JP6050912B1 - How to prevent cracks in drawn products of high-strength steel sheets - Google Patents

Abstract

An object of the present invention is to provide a method for preventing cracks occurring in a drawn product of a high-tensile steel plate. A high-strength steel sheet having a tensile strength of 400 MPa to 800 MPa is first or prototypely drawn, and a place where a crack (P) occurs in the drawn product is specified in advance. After the same secondary or full-scale drawing of a high-tensile steel plate, only the part (P) that has been specified in advance of the drawn product is quenched in the optical micrograph of the cut surface. The boundary (c), which is an intermediate layer of a structure change different from the part (a) and the non-quenched unheated remaining part (b), extends along a direction (HH) substantially parallel to the plate surface (d). Partially heat until it appears as a strip. [Selection] Figure 1

Description

  The present invention relates to a method for preventing cracks in a product obtained by drawing a high-tensile steel plate.

  Generally, when a metal plate is drawn by a press die, depending on the processing, a tensile stress may remain inside the material, so that the drawn product may be left for several hours to several days after processing. Phenomenon in which cracks occur naturally from the open end (called cracks, time cracks or aging cracks), and damage due to hydrogen absorption in processes such as corrosion, welding, pickling and electroplating (called delayed fractures and plating cracks) There are problems that occur.

  In particular, high-tensile steel sheets obtain high tensile strength by adding alloy elements such as nickel (Ni), silicon (Si), and manganese (Mn) in addition to carbon (C) and devising the heat treatment process. Since it is also useful for weight reduction, it is widely used, but its ductility is lower than that of general steel. Even if defective products such as cracks are not generated during processing and a good processed product is obtained, the above-mentioned cracks, plating cracks, delayed fracture, etc. occur thereafter, which is troublesome.

  As a solution to such a problem, Japanese Patent No. 29882494 has been proposed.

Japanese Patent No. 2982494

  However, in the configuration of the pressing method disclosed in the above-mentioned Patent Document 1, the normal ironing mainly for uniformizing the thickness of the material, and the reverse ironing for reducing the tensile residual stress of the material and causing the compressive stress to remain. Even if the material is a high-strength material such as high-strength steel, the generation of compressive residual stress prevents the occurrence of the cracking phenomenon. Even if the above-described reverse ironing can be performed without any problem, when pressing a complicated shape, the direction of ironing is restricted, so that applicable objects are very narrowly limited. It is inferior in versatility.

  The present invention aims to drastically solve such problems, and in order to achieve the object, in claim 1, a high-strength steel sheet having a tensile strength of 400 MPa to 800 MPa is first or prototypely drawn. Then, specify the place where cracks occur in the drawn product beforehand,

  After the same secondary or full-scale drawing of the high-tensile steel plate, only the above-mentioned part specified in advance of the drawn product is subjected to quenching in the optical micrograph of the cut surface and non-quenched Partial heating is performed until the boundary, which is an intermediate layer of a structure change different from the unheated remaining portion in the state, appears in a strip shape extending along a direction substantially parallel to the plate surface.

  Further, in the second aspect, the partial heating temperature is 400 ° C. to 1,000 ° C., and the heating and holding time is 0.1 second to 1.0 minute.

  According to a third aspect of the present invention, the partial heating method is a method of energizing a pair of electrodes facing each other while sandwiching a predetermined crack generation site.

  According to a fourth aspect of the present invention, the partial heating method is a method of irradiating an energy beam to a predetermined crack generation site.

  Further, according to a fifth aspect of the present invention, the drawn product of the high-strength steel sheet has a U-shaped cross section, and only the bending corner portion specified in advance as the place where the drawn product is placed is partially heated.

  According to the above configuration of the first aspect, the problems of the prior art described at the beginning can be surely solved, and a high-tensile steel plate can be widely used as a material for drawing by a press die.

  That is, the first crack or the prototype drawing of the high-strength steel plate specifies (acquires) the crack generation site of the drawn product in advance, and the high-strength steel plate is secondarily the same. Or, after full-scale drawing, it is a method of partially heating only the above-mentioned place where the drawing crack has been specified (knowledge) of the drawn product, so versatility for various target shapes of the drawn product Excellent.

