JPWO2020138468A1 - Manufacturing method of steel parts - Google Patents

Abstract

The present invention provides a method for manufacturing a steel member having one or both methods of nuts and bolts, which can stably obtain high joint strength. The steel member is manufactured by projection welding, and the method for manufacturing the steel member includes, in order, a pre-energization step, a cool step having a non-energization time of 70 ms or more, and a main energization step in which a current higher than that of the pre-energization step is passed.

Description

The present invention relates to a method for manufacturing a steel member, specifically, 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, there have been cases where hot stamping materials obtained by hot stamping high-strength steel plates are used as skeleton members of vehicle bodies. Further, for example, in structural members for automobiles such as front side members, center pillars, and hinge reinforcements, steel members in which nuts and bolts are welded to parts made of hot stamping materials are used.

In the production of steel members to which nuts and bolts are welded, a method of joining nuts or bolts to the surface of a steel plate by a projection welding method is common. Structural members for automobiles obtained by projection welding are required to have high joint strength between steel plates and nuts and bolts, and to have small variations.

Patent Document 1 describes 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 a welded portion. A configuration is disclosed in which a recess is locally provided around the projection of the bolt.

Patent Document 2 describes for automobiles in which a high-strength steel plate and a nut or bolt are joined by projection welding as a structural member for an automobile having a nut or a bolt capable of obtaining excellent delayed fracture characteristics and high static strength. Disclosed is a structural member having a structure in which the depth of a heat-affected zone in a high-strength steel plate in the plate thickness direction is within a predetermined range.

Patent Document 3 specifies the chemical composition of nuts or bolts, as well as the tensile strength, plate thickness, and carbon equivalent of high-strength steel plates to appropriately control the hardness and toughness of joints, and further, with nuts or bolts. By specifying the ratio of the area of the joint with the high-strength steel plate to the area of the nominal diameter of the nut or bolt within an appropriate range, projection with excellent static strength such as torque peeling strength and indentation peeling strength A method of manufacturing a welded joint is disclosed.

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

For example, a hot stamping material obtained by hot-pressing a galvanized steel sheet may be used as a structural member for an automobile. When nuts or bolts are joined by projection welding to a hot stamping material with zinc-based plating on the surface, the joining strength is higher than when projection welding is performed on a hot stamping material without zinc-based plating on the surface. Variation is likely to occur. In addition to structural members for automobiles, when nuts or bolts are joined to a steel material having zinc-based plating formed on its surface by projection welding, variations in joining strength are likely to occur as well. Here, zinc-based plating means plating containing zinc (hereinafter, the same applies).

If other joining processes such as arc welding are individually used to secure a certain joining strength, the cost will increase accordingly. As a result, the member hinders the use of high-strength steel sheets.

The present invention solves the above-mentioned problems, and when a nut or a bolt is joined to a steel material having zinc-based plating by projection welding, a nut capable of stably obtaining a high joining strength (specifically, indentation peeling strength). An object of the present invention is to provide a method for manufacturing a steel member having one or both of the bolts and bolts.

The present inventors have diligently studied a method for stably obtaining high joint strength when nuts and bolts are joined to a zinc-based plated steel material by projection welding. As a result, it was confirmed that the cause of the variation in the bonding strength is the oxide film, which is a high resistor existing on the surface of the steel material.

The present inventors further studied and made the present invention. The summary is as follows.

(1) Manufacture of a steel member having one or both of nuts and bolts by joining a steel material having zinc-based plating on the surface and nuts or bolts having a plurality of projections by projection welding in which energization heating is performed while pressurizing. The method includes a pre-energization step, a cooling step, and a main energization step in order. In the pre-energization step, the heights of all the plurality of projections are 0.7 to 0 as compared with those before the pre-energization step. The nut is energized so as to be 9.9 times, and the cooling step has a time of 70 ms or more to be de-energized, and the main energizing step causes a higher current to flow than the pre-energizing step. And a method of manufacturing a steel member having one or both of bolts.

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

(3) A method for manufacturing a steel member having one or both of the nuts and bolts of the above (1) or (2), wherein the tensile strength of the steel material before joining is 1100 MPa or more.

(4) A method for manufacturing a steel member having one or both of the nuts and bolts according to any one of (1) to (3) above, wherein the contact resistance of the steel material at room temperature is 1 mΩ or more.

According to the present invention, even when nuts and bolts are joined to a steel material having zinc-based plating by projection welding, stable and high joining strength (specifically, indentation peeling strength) can be obtained.

