JP6316912B1 - Manufacturing method for high-strength steel sheet press products - Google Patents

Abstract

[PROBLEMS] To provide a press-worked product of a high-strength steel plate that does not cause cracking or delayed fracture. A press-worked product formed by drawing from a high-tensile steel plate having a tensile strength of 400 MPa to 800 MPa using a press die, wherein only a predetermined crack occurrence site (P) of the press-worked product is specified. The boundary (c), which is an intermediate layer of a structural change that is different from both the heated portion (a) in the quenched state by partial heating and the unheated remaining portion (b) in the non-quenched state, is substantially parallel to the plate surface (d) of the steel plate. It is characterized in that it is shown in the optical micrograph of the cut surface at the place where cracks are generated (P) as a band extending along a specific direction (HH). [Selection] Figure 1

Description

The present invention relates to the pressed product of the process for the preparation of high-tensile steel sheet that does not cause such as season cracking or delayed fracture.

  In general, when a metal plate is drawn by a press die, depending on the processing, tensile stress remains in the material, so that the opening of the drawn product is left for several hours to several days after the completion of processing. Phenomenon that cracks spontaneously occur from the edge (called cracks, time cracks or aging cracks), or damage caused by hydrogen absorption in processes such as corrosion, welding, pickling and electroplating (called delayed cracks and plating cracks). There is a problem.

  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. It is also widely used in automobiles, etc., because it is also useful for weight reduction. However, its ductility is lower than that of general steel. Prone to occur. 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 extremely 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, claim 1 is a method for producing a press-formed product of a high-tensile steel sheet having a tensile strength of 400 MPa to 800 MPa. And

  In order to preliminarily specify the place where cracks occur in the press-processed product, the high-tensile steel sheet is first or prototypely drawn by a press die, or an alternative press die design stage. A preliminary process of drawing analysis by simulation in

  Continuously or separately from the preliminary process, using the same press mold as the preliminary process, a production process that is secondarily or fully drawn under the same processing conditions;

  Only in the pre-specified place where cracking occurs in the press-processed product that has been drawn in the production process, the temperature range is 400 ° C. or more and less than 1,000 ° C., and the holding time is 0.1 second to The boundary of the steel sheet is an intermediate layer of a structural change different from both the heated part in the quenched state and the unheated remaining part in the non-quenched state. As a band extending along a direction substantially parallel to the plate surface, it is expressed in an optical micrograph of a cut surface at the place where the crack is generated.

  According to a second aspect of the present invention, partial heating in the heating step is performed in a temperature range of 600 ° C. or higher and lower than 900 ° C. and a holding time of 0.2 to 20 seconds.

  In the third aspect, the partial heating method in the heating step applies a voltage of 0.2 to 20 seconds at 10 kV to 45 kV while sandwiching a place where a crack is generated in advance with a pair of electrodes in a spot welder. It is a method.

According to the manufacturing method of claim 1, only the place where the crack is generated (acquired) in the preliminary process in the press-processed product is subjected to full-scale or secondary drawing in the actual process. After the completion of the process, partial heating is performed under a predetermined heating temperature and its holding time. There is versatility to easily obtain various types of pressed products.

Moreover, the heating process for performing partial heating at the heating temperature and the holding time can be easily incorporated into the production line as a subsequent process of the production process for secondary or full-scale drawing of a pressed product. As a result, there is also an effect that the productivity can be remarkably improved in combination with partial heating of only the place where the place crack occurs in the press-processed product.

In that case, if the structure of Claim 2 is employ | adopted, there exists an effect which can prevent the generation | occurrence | production of the setting crack and delayed fracture in the said press-worked product more efficiently and reliably.

Furthermore, if the configuration of claim 3 is adopted, even if there is a change in the drawn target shape of the press-processed product, the place where the crack occurs can be easily and reliably sandwiched by the electrode of the spot welder, In addition, there is an effect that can be quickly heated from the center of the material toward the surface (plate surface), and in this sense, it helps to improve productivity.