  Moreover, in the partial heating, an optical micrograph obtained by photographing the cut surface of the place where the crack is generated has a boundary that is an intermediate layer of a structural change different from the heated part in the quenched state and the unheated remaining part in the non-quenched state. Since it is performed until it appears in a strip shape extending in a direction substantially parallel to the direction, the effect of preventing cracking and delayed fracture can always be obtained stably.

  In that case, if the structure of claim 2 is adopted, since the boundary (intermediate layer) of the above-mentioned structure change is surely displayed in the optical micrograph of the partially heated place where the crack is generated, anyone can use the appropriate part. It can be determined that it is heating, and is excellent in objectivity.

  If the construction of claim 3 or claim 4 is adopted, it is easy to cope with the change of the shape when drawing products of various target shapes are partially heated. Because it can be heated quickly and smoothly from the center of the high-tensile steel plate, which is the material of the product, to the surface, versatility and productivity are further improved.

  If the structure of Claim 5 is employ | adopted, there exists an effect which can relieve | eliminate or remove the residual stress of the bending-corner site | part which is easy to generate | occur | produce the place crack and delayed fracture of the drawing processed goods shape | molded by the U-shaped cross section by the partial heating. .

It is the schematic diagram of the structure | tissue shown in the optical micrograph which image | photographed the cut surface of the place crack generation | occurrence | production part partially heated by this invention, (a) shows a structure | tissue at the time of heating with a spot welder, (b) Indicates a tissue when heated by a laser device. It is a side view of the caster shown as an example of the drawing processed product which is the object of the present invention. FIG. 3 is a front view of FIG. 2. It is explanatory drawing which shows the series of process order, using the bearing main body of a caster as a drawing processed product. It is a perspective view which shows the placement crack generation | occurrence | production site | part of the said bearing main body. It is a schematic slope view of the state which partially heats the place crack generation | occurrence | production site | part of FIG. 5 with a spot welder. It is explanatory drawing of the partial heating state corresponding to FIG.

  Hereinafter, the configuration of the present invention will be described in detail. The present invention is a general-purpose crack prevention method for products that are drawn into various target shapes by a press die using a high-tensile steel plate.

  The high-tensile steel plate to which the present invention is applied has a tensile strength of 400 MPa to 800 MPa. Those having a tensile strength of less than 400 MPa can be manufactured under almost the same processing conditions as general steel, without needing to apply the present invention, while those having a tensile strength exceeding 800 MPa are ordinary. Manufacturing by the press working method is difficult, and the cause of occurrence of cracking and delayed fracture is greatly different. Therefore, it is difficult to apply the present invention.

  As a result of the inventor drawing high-strength steel sheets into various target shapes using a press die and investigating and studying their forming characteristics, the occurrence sites of cracks and delayed fracture are always concentrated at specific locations. It was found that the specific location is mainly a bending corner where tensile stress tends to remain. Therefore, it is thought that removing or mitigating the residual stress will solve the problem.

  Therefore, in order to obtain a drawn product with a target shape, the above-described high-tensile steel plate is drawn first or as a prototype, thereby specifying the place where the drawn crack is placed in advance (knowledge). I will keep it. Instead of performing the drawing as the prototype, the place where the placement crack is generated may be specified in advance by a press forming analysis method.

  Then, the high-strength steel sheet of the same material is drawn secondarily or in earnest, and only the part where the crack is generated is specified under certain conditions. Heat. The fixed condition on the partial heating is “an intermediate layer having a structure change different from that of the heated part in the quenched state and the unheated remaining part in the non-quenched state in the optical micrograph of the cut surface at the place where the crack occurred after heating. Heating until the boundary appears in a strip extending along a direction substantially parallel to the plate surface.

  That is, as a method of partial heating, with a pair of electrodes facing each other in a spot welder (AC resistance welding machine), etc., energize while sandwiching a predetermined crack generation site of the drawn product, Beginning with the method of heating from the center of the high-strength steel plate that is the material, the laser beam or other energy beam is applied to the part of the drawn product that has been specified in advance and heated from the surface of the material. A wide range of methods, induction heating methods using high frequency, and methods using gas flames and arcs can be widely used. In particular, the direct current heating method that can quickly heat from the center of the material to the surface of the cracking site as a whole. Is most preferred.

