JP7131634B2 - Steel member manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a steel member, and more particularly to a method for manufacturing a steel member having one or both of a nut and a bolt.

In recent years, especially in the field of automobiles, etc., there have been cases of using hot-stamped materials obtained by hot-pressing high-strength steel plates for frame members of vehicle bodies. Further, for example, structural members for automobiles, such as front side members, center pillars, and hinge reinforcements, use steel members in which nuts and bolts are welded to parts made of hot-stamped materials.

A method of joining a nut or a bolt to the surface of a steel plate by a projection welding method is generally used to manufacture a steel member to which a nut or a bolt is welded. Automobile structural members obtained by projection welding are required to have high joint strength between steel plates, nuts, and bolts, and small variation.

Patent Document 1 discloses a structural member for automobiles in which a high-strength steel plate and a nut or bolt are joined by projection welding as a structural member for automobiles having excellent delayed fracture characteristics and static strength characteristics of the welded portion, wherein the nut or An arrangement is disclosed in which a local recess is provided around the projection of the bolt.

Patent Document 2 describes an automobile structural member having a nut or bolt that can obtain excellent delayed fracture properties and high static strength, and a high-strength steel plate and a nut or bolt are joined by projection welding. It is a structural member, and the structure which makes the depth of the plate|board thickness direction of the heat affected zone in a high strength steel plate a predetermined range is disclosed.

In Patent Document 3, the chemical composition of the nut or bolt, as well as the tensile strength, plate thickness, and carbon equivalent of the high-strength steel plate are specified to appropriately control the hardness and toughness of the joint. A projection with excellent static strength such as torque peel strength and indentation peel strength by specifying the ratio of the area of the joint with the high-strength steel plate and the area of the nominal diameter part of the nut or bolt within an appropriate range. A method of manufacturing a welded joint is disclosed.

Japanese Patent No. 5626025 Japanese Patent No. 5613521 Japanese Unexamined Patent Application Publication No. 2012-157900

For example, in automotive structural members, hot stamped materials obtained by hot pressing galvanized steel sheets are sometimes used. When joining a nut or bolt to a hot stamped material with zinc-based plating on the surface by projection welding, the joint strength is greater than when performing projection welding on a hot stamped material without zinc-based plating on the surface. variation is more likely to occur. In addition, when a nut or bolt is joined to a steel material having a zinc-based plating on its surface by projection welding, the joint strength is likely to vary, even in other than structural members for automobiles. Here, zinc-based plating means plating containing zinc (the same shall apply hereinafter).

If a separate joining process such as arc welding is used to ensure a constant joining strength, the cost will increase accordingly. As a result, the use of high-strength steel sheets is hindered by the members.

The present invention solves the above problems, and provides a nut that can stably obtain a high joint strength (specifically, indentation peel strength) when joining nuts and bolts to steel materials having zinc-based plating by projection welding. and bolts.

The present inventors diligently studied a method for stably obtaining high joint strength when joining nuts and bolts to galvanized steel materials by projection welding. As a result, it was confirmed that the cause of the variation in joint strength was the oxide film, which is a high resistance material present on the surface of the steel material.

The present inventors have made further studies and completed the present invention. The gist of it is as follows.

(1) Manufacture of steel members having either or both nuts and bolts by joining steel materials with zinc-based plating on the surface and nuts or bolts with multiple projections by projection welding in which electrical heating is performed while applying pressure. The method comprises a preliminary energizing step, a cool step, and a main energizing step in order, and in the preliminary energizing step, the heights of all the plurality of projections are 0.7 to 0 compared to before the preliminary energizing step. A nut characterized in that the energization is performed so as to increase the current by 9 times, the cool process has a non-energization time of 70 ms or more, and the main energization process applies a higher current than the preliminary energization process. and bolts.

(2) The method of manufacturing a steel member having one or both of a nut and a bolt according to (1), wherein the non-energizing time in the cooling step is 400 ms or less.

(3) The method of manufacturing a steel member having one or both of a nut and a bolt according to (1) or (2), wherein the tensile strength of the steel before joining is 1100 MPa or more.

(4) The method of manufacturing a steel member having one or both of a nut and a bolt according to any one of (1) to (3), wherein the steel member has a contact resistance of 1 mΩ or more at room temperature.

According to the present invention, high joint strength (specifically, indentation peel strength) can be stably obtained even when nuts and bolts are joined to steel materials having zinc-based plating by projection welding.