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 press work product concerning the present invention. FIG. 3 is a front view of FIG. 2. It is explanatory drawing which shows the series of processing order as the press main body of the bearing main body of a caster. It is a perspective view which shows the place crack generation | occurrence | production site | part of the bending corner location in the said bearing main body. FIG. 6 is a schematic slope view of a process of partially heating the place where cracks occur in FIG. 5 with a spot welder. Similarly, it is explanatory drawing of the heating process energized while pinching a crack generation part. It is a front view which shows the heating state of the said crack generation part.

Hereinafter, the configuration of the present invention will be described in detail. The present invention is a press-processed product obtained by drawing a high-tensile steel sheet into various target shapes using a press die, and a general-purpose manufacturing method thereof, and aims to prevent occurrence of cracks and delayed fracture.

The high-tensile steel plate employed in the present invention 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 adopting the present invention, while those having a tensile strength exceeding 800 MPa are ordinary. Since the production by the press working method is difficult and the cause of the occurrence of the cracking and delayed fracture is greatly different, the present invention cannot be adopted.

As a result of investigating and examining the forming characteristics of press-worked products in which the inventor has drawn high-strength steel sheets into various target shapes using a press die, It was found that it was concentrated at the location, and that specific location was mainly the bending corner where tensile stress remained easily. Therefore, it is considered that removing or relieving the residual stress at the bending corner portion will solve the problem.

Therefore, in order to obtain press-processed products with various target shapes that do not cause settling or delayed fracture, the high-tensile steel plate with the above-mentioned tensile strength, which is the material, is drawn first or as a prototype using a press die. The place where cracks in the drawn product are generated is specified (knowledge) in advance.

Instead of executing the prototype drawing process, a drawing forming analysis by computer simulation may be performed at the press die design stage to specify the place where the set crack is generated in advance.

In any case, after performing the preliminary process as described above, using the same press die as the preliminary process, under the same processing conditions, the high-strength steel plate of the same material is secondarily or fully developed. Draw molding. Such a production process may be executed subsequent to the preliminary process, or may be executed independently in time or place.

The temperature range is 400 ° C. or higher as a heating step after a certain period of time such as 30 minutes from the end of the press-formed product that has been drawn in the production process. Partial heating is performed at a temperature lower than 1000 ° C. and a holding time of 0.1 seconds to 1.0 minutes.

As a partial heating method in the heating process, a pair of electrodes facing each other equipped with a spot welder (AC resistance welder) or the like is used to place a press-processed product that has been drawn in advance. Starting with the method of heating from the center of the high-tensile steel plate that is the raw material while sandwiching the crack generation site, an energy beam such as a laser is applied to the predetermined crack generation site of the pressed product. Irradiation and heating from the surface of the high-strength steel plate that is the material, induction heating method using high frequency, gas flame and arc method, etc. can be widely adopted, especially materials such as the above spot welder It is most preferable to use a direct current heating method that can quickly heat the entire surface from the center of the substrate toward the surface of the place where the crack occurs.

Further, the temperature range of partial heating is set to the above 400 ° C. or higher and lower than 1000 ° C., preferably 600 ° C. or higher and lower than 900 ° C. Even if it is partial heating, if the temperature is less than 400 ° C., it is not possible to obtain a large structural change as expected, and the residual stress cannot be relaxed sufficiently. is there. A structure change boundary (intermediate layer) which is different from any of a heated portion and a non-heated remaining portion described later does not appear in a strip shape.

For detection of the heating temperature, various general sensors for temperature measurement can be used, and a determination method based on a color change in the heated portion can also be employed.

On the other hand, when the heating temperature is higher than 1000 ° C., the influence of heat spreads over a wide range, and the entire structure is changed into a quenched state, and the heated portion becomes hard and brittle, so that the impact resistance is reduced. Invite. Even under this heating condition, the boundary (intermediate layer) of the structural change that is different from any of the heating part and the non-heated residual part, which will be described later, does not appear in a strip shape. When the temperature is further increased, the base material is melted, resulting in a problem that the shape cannot be maintained.

In the case of partial heating by the direct current heating method such as the spot welding machine, a voltage of 10 kV to 45 kV is applied although it depends on the thickness of the material. If the voltage is lower than 10 kV, partial heating becomes insufficient and it is difficult to form a quenched portion. If the voltage is higher than 45 kV, the entire heated portion is baked and hard and brittle. It causes problems such as becoming melted.