It is a figure which shows the difference in the magnitude of the electric current flowing through each projection schematically in the projection welding, and (a) is the case of this invention, and the current flows from the projection to the steel sheet by the pre-energization, and the surface of the steel sheet is oxidized. It is a figure which shows how the film is broken and removed or thinned, the resistance in the region where the current from a projection flows is made uniform, and the current flows uniformly to each projection by the subsequent main current energization. FIG. (B) is a diagram showing how the thickness of the oxide film on the surface of the steel sheet varies and the current flowing through each projection differs in the conventional case. It is a figure for demonstrating the relationship between the variation in the thickness of the oxide film on the surface of a steel sheet, and the flow of a current in each projection, and the nut hole and the screw thread and the screw hole preformed in the steel sheet are omitted. It is a figure explaining the height of the projection of a nut. The screw holes formed in the nut are omitted. It is a figure explaining the measuring method of contact resistance. It is a photograph which shows the fracture surface after the indentation peeling test in an Example. It is a graph which shows the relationship between the non-energized time of a cool process, and one-side process capability index in an Example.

First, the contents of the study that the present inventors have led to the present invention will be described. Hereinafter, a case where a nut is joined to a zinc-based plated hot stamping material by projection welding will be described, but the same applies even when a bolt is joined. Further, the present invention is not limited to the zinc-based plated hot stamping material, and the same applies to the case of joining to a steel material having zinc-based plating. The steel material refers to a steel product including a steel plate and a molded product obtained by molding the steel plate.

The present inventors considered that the reason why the bonding strength varies when a nut is bonded to a zinc-based plated hot stamping material by projection welding is that an oxide film, which is a high resistor, is present on the surface of the steel sheet. More specifically, it is as follows.

In particular, when a zinc-based plated 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 constant thickness, but is formed with a certain degree of shading, that is, variation in thickness. For example, when the nut has multiple 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 resistor, so the projection that contacts the position where the oxide film is relatively thin. The current easily flows through, and the magnitude of the current flowing through each projection differs.

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 a plurality of projections 15 possessed by the nut 14 are brought into contact with the steel plate 12. Welding is performed by passing an electric current 16 from 11a toward the lower electrode 11b. FIG. 1A 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 flows uniformly to each projection 15 by the main energization. FIG. 1B is a diagram in which the oxide film 13 has variations in shade, that is, thickness, and there is a difference in the current flowing through each projection 15.

As shown in FIG. 1 (b), if there is a difference in the magnitude of the current 16 flowing through each projection 15, the pressure contact diameter will be different as a result, and the bonding strength will vary. That is, the projection 15 in which the current 16 is concentrated is crushed at the initial stage of welding, and the projection 15 in which the current 16 is difficult to flow is not crushed until the end of welding. Makes a difference. The pressure contact diameter refers to the diameter of the joint surface of the projection 15 that is melted by the electric current during projection welding and joined to the crushed steel plate 12 with the steel plate 12.

In order to confirm this, the present inventors have attached a nut to a zinc-based plated hot stamping material that has been hot-pressed on a zinc-based plated steel sheet by projection welding as it is, and an oxide film on the surface by shot blasting. When the nut was attached by projection welding after removing the above, the joint strength was compared by the indentation peeling test.

As a result, it was confirmed that when the oxide film was removed by shot blasting, the variation in the bonding strength became small and the average value of the bonding strength became large when a large number of projection weldings were performed. In addition, as a result of observation, it was confirmed that the joint area (pressure contact diameter) at each projection position was non-uniform when the nut was attached by projection welding as it was without shot blasting.

From the above, it was confirmed that the variation in the joining strength when the welding nut was joined to the zinc-based plated hot stamping material by projection welding was due to the presence of the oxide film on the surface.

However, the method of removing the oxide film by shot blasting requires an additional step and cannot solve the problem of high cost. Therefore, the present inventors change the welding conditions to carry out the main energization in projection welding. The present invention has been made by examining a method of removing or thinning the oxide film before the step to make the thickness of the oxide film uniform in the region where current flows from each projection. Hereinafter, the present invention will be described.

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

In the pre-energization step, the oxide film 13 of the steel sheet 12 in contact with each projection 15 is destroyed and removed or thinned. Since the oxide film 13 has a high intrinsic resistance value and the distribution is not always uniform, the oxide film 13 is destroyed and removed or thinned in the pre-energization step to oxidize the region where the current 16 flows from each projection 15. The thickness of the coating film 13 is made uniform, and the resistance in the current path at the contact point of each projection 15 is made uniform. Therefore, in the main energization step for forming the joint portion, as shown in FIG. 1A, the density of the current 16 in each projection 15 becomes uniform, and a uniform joint portion can be formed at each point.

In order to make the current density uniform in the main energization, the projection height (see FIG. 2) of all the projections of the nut should be 0.7 to 0.9 times that before the start of the pre-energization. .. In order to make the projection height of all multiple projections 0.7 to 0.9 times that before the start of pre-energization, adjust the current magnitude, energization time, pressurization, etc. in the pre-energization process. And do it.