In projection welding, it is a diagram schematically showing the difference in the magnitude of the current flowing through each projection. FIG. 10 is a diagram showing how the film is destroyed and removed or thinned, the resistance in the region where the current from the projection flows is uniformed, and the current uniformly flows to each projection in the subsequent main energization. (b) is a diagram showing how the thickness of the oxide film on the surface of the steel plate varies and the current flowing through each projection differs in the conventional case. FIG. 4 is a diagram for explaining the relationship between variations in the thickness of the oxide film on the surface of the steel sheet and how current flows in each projection, and the nut holes, screw threads, and screw holes preformed in the steel sheet are omitted. It is a figure explaining the height of the projection of a nut. The threaded holes molded into the nut are omitted. It is a figure explaining the measuring method of contact resistance. 1 is a photograph showing a fracture surface after a push-peeling test in an example. 4 is a graph showing the relationship between the non-energization time in the cool process and the one-side process capability index in the example.

First, the contents of the studies that led to the present invention by the inventors will be described. The case of joining a nut to a zinc-based plated hot stamp material by projection welding will be described below, but the same applies to the case of joining a bolt. Moreover, it is not limited to zinc-based plated hot stamp materials, and the same applies when joining to steel materials having zinc-based plating. The steel material refers to a steel product including a steel plate and a molded product obtained by forming the steel plate.

The present inventors considered that the reason why the joint strength varies when a nut is joined to a galvanized hot stamped material by projection welding is that an oxide film, which is a high resistance body, exists on the surface of the steel plate. More specifically, it is as follows.

In particular, when a galvanized steel sheet is hot pressed, an oxide film is formed on the surface of the steel sheet. The oxide film is not formed with a uniform thickness, but is formed with a certain degree of density, that is, with variations in thickness. For example, if a nut has a plurality of projections, if there is a difference in the thickness of the oxide film at the position where each projection contacts the steel plate, the oxide film is a high resistance body, so the projection contacting the position where the oxide film is relatively thin current flows more easily, and there is a difference in the magnitude of the current flowing in each projection.

FIG. 1 schematically shows this. In the projection welding shown in FIG. 1, the upper electrode 11a is brought into contact with the nut 14, the lower electrode 11b is brought into contact with the steel plate 12, and the plurality of projections 15 of the nut 14 are brought into contact with the steel plate 12, so that the upper electrode is welded. Welding is performed by applying an electric current 16 from 11a toward the lower electrode 11b. FIG. 1(a) is a diagram showing how the thickness of the oxide film 13 in the region where the current flows from the projection 15 to the steel plate 12 is made uniform by the preliminary energization, and the current uniformly flows to each projection 15 by the main energization. FIG. 1(b) shows that the oxide film 13 has variations in density, ie, thickness, and the current flowing through each projection 15 is different.

As shown in FIG. 1(b), if there is a difference in the magnitude of the current 16 flowing through each projection 15, this results in a difference in pressure contact diameter, which causes variations in bonding strength. That is, the projection 15 where the current 16 is concentrated collapses in the early stage of welding, and the projection 15 where the current 16 does not easily flow does not collapse until the final stage of welding. difference occurs. The press-contact diameter refers to the diameter of the joining surface between the projection 15 and the steel plate 12 , which is melted by the current during projection welding and joined to the crushed steel plate 12 .

In order to confirm this, the inventors of the present invention performed a zinc-based plated hot-stamped material obtained by subjecting a zinc-based plated steel sheet to hot pressing. After removing the nut, the joint strength was compared by the indentation peeling test when the nut was attached by projection welding.

As a result, it was confirmed that when the oxide film was removed by shot blasting, the variation in joint strength was reduced and the average value of joint strength was increased when a large number of projection welding was performed. As a result of observation, it was confirmed that when the nut was attached by projection welding without shot blasting, the joint area (press contact diameter) at each projection position was uneven.

From the above, it was confirmed that the variation in joint strength when a weld nut is joined to a galvanized hot stamped material by projection welding is caused by the presence of an oxide film on the surface.

However, the method of removing the oxide film by shot blasting adds one step and cannot solve the problem of high cost. The inventors have studied a method of removing or thinning the oxide film before the process to make the thickness of the oxide film uniform in the regions where the current flows from each projection, and made the present invention. The present invention will be described below.

