12 to 14% with respect to the total area. As shown in FIG. 8, it can be seen that it is difficult to be destroyed even in a corrosive environment. That is, according to the spring processed product of the present invention, it can be understood that delayed fracture in a corrosive environment can be effectively suppressed.

It is a perspective view which shows the hose clamp to which the spring processed goods of this invention are applied. It is a conceptual diagram which shows the structure | tissue state of the spring processed goods of the same invention. It is a Fe-C type equilibrium state figure explaining heat treatment conditions in a manufacturing method of a spring processed article of the present invention. The structure photograph of the state which made the bainite phase and the ferrite phase appear is shown, (a) is the structure photograph of Example 1, (b) is the structure photograph of Example 2, (c) is the structure of Example 3. It is a photograph. The structure photograph of the state which made the grain interface of a prior austenite grain appear is shown, (a) is the structure photograph of Example 1, (b) is the structure photograph of Example 2, (c) is the structure photograph of Example 3. Organization photo The structure photograph of the state which made the bainite phase and the ferrite phase appear is shown, (a) is the structure photograph of the comparative example 1, (b) is the structure photograph of the comparative example 2, (c) is the structure of the comparative example 3. It is a photograph. The structure photograph of the state which made the grain interface of a prior austenite grain appear is shown, (a) is the structure photograph of comparative example 1, (b) is the structure photograph of comparative example 2, and (c) is the comparative example 3. It is an organization photograph. It is explanatory drawing which shows the result of the characteristic evaluation test of hydrogen embrittlement resistance.

Explanation of symbols

10 Hose clamp α Ferrite phase γ Austenitic phase γs Grain interface B Bainite phase

Claims (3)

In mass%, C: 0.5 to 0.7%, Si: 0.15 to 0.35%, Mn: 0.6 to 0.9%, the balance consists of Fe and inevitable impurities,
The structure is a mixed structure of a bainite phase and a ferrite phase. Further, the grain size number of the prior austenite grains is 10 or more, and the grain interface of the prior austenite grains with respect to the area of the entire mixed structure. A spring processed product characterized in that the area of the ferrite phase present is 5 to 20%.   The spring processed product according to claim 1, which is applied as a hose clamp for an automobile. From a composition containing, in mass%, C: 0.5 to 0.7%, Si: 0.15 to 0.35%, Mn: 0.6 to 0.9%, the balance being Fe and inevitable impurities After hot rolling the material to be formed, a spheroidizing process is performed to form a base material with a processing rate of 5% or more, and this base material is processed into a predetermined shape to form a processed product,
Furthermore, the workpiece, at A ₃ transformation point or higher, and, after heating maintained at 40 ° C. higher temperature or lower than the A ₃ transformation point, the production method of the spring workpiece, characterized in that performing austempering.