Furthermore, the holding time for the partial heating is set to 0.1 second to 10 minutes, 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 the residual stress is relieved is insufficient. May occur. There is also a possibility that the strength of the pressed product may vary.

On the other hand, when the heating holding time is longer than 1.0 minutes, it is sufficient for the relaxation of residual stress, but as in the case where the heating temperature exceeds 1000 ° C., heat is conducted in a wide range, There is a problem that the strength change becomes excessive, and the design strength of the pressed product cannot be secured. Moreover, as a result of requiring a long time for post-processing, the productivity is also lowered.

Therefore, in short, the direct current heating method and the energy beam are applied only to the cracked part specified in the previous preliminary process for the press-processed product that has been subjected to the drawing by the press die in the heating process. Depending on the method, the partial heating is performed at a temperature range of 400 ° C. or higher and lower than 1000 ° C. and a holding time of 0.1 seconds to 1.0 minutes.

Then, as shown in the schematic diagrams of FIGS. 1 (a) and 1 (b), the cut surface of the crack generation site (P) in the press-processed product (P) is then mechanically polished to form a 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 structure changes different from the quenched part (a) and the non-quenched remaining part (b). The boundary (c), which is the intermediate layer, appears as a strip extending along a direction (HH) substantially parallel to the plate surface (d) of the high-tensile steel plate that is the material.

In other words, in the optical micrograph of the cut surface at the place where cracks occur (P) after heating, both the heated part (a) in the quenched state and the unheated remaining part (b) in the unquenched state The place where cracks occur in the press-processed product until the boundary (intermediate layer) (c) of different texture changes appears in a strip shape extending along a direction (HH) substantially parallel to the plate surface (d) ( Only P) is partially heated.

It is manufactured through the production process and the heating process, and in the optical micrograph of the cut surface at the crack generation site (P), the quenched part (a) and the non-quenched unheated remaining part (b) The number of post-manufactured press-processed products in which the boundary (intermediate layer) (c), which is different from any one, appears in a strip shape extending along the direction (HH) substantially parallel to the plate surface (d) Even after visually observing after being left for days, no cracking or delayed fracture occurred. As a result, by controlling and controlling the partial heating conditions, various press-worked products that do not cause cracking and delayed fracture can be manufactured for general use.

As understood from the above description, the manufacturing method of the press-processed product of the high-strength steel sheet according to the present invention generally uses the high-tensile steel sheet as a material before drawing in order to alleviate general residual stress. It is not a method of heating, nor is it a method of partially heating the steel plate before or during drawing so as to provide a softened portion in the high-strength steel plate of the same material.

It is a method of partially heating the press-processed product that has been drawn in the production process to the last, and therefore there is a risk of a decrease in productivity, but the execution of primary or prototype drawing of the high-tensile steel plate, In the preliminary process that performs drawing forming analysis by simulation at the press die design stage instead of the execution, only the part where the crack has been identified (understood) is then partially heated. Therefore, even if the target shape of the various press-worked products changes in a complicated manner, it has versatility that it can be heated accurately at all times. There is no possibility of causing excessive deformation, and there is no reduction in strength or productivity.

Furthermore, since the heating process is a partial heating only for the place where cracks have been specified in advance, the heating and holding time is only 0.1 seconds to 1.0 minutes, so it is combined with the tact of the production process. In addition, when drawing in the production process is performed a plurality of times, it is easy to join in the middle of the process, and in this sense, it is possible to prevent the productivity from being lowered.

<Example 1>
In the following, embodiments of the present invention applied to a bearing body of a caster will be described. However, the press-formed product of the high-tensile steel sheet is not limited to the bearing body of the caster, but can be drawn to various target shapes. Targets molded press-formed 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.

Although the press-processed product immediately after the processing was visually observed, defects such as cracks and tears could not be confirmed. Therefore, after leaving for 12 hours as it was, the press-processed product was visually observed again. Cracks were found at the bending corners indicated by Moreover, about the press-processed product in which the crack was not discovered, when the trivalent unichrome plating (galvanization) was given, the crack was found in the bending corner part shown with the code | symbol (P) of the same FIG. 5, and a bearing main body (12) It was in a state that could not be used as.