In the method for manufacturing a steel member of the present invention, a steel material having a zinc-based plating on its surface and a nut or bolt having a plurality of projections are joined by projection welding in which electric heating is performed while applying pressure, and one of the nut and the bolt is joined. Or a method of manufacturing a steel member having both. At that time, before the process of applying current to join the nuts or bolts by projection welding (main current application process), as a preliminary current application process, pressure is applied while the steel material and the multiple projections of the nuts or bolts are in contact with each other. , a step of passing a current lower than that of the main energization, and a step of not energizing as a cool step are provided.

In the pre-energization step, the oxide film 13 of the steel plate 12 in contact with each projection 15 is destroyed and removed or thinned. The oxide film 13 has a high resistivity value and the distribution is not always uniform. The thickness of the film 13 is made uniform, and the resistance in the current path of the contact point of each projection 15 is made almost uniform. Therefore, in the main energization step for forming the joints, as shown in FIG. 1A, the density of the current 16 in each projection 15 is uniform, and uniform joints can be formed at each point.

In order to equalize the current density in the main energization, the projection heights of all the multiple projections on the nut (see Fig. 2) should be 0.7 to 0.9 times the height before the start of the preliminary energization. . In order to make the projection heights of all multiple projections 0.7 to 0.9 times higher than before the start of preliminary energization, the magnitude of current, energization time, pressure, etc. in the preliminary energization process are adjusted. and do.

Here, a method of performing the preliminary energization process so that the projection heights of all the multiple projections are 0.7 to 0.9 times the height before the start of the preliminary energization will be described. As such a method, there is a method in which test welding is performed only by pre-energization using a steel plate which is a flat plate, and various conditions for the pre-energization step are determined in advance. In addition, the projection height before and after the pre-energization process in the test welding is performed using a steel plate that is a flat plate, and the test welding is performed only with pre-energization. Just measure. By this test welding, the preliminary energizing current, energizing time, and applied pressure that make the height of all projections 0.7 to 0.9 times before the start of preliminary energizing are determined, and the preliminary energizing current, energizing time, and Subsequent projection welding may be performed using the applied pressure.

For the projection height before and after the preliminary energization process, for example, with the nut or bolt in contact with the steel plate, measure the total thickness of the steel plate and the nut or bolt including the projection at each projection position, It is obtained by subtracting the thickness of the steel plate and the thickness of the portion excluding the projection of the nut or bolt from the total thickness.

If the projection height of the projection after preliminary energization is in the range of more than 0.9 to 1.0 times that before the start of preliminary energization, the effect of destroying, removing, or thinning the oxide film by preliminary energization is hardly obtained. Therefore, the current density at each projection position (often 3 or 4) is not uniform. If the projection height of the projection after preliminary energization is less than 0.7 times the height before the start of preliminary energization, the projection collapses too much and the required current density cannot be obtained in the main energization step, resulting in improper welding. Gone.

The current value in the preliminary energizing step is lower than the current value in the main energizing step, and can be, for example, 1/3 to 1/5 of the current value in the main energizing step. Specifically, the current value per projection can be set to 1.0 to 1.4 kA in the pre-energization step. The time required for the preliminary energization process is about several tens of milliseconds, and since it is performed by a device for performing projection welding, it does not greatly affect the cost.

Between the preliminary energizing step and the main energizing step, there is provided a step (cooling step) having a non-energizing time (that is, no energizing) for 70 ms or more. If the current is set to 0 in the cooling process, the pressurized state may be maintained. By providing the cooling process, the temperature of each projection is made uniform, and the easiness of current flowing through the projection is also made uniform. As a result, the current flowing through each projection becomes uniform in the main energization, the variation in the joint strength between the steel plate and the nut can be reduced, and the indentation peel strength Cp (one-sided process capability index) increases. It is more effective to set the non-energization time to 80 ms or longer, 180 ms or longer, or 200 ms or longer.

If the time for the cooling process is lengthened, there are not a few demerits in terms of characteristics. However, if the applied pressure is kept constant from the pre-energization process, the longer the cool process time, the lower the Cp and the projection height will decrease, so the heat generation and temperature rise at the joint in the main energization process will occur. Since it becomes difficult to obtain, it is preferably 400 ms or less, more preferably 360 ms or less.