  The temperature of the partial heating is set to 400 ° C to 1000 ° C, preferably 600 ° C to 900 ° C. Even in the case of partial heating, if the temperature is lower than 400 ° C., a large structural change cannot be obtained, and the residual stress cannot be sufficiently relaxed, so that it does not help prevent cracking and delayed fracture. The boundary (intermediate layer) of the structure change different from the heated part and the unheated remaining part does not appear.

  On the other hand, when the temperature is higher than 1000 ° C., the influence of heat spreads over a wide range, and the entire structure is changed, and the heated portion becomes hard and brittle. Even under this condition, a boundary (intermediate layer) of the structural change different from the heated part and the unheated remaining part does not appear. Further, when the temperature becomes higher, the base material is melted and the shape cannot be maintained.

  In the case of heating by the direct current heating method, although it depends on the plate thickness, it is preferable to apply a voltage of 10 to 45 kV. When the voltage is lower than 10 kV, partial heating is insufficient and it is difficult to form a quenched portion. When the voltage exceeds 45 kV, the entire heated portion is hardened and becomes hard and brittle, or the heated portion is melted. Produce.

  Regarding the temperature of the heated portion, various sensors generally used for temperature measurement can be used, and a method based on a color change of the heated portion can be used.

  Furthermore, the holding time for the partial heating is set to 0.1 second to 1.0 minute, preferably 0.2 second to 20 seconds. If the heating and holding time is less than 0.1 seconds, as in the case where the heating temperature is less than 400 ° C., the structure change is still small and the time until stress is relaxed is insufficient. It cannot be prevented. There is also a risk of variations in product strength.

  On the other hand, in the case of a long time exceeding 1.0 minutes, it is sufficient for relaxation of stress, but as in the case where the heating temperature exceeds 1000 ° C., the heat is conducted in a wide range, so that the strength change becomes large and the design is increased. There is a problem that the strength cannot be achieved. Moreover, since a long time is required for post-processing, the productivity is reduced.

  In short, the combination of the above heating temperature and the heating holding time is optimally set, and the optimum heating method according to the target shape of the drawn product is adopted, and only the place where the drawn crack is generated is shown. As shown in the schematic diagrams of 1 (b) and (b), in the optical micrograph of the cut surface at the cracked portion (P) after the heating, the heated part (a) in the quenched state and the unheated remaining in the unquenched state Partial heating until the boundary (intermediate layer) (c) of the structure change different from any part (b) appears in a strip shape extending along a direction (HH) substantially parallel to the plate surface (d) To do.

  In other words, after partially heating only the specified crack generation site (P) of the drawn product, the cut surface of the site (P) is mechanically polished, and the nital (etching agent: In the structure photograph, which was corroded with a mixed solution of nitric acid and alcohol), observed with an optical microscope, and photographed, the middle of the structural change different from the heated part (a) in the quenched state and the unheated remaining part (b) in the non-quenched state If it can be confirmed that the boundary (c), which is a layer, is exposed in a strip shape extending along a direction (HH) substantially parallel to the plate surface (d), it is determined that the layer has been appropriately partially heated. (Evaluation).

  According to the above configuration of the present invention, it is possible to prevent the occurrence of cracks, plating cracks, and delayed fracture in the drawn product of the high-tensile steel sheet, so that there is no risk that a good processed product will become a defective product later. There is no need to modify the press die or create a new press die to prevent the cracking.

  Furthermore, the crack prevention method of the present invention is not a method of heating the high-strength steel sheet, which is the material before drawing, as a whole, but to soften the high-strength steel sheet, which is the same material, in order to relieve general stress. It is not a method of partially heating the steel plate before or during drawing to provide a portion. This is only a method of partially heating the product after drawing, and as a result there is a risk that productivity will decline, but it is specified (understood) in advance by the primary or prototype drawing of the high-tensile steel plate. Since only the part where the crack was generated is partially heated, when the current heating method that can be heated quickly from the center of the material by a pair of electrodes facing each other is used as the heating method, the target shape of the drawing process is Even if it changes complicatedly, it has versatility that it can always be heated quickly and accurately, and there is no risk of large deformation caused by heat, compared to the overall heating of the above materials, reducing strength and productivity. There is no invitation.

<Example 1>
Hereinafter, the embodiment of the present invention applied to the bearing body of the caster will be described. However, the drawn product of the high-tensile steel plate is not limited to the bearing body of the caster, and has been drawn into various target shapes. For products.