Here, a method of carrying out the pre-energization step will be described so that the projection height of all the plurality of projections is 0.7 to 0.9 times that before the start of the pre-energization. Examples of the method include a method in which test welding of only pre-energization is performed using a flat steel plate, and various conditions of the pre-energization process are determined in advance. For the projection height before and after the pre-energization process in test welding, test welding with only pre-energization is performed using a flat steel plate, and before and after pre-energization, for example, with a caliper or a laser-type measuring instrument. You just have to measure. This test weld determines the pre-energization current, energization time, and pressure that all projection heights are 0.7 to 0.9 times higher than before the start of pre-energization, and the pre-energization current, energization time, and energization time. Subsequent projection welding may be performed using the pressing force.

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

When the projection height of the projection after the pre-energization is in the range of more than 0.9 to 1.0 times that before the start of the pre-energization, the effect of destroying, removing or thinning the oxide film by the pre-energization is hardly obtained. Therefore, the current densities at each projection (often 3 or 4) positions are not uniform. If the projection height of the projection after the pre-energization is less than 0.7 times that before the start of the pre-energization, the projection collapses too much, the current density required in the main energization process cannot be obtained, and appropriate welding is performed. It disappears.

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

Between the pre-energization step and the main energization step, a step (cool step) having a non-energization time, which is a time for de-energizing (that is, not energizing) for 70 ms or more, is provided. 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 ease of flow of the current flowing through the projection is also made uniform. As a result, the current flowing through each projection becomes uniform during the main energization, the variation in the joint strength between the steel plate and the nut can be reduced, and the Cp (one-side process capability index) of the indentation peel strength increases. It is more effective when the non-energized time is 80 ms or more, 180 ms or more, or 200 ms or more.

If the cooling process time is lengthened, there are many disadvantages in terms of characteristics. However, when the pressing force is constant from the pre-energization process, if the cooling process time is lengthened, Cp decreases and the projection height decreases, resulting in heat generation and temperature rise at the joint in the main energization process. Since it is difficult to obtain, it is preferably 400 ms or less, more preferably 360 ms or less.

As described above, in the pre-energization step, the oxide film of the steel sheet in contact with each projection is destroyed and removed or thinned, and the temperature of each projection is made uniform in the cool step. It is possible to apply a uniform current to the steel sheet, and it is possible to obtain a steel member having a high joint strength between the steel plate and the nut and a small variation in the joint strength. In the case of a nut or bolt having a general projection, the projection height before projection welding is 0.9 to 1.0 mm, and the projection height after main energization is preferably around 0 mm.

The step before the pre-energization step and the step after the main energization step are not particularly limited. Similar to general projection welding, there may be a step of applying only pressure and holding the current without flowing.

The steel material used in the method for producing a steel member of the present invention is not particularly limited as long as it is a steel material having zinc-based plating on its surface. Hot press is one of the things that easily form an oxide film on the surface of steel materials, and since hot press is often applied to high-strength steel sheets, especially for high-strength galvanized steel sheets used for hot stamping materials. Is suitable. Examples of the steel material having high-strength zinc-based plating include zinc-based plated steel materials having a tensile strength of 1100 MPa or more before joining.

Further, since the variation in joint strength due to projection welding is often caused by the oxide film of the high resistor on the surface of the steel material, it is suitable for the steel material having a contact resistance of 1 mΩ or more at room temperature. For example, when a zinc-based plated steel sheet is hot-pressed, an oxide film is likely to be formed and the contact resistance is likely to increase, which is suitable for the present invention.

The method of measuring the contact resistance is shown in FIG. A single 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 applied to the electrode. The voltage V1 between the upper electrode 31a and the steel plate 32 and the voltage V2 between the lower electrode 31b and the steel plate 32 are measured.

The electric resistance between the upper electrode 31a and the steel plate 32 is R1, the electric resistance between the lower electrode 31b and the steel plate 32 is R3, and the resistance caused by the intrinsic resistance of the bulk (base material) of the steel plate 32 itself is R2. R2 can be approximated to zero. Further, the resistances of the upper and lower electrodes 31a and 31b can be approximated to zero. Therefore, the relationship between the measured voltages V1 and V2 and the electrical resistances R1 and 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 larger resistance value of R1 and R3 is defined as the contact resistance in the present invention. Although FIG. 3 shows a steel sheet 32 in which zinc-based plating 33 is formed on both sides, in the present invention, a steel sheet in which zinc-based plating is formed on only one side may be used.