As described above, in the preliminary energizing step, the oxide film on the steel sheet in contact with each projection is destroyed and removed or thinned, and in the cooling step, the temperature of each projection is made uniform. It is possible to uniformly apply an electric current to the steel plate, and it is possible to obtain a steel member with high joint strength between the steel plate and the nut and small variations in joint strength. In the case of a nut or bolt with a general projection, the projection height before projection welding is 0.9 to 1.0 mmmm, and the projection height after main energization is preferably close to 0 mm.

In addition, the process before the preliminary energization process and the process after the main energization process are not particularly limited. As in general projection welding, there may be a step of applying only pressure and holding without current.

The steel material used in the steel member manufacturing method of the present invention is not particularly limited as long as it is a steel material having zinc-based plating on its surface. Hot pressing is one of the ways in which an oxide film tends to form on the steel surface. Hot pressing is often applied to high-strength steel sheets. is suitable. As a steel material having a high-strength zinc-based plating, for example, a zinc-based plated steel material having a tensile strength of 1100 MPa or more before joining can be exemplified.

In addition, the occurrence of variations in joint strength due to projection welding is often caused by the oxide film of the high resistance material on the surface of the steel material. For example, when hot pressing is applied to a galvanized steel sheet, an oxide film tends to form and the contact resistance tends to increase, which is suitable for the present invention.

FIG. 3 shows a method for measuring contact resistance. A steel plate 32 having a zinc-based plating 33 on its surface is sandwiched between an upper electrode 31a and a lower electrode 31b for spot welding. A current I of 1 A is passed through the electrodes. A voltage V1 between the upper electrode 31a and the steel plate 32 and a voltage V2 between the lower electrode 31b and the steel plate 32 are measured.

Let R1 be the electrical resistance between the upper electrode 31a and the steel plate 32, R3 be the electrical resistance between the lower electrode 31b and the steel plate 32, and R2 be the resistance due to the specific resistance of the bulk (base material) of the steel plate 32 itself. R2 can be approximated as zero. Also, the resistance of the upper and lower electrodes 31a and 31b can be approximated to zero. Therefore, the relationship between the measured voltages V1, V2 and the electrical resistances R1, R3 can be approximated as follows.

V1=(R1+R2)×I≈R1×I=R1×1(A)=R1
V2=(R2+R3)×I≈R3×I=R3×1(A)=R3

The contact resistance in the present invention is the larger one of R1 and R3. Although FIG. 3 shows the steel sheet 32 with the zinc-based plating 33 formed on both sides, a steel sheet with the zinc-based plating formed only on one side may be used in the present invention.

The present invention will be described in more detail below using examples. The following examples are examples of embodiments of the present invention, and the present invention is not limited to the following embodiments.

A nut having a projection is welded to a hot-stamped steel plate having a tensile strength of 1.5 GPa, a plate thickness of 2.3 mm, a contact resistance of 12 mΩ at room temperature, and zinc-based plating on both sides, under the conditions shown in Table 1. joined.

As the nut, a nut conforming to JIS B 1196:2010 and having a substantially hemispherical projection on the joint surface was used. More specifically, in the strength class 8T, the projection distance (projection height) of 3 or 4 projections (Comparative Example No. 10 is an example with 4 projections) is 0.90 mm or 1.00 mm. A nut was used.

In this embodiment, before performing projection welding, first, a pierce hole was formed in the steel plate with a drill, and then a guide pin was used to align the center of the pierce hole with the center of the threaded hole of the nut. With the steel plate and the nut superimposed on each other, electric heating was performed while applying pressure with the upper and lower electrodes to perform projection welding.

A stationary projection welder was used for projection welding, and after initial pressurization of 0.5 s, the electrode was held for 0.17 s through a preliminary energizing step, a cooling step, and a main energizing step under the conditions described in Table 1. After that, the pressure was released. The conditions shown in Table 1 were applied to the pressure applied by the upper and lower electrodes throughout the entire process. For each of the conditions listed in Table 1, projection welding was performed under the same welding conditions to prepare 15 specimens.

For the projection height after the preliminary energizing process and after the main energizing process, the total thickness of the steel plate and the nut including the projection is measured with a laser displacement meter, and the thickness of the steel plate and the projection of the nut are calculated from this total thickness. It was obtained as a value obtained by subtracting the thickness of the removed portion. Table 1 lists 15 average values for each of 3 or 4 projection heights. In invention examples (No. 1 to No. 7), it was confirmed that the ratios of the 15 projection heights before and after preliminary energization were all within the range of 0.7 to 0.9 of the present invention. did.