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. In the results of Table 1, the partial heating according to the present invention at the bending corner portion (placement crack occurrence portion) specified in advance contributed to the prevention of the placement crack, plating crack, and delayed fracture of the pressed product. Conceivable.

The partially heated bending corner part (placement cracking part) (P) of the bearing body (12) is cut along the line ZZ in FIG. 8, and the cut surface is polished and then subjected to corrosion treatment with nital. The structure shown in the photographed optical micrograph is as shown in the schematic diagram of FIG. 1 (a). The photograph shows the result of the partial heating (evidence) in the quenched state (a) And the boundary (c), which is an intermediate layer of the structure change different from any of the unheated remaining part (b) in the non-quenched state, extends along a direction (HH) substantially parallel to the plate surface (d) It was expressed in a belt shape. As a result of the partial heating at the heating temperature and the holding time at which the structural change occurs as the belt-like boundary (intermediate layer) (c), the mechanical performance of the bearing body (12), which is a press-formed product that has been drawn. The strength was not reduced.

<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, in which only the standing time after drawing is 180 minutes among the heating conditions in Example 1.

As is clear from the results described in Table 1 in Examples 2 to 4, the same belt-like boundary (intermediate layer) (c) in FIG. It was possible to prevent the occurrence of cracks and delayed fracture.

<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 the caster (10) was allowed to stand for 48 hours after the heating, the bearing body (12) in the caster (10) was visually observed for defects. However, 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 (10) were also conducted, but no defects such as cracks and tears were confirmed.

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 structural change that is different from any of the heated part (a) in the quenched state and the unheated remaining part (b) in the unquenched state, is also the plate surface (d ) In a strip shape extending along a direction (HH) substantially parallel to ().

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. As described above, measurement may be performed as described above.

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) (c) of the structure 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. However, when the trivalent unichrome plating (zinc plating) was applied to the bearing body (12), some of the cracks were cracked. (Plating crack) was observed. Furthermore, when a running test was conducted on the caster (10) in which wheels and bearings were assembled and completed on the bearing body (12) which did not cause plating cracking, cracking occurred at the time of running for 60 km.

The reason is considered that the residual temperature of the pressed product could not be relieved or removed reliably because the heating temperature was extremely low.

<Comparative example 2>
In this case, the heating temperature becomes too high, 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 (3)

A method for producing a press-formed product of a high-tensile steel plate having a tensile strength of 400 MPa to 800 MPa,
In order to preliminarily specify the place where cracks occur in the press-processed product, the high-tensile steel sheet is first or prototypely drawn by a press die, or an alternative press die design stage. A preliminary process of drawing analysis by simulation in
Continuously or separately from the preliminary process, using the same press mold as the preliminary process, a production process that is secondarily or fully drawn under the same processing conditions;
Only in the pre-specified place where cracking occurs in the press-processed product that has been drawn in the production process, the temperature range is 400 ° C. or more and less than 1,000 ° C., and the holding time is 0.1 second to By comprising a heating step for performing partial heating for 1.0 minute,
The boundary, which is an intermediate layer of a structural change different from both the heated part in the quenched state and the unheated remaining part in the unquenched state, is formed as a band extending along a direction substantially parallel to the plate surface of the steel plate. A method for producing a press-worked product of a high-strength steel sheet, characterized by being shown in an optical micrograph of a cut surface at a crack occurrence site.   2. The high-tensile steel press-formed product according to claim 1, wherein the partial heating in the heating step is performed at a temperature range of 600 ° C. or more and less than 900 ° C. and a holding time of 0.2 to 20 seconds. Manufacturing method.   The partial heating method in the heating step is a method of applying a voltage of 0.2 to 20 seconds at 10 kV to 45 kV while sandwiching a place where a crack is generated in advance with a pair of electrodes in a spot welder. The manufacturing method of the press-worked product of the high strength steel plate of Claim 1 characterized by these.

Images