  As a material of the main body (12) for bearing the wheel (11) of the caster (10) as shown in FIGS. 2 and 3, a high-strength steel plate having a tensile strength of 590 MPa and a plate thickness of 2.0 mm (JSC590R of JFE Steel Corporation) -SD), 150-200T press machine (manufactured by Amada Co., Ltd.), outline drawing / drilling as shown in FIGS. 4 (i)-(vi) → pre-drawing / engraving → drawing → drilling → groove 6 steps of press working, machining → axle drilling.

  The product immediately after the processing was visually observed, but defects such as cracks and tears could not be confirmed. Therefore, after standing for 12 hours as it was, the product was visually observed again, and the bending indicated by the symbol (P) in FIG. Cracks were found at the corners. In addition, for products in which no cracks were found, trivalent unichrome plating (zinc plating) was applied, but cracks were found at the bending corners indicated by reference (P) in FIG. It was.

  Therefore, a spot welder as shown in FIGS. 6 and 7 after 30 minutes has elapsed after the press working at the bending corner portion where the crack (P) in FIG. 5 has occurred in the bearing body (12). It was sandwiched between a pair of electrodes (13) facing each other with a diameter of 2 mm in (W) and partially heated by applying a voltage at 30 kV for 0.4 seconds. During the application, red heat was seen at the portion sandwiched between the electrodes (13), and the heating temperature determined from the degree of color development was about 800 ° C.

  After the heating, the bearing main body (12) was visually observed for defects, but no defects such as cracks or tears were found. Further, when the sample was left for 48 hours after the heating, the defect of the bearing body (12) was visually observed again, but the defect could not be confirmed.

  Furthermore, after trivalent unichrome plating (zinc plating) was performed on the bearing body (12), the presence or absence of the defect was visually observed, but the defect could not be confirmed.

  The bearing body (12) subjected to the above plating process was assembled with necessary parts such as wheels (11) and bearings, and a running test of the completed caster (10) was conducted. The above bug was not found.

  The heating conditions and results of Example 1 are shown in Table 1. From the results of Table 1, it is considered that the partial heating according to the present invention at the bending corner portion specified in advance contributed to the prevention of setting cracks, plating cracks, and delayed fracture.

  The partially heated bending corner portion (P) of the bearing body (12) is cut, and the cut surface is polished, and then the structure is exposed to a corrosion treatment with nital and shown in the photographed optical micrograph. ), And the photograph shows the structure change different from that of the heated part (a) in the quenched state and the unheated remaining part (b) in the non-quenched state, which means the result of the partial heating (evidence). The boundary (c), which is an intermediate layer, was exposed in a strip shape extending along a direction (HH) substantially parallel to the plate surface (d). As a result of the partial heating at the heating temperature and the holding time at which the structural change as the boundary (intermediate layer) (c) occurs, the mechanical strength of the bearing body (12) as the drawn product is reduced. It never happened.

<Example 2>
This is an example in which only the voltage (40 kV) and the heating temperature (950 ° C.) are changed and partial heating is performed by the same spot welder (W) among the heating conditions in Example 1.

<Example 3>
This is an example in which, of the heating conditions in Example 1, only the heating temperature (900 ° C.) and the heating holding time (0.6 seconds) were changed, and partial heating was performed by the spot welder (W).

<Example 4>
This is an example of partial heating by the spot welder (W) after a long period of time, with the standing time after drawing being 180 minutes, of the heating conditions in Example 1.

  As is clear from the results described in Table 1 in Examples 2 to 4, the same boundary (c) in FIG. 1 (a) as that in Example 1 was revealed, and no reduction in strength was caused. The effect of preventing cracking and delayed fracture could be achieved.

<Example 5>
In place of the spot welder (W) employed in Examples 1 to 4, a laser heating device (manufactured by Amada Co., Ltd.) (not shown) was used and heated at an output of 240 W at 2 cm / second. It was determined that the surface irradiated with the energy beam was red-hot and heated to 850 ° C. from the degree of color development.

  When left for 48 hours after the heating, defects of the bearing body in the caster were visually observed, but defects such as cracks and tears could not be confirmed. Further, the same plating process as in Example 1 and a running test of the completed caster were also performed, but no defects such as cracks and tears were found.