Hereinafter, the present invention will be described in more detail with reference to 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 with projection by projection welding on a hot stamped steel sheet with tensile strength of 1.5 GPa, plate thickness of 2.3 mm, contact resistance at room temperature of 12 mΩ, and zinc-based plating on both sides, under the conditions shown in Table 1. Was joined.

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

In this embodiment, before the projection welding is performed, first, a pierced hole is formed in the steel plate by a drill, and then the center of the pierced hole is roughly aligned with the center of the screw hole of the nut by using a guide pin. In this state, the steel plate and the nut were overlapped with each other, and the upper and lower electrodes were pressurized while energizing and heating to perform projection welding.

A stationary projection welder was used for projection welding, and after an initial pressurization of 0.5 s, the 0.17 s electrode was held through a preliminary energization step, a cool step, and a main energization step under the conditions shown in Table 1. Later, the pressure was released. The pressure applied by the upper and lower electrodes was set as the condition shown in Table 1 throughout the entire process. For each condition shown in Table 1, projection welding was performed under the same welding conditions to prepare 15 test materials.

For the projection height after the pre-energization process and after the main energization 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 calculated as the value obtained by subtracting the thickness of the excluded part. Table 1 shows the average value of 15 pieces of each projection height having 3 or 4 pieces. In the examples of the invention (No. 1 to No. 7), it was confirmed that the ratios of the 15 projection heights before and after the pre-energization were all within the range of the present invention of 0.7 to 0.9. bottom.

The prepared test material was subjected to the indentation peeling test specified in JIS B 1196: 2010. At this time, the bolt is screwed through the piercing hole from the steel plate side of the test material in which 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 (pushing peel strength) is measured. bottom. Table 1 shows the average value of the indentation peel strength of the 15 test materials. In addition, the one-sided process capability index (Cp) when the lower limit of the standard was set to 10.5 kN was obtained from the indentation peel strength of 15 test materials and is shown in Table 1. The one-sided process capability index (Cp) is an index indicating how uniformly the target process has less variation and can produce a product having the required performance.

In this example, the average indentation peel strength of 10.5 kN or more and Cp of 1.33 or more were set as "OK", and those not satisfying this standard were set as "NG".

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

When manufactured by the conventional method (No. 8) without the 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 judgment standard. However, the variation between the test materials was large, and the judgment of the one-side process capability index was "NG".

When manufactured by the method (No. 9, 10) having a pre-energization step but no cooling step, the average value of the indentation peeling strength was the same as that in the case of the manufacturing method of the present invention, but between the test materials. The variation was large, and the judgment of the one-sided process capability index was "NG".

If the product is manufactured by a method that has a pre-energization process but does not have a cool process, and the change in projection height in the pre-energization process is small (No. 11), the indentation peel strength exceeds the judgment criteria but is low, and between the test materials. The variation was large, and the judgment of the one-sided process capability index was "NG".

When the product is manufactured by a method that has a pre-energization process but no cool process, and the projection height in the pre-energization process is significantly changed (No. 12), the indentation peel strength exceeds the judgment standard but becomes a low value, and the test material is used. The variation between them was large, and the judgment of the one-sided process capability index was "NG".

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

A pre-energization and cooling process is provided, but when the non-energization time is short in the cooling process (No. 14), the average value of the indentation peeling strength exceeds the judgment standard, but the variation between 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 peeling test. (A) is the invention example (No. 1), and (b) is the comparative example (No. 8) in which the pre-energization and cooling steps were not performed. In the example of the invention, each (three) projection joints were broken almost evenly, but in (b), it was confirmed that there was a difference in the broken state and zinc oxide remained in the joints. rice field.

FIG. 5 shows the relationship between the non-energized 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 more.

11a Upper electrode 11b Lower electrode 12 Steel plate 13 Oxide film 14 Nut 15 Projection 16 Current 31a Upper electrode 31b Lower electrode fs
32 Steel plate 33 Zinc-based plating

Claims (4)

A method for manufacturing a steel member having one or both of nuts and bolts, in which a steel material having zinc-based plating on the surface and nuts or bolts having multiple projections are joined by projection welding in which energization heating is performed while pressurizing. hand,
A pre-energization process, a cool process, and a main energization process are provided in order.
In the pre-energization step, energization is performed so that the heights of all the plurality of projections are 0.7 to 0.9 times as high as those before the pre-energization step.
The cooling step has a time of 70 ms or more to be de-energized.
A method for manufacturing a steel member having one or both of a nut and a bolt, characterized in that a higher current is passed in the main energization step than in the pre-energization step. The method for manufacturing a steel member having one or both of a nut and a bolt according to claim 1, wherein the time for de-energizing in the cooling step is 400 ms or less. The method for 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 producing a steel member having one or both of a nut and a bolt according to any one of claims 1 to 3, wherein the contact resistance of the steel material at room temperature is 1 mΩ or more.

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