The indentation peel test specified in JIS B 1196:2010 was performed on the prepared test material. At this time, a bolt is screwed through the pierce hole from the steel plate side of the test material where the steel plate and the nut are joined, a compressive load is applied from the head of the bolt, and the load when the nut is peeled (indentation peel strength) is measured. did. Table 1 lists the average value of the indentation peel strength of 15 test materials. In addition, from the indentation peel strength of 15 test materials, the one-side process capability index (Cp) was obtained when the lower limit of the specification was 10.5 kN, and is shown in Table 1. The one-side process capability index (Cp) is an index that indicates how uniformly the target process can produce products with less variation and with the required performance.

In this example, an average indentation peel strength of 10.5 kN or more and a Cp of 1.33 or more were evaluated as "OK", and those that did not satisfy these criteria were evaluated as "NG".

It was confirmed that according to the method for manufacturing a steel member of the present invention, the indentation peel strength of the nut can be stably obtained at a high value without variation.

In the case of manufacturing by the conventional method (No. 8) without pre-energization and cooling steps, the average value of the indentation peel strength was lower than that of the manufacturing method of the present invention, but exceeded the criterion. However, the variation among the test materials was large, and the judgment of the one-sided process capability index was "NG".

In the case of manufacturing by the method (No. 9, 10) including the pre-energization step but not the cooling step, the average value of the indentation peel strength was the same as in the case of the manufacturing method of the present invention. The variation was large, and the judgment of the one-sided process capability index was "NG".

In the case of manufacturing using a method that includes a preliminary energizing step but no cooling step, and the change in projection height in the preliminary energizing step is small (No. 11), the indentation peel strength exceeds the criterion but is a low value. , and the judgment of the one-sided process capability index was "NG".

When the production method includes a preliminary energizing process but no cooling process, and the projection height in the preliminary energizing process is greatly changed (No. 12), the indentation peel strength exceeds the criterion but is low, and the test material There was a large variation between them, and the judgment of the one-sided process capability index was "NG".

Although the preliminary energization and cooling steps are provided, when the projection height is greatly changed in the preliminary energization step (No. 13), the average value of the indentation peel strength is the same as in the manufacturing method of the present invention. , the variation between the test materials was large, and the judgment of the one-sided process capability index was "NG".

Preliminary energization and a cooling process are provided, but when the non-energization time in the cooling process is short (No. 14), the average value of the indentation peel strength exceeds the criterion, but the variation among the test materials is large. , the judgment of the one-sided process capability index was "NG".

FIG. 4 shows the fracture surface of the joint after the indentation peel test. (a) is that of an invention example (No. 1), and (b) is that of a comparative example (No. 8) in which the preliminary energization and the cooling process were not performed. In the invention example, each (three) projection joints were broken almost evenly, but in (b), there was a difference in the broken state, and it was confirmed that zinc oxide remained in the joints. rice field.

FIG. 5 shows the relationship between the non-energization time in the cool process and the one-side process capability index. It was confirmed that the one-side process capability index can be increased by setting the non-energization time of the cool process to 70 ms or longer.

11a upper electrode 11b lower electrode 12 steel plate 13 oxide film 14 nut 15 projection 16 electric current 31a upper electrode 31b lower electrode fs
32 Steel plate 33 Zinc-based plating

Claims (4)

A method of manufacturing a steel member having one or both of a nut and a bolt, wherein a steel material having zinc-based plating on the surface and a nut or bolt having a plurality of projections are joined by projection welding in which electrical heating is performed while applying pressure. hand,
A preliminary energization process, a cool process, and a main energization process are provided in order,
In the preliminary energization step, energization is performed so that the heights of all the plurality of projections are 0.7 to 0.9 times higher than before the preliminary energization step,
The cool step has a non-energizing time of 70 ms or more,
A method of manufacturing a steel member having one or both of a nut and a bolt, wherein a current higher than that in the preliminary energizing step is applied in the main energizing step. 2. The method of manufacturing a steel member having one or both of a nut and a bolt according to claim 1, wherein said non-energizing time in said cooling step is 400 ms or less. 3. The method of manufacturing a steel member having one or both of a nut and a bolt according to claim 1 or 2, wherein the tensile strength of the steel material before joining is 1100 MPa or more. The method for manufacturing a steel member having one or both of a nut and a bolt according to any one of claims 1 to 3, wherein the steel member has a contact resistance of 1 mΩ or more at room temperature.

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