  And the part heated partially by the energy beam of the said laser heating apparatus was cut, the cut surface was corroded with nital, and when the change state of the structure | tissue was observed with the optical microscope, the photograph of FIG. As shown in the schematic diagram, the boundary (c), which is an intermediate layer of the structure change different from the quenched portion (a) in the quenched state and the unheated remaining portion (b) in the unquenched state, is substantially parallel to the plate surface (d). It was exposed in the shape of a band extending along the direction (HH).

  In Examples 1 and 5, the partial heating temperature was judged sensuously based on the correspondence data (not shown) between the heating temperature and the heating color, but a commercially available appropriate temperature sensor was used. Of course, it may be measured.

  Furthermore, since the heating conditions of Comparative Examples 1 to 4 written together in Table 1 are all out of the numerical range defined by the present invention, it is possible to achieve the desired effect of preventing cracks and delayed fracture. could not. Needless to say, the belt-like boundary (intermediate layer) of the texture change confirmed in Examples 1 to 5 was not exposed under the heating conditions of Comparative Examples 1 to 4.

<Comparative Example 1>
In this case, defects such as cracks and tears were not confirmed when left for 48 hours after heating, but when the trivalent unichrome plating (zinc plating) was applied to the bearing body, some of the cracks (plating cracks) ) Was observed. Furthermore, when a running test was performed on a caster in which wheels and bearings were assembled and completed on a bearing body that did not cause plating cracking, cracking occurred when traveling for 60 km.

  The reason for this is considered to be that the heating stress was extremely low, so that the residual stress of the drawn product could not be relaxed or removed reliably.

<Comparative example 2>
In this case, the heating temperature becomes too high, and the heated portion is melted and the predetermined shape cannot be maintained.

<Comparative Example 3>
In this case, cracking was not confirmed even after standing for 48 hours after heating, but when trivalent unichrome plating (zinc plating) was applied thereafter, cracking occurred in all.

  The reason for this is considered that the heating and holding time is remarkably short and the heating is insufficient to relieve the residual stress of the drawn product.

<Comparative example 4>
In this case, no cracks were confirmed even after standing for 48 hours after heating, and no cracks were observed even after the trivalent unichrome plating (zinc plating) was applied. Cracks occurred at that time.

  As the cause, contrary to the comparative example 3, since the heating and holding time is remarkably long, the influence of the heat spreads over a wide range, causing the entire structure to change and becoming hard and brittle. .

(10) ・ Caster (11) ・ Wheel (12) ・ Bearing body (13) ・ Electrode (P) ・ Place where cracks occur (w) ・ Spot welder (a) ・ Heat-hardened part (b) ・ Non Quenched unheated remaining part (c), boundary (intermediate layer)
(D) ・ Plate surface

Claims (5)

A tensile strength of 400 MPa to 800 MPa high-strength steel sheet is first or prototypely drawn, and the place where cracks occur in the drawn product is specified in advance,
After the same secondary or full-scale drawing of the high-tensile steel plate, only the above-mentioned part specified in advance of the drawn product is subjected to quenching in the optical micrograph of the cut surface and non-quenched Drawing of a high-tensile steel sheet, which is partially heated until the boundary, which is an intermediate layer of a structural change different from the unheated remaining part in the state, appears in a strip shape extending in a direction substantially parallel to the plate surface How to prevent cracks in products.   The partial cracking of a drawn product of a high-strength steel sheet according to claim 1, wherein the partial heating temperature is 400 ° C to 1,000 ° C, and the heating and holding time is 0.1 second to 1.0 minute. Prevention method.   3. A crack in a drawn product of a high-strength steel sheet according to claim 1 or 2, wherein the partial heating method is a method of energizing a pair of electrodes facing each other while sandwiching a predetermined crack generation site. Prevention method.   The method for preventing cracks in a drawn product of a high-strength steel sheet according to claim 1 or 2, wherein the partial heating method is a method of irradiating an energy beam to a crack generation site specified in advance.   The drawn product of the high-strength steel sheet has a U-shaped cross section, and only the bending corner portion specified in advance as a place where a crack occurs in the drawn product is partially heated. A method for preventing cracks in drawn products of high-tensile steel sheets.

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