JP2004291851A - Manufacturing method for vehicle body member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain a nitrided steel sheet from being cracked during a stage from a painting process to a drying process, and to ensure the nitrided steel sheet with stable strength after the drying process. <P>SOLUTION: This manufacturing method for a vehicle body member includes: a plastic working process of performing plastic working for the steel sheet; a nitriding process of nitriding at least part of the plastically worked steel sheet; a welding process of welding the nitrided portion of the steel sheet to another plate material to prepare the vehicle body member constituting at least part of a vehicle body; a painting process of painting at least part of the vehicle body member; and a heating process of heating at least a painted portion of the vehicle body member to a temperature of 190 °C max. to dry a painted film on the painted portion. The method comprises a tempering process of tempering the nitrided portion of the steel sheet at temperatures of over 200 to and 250 °C incl. at timings from the welding process to the painting process. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a vehicle body member, and particularly to a technical field in which a steel plate subjected to a nitriding treatment is used to increase strength.
[0002]
[Prior art]
2. Description of the Related Art In recent years, high-strength steel sheets have been used as plate materials for vehicle bodies from the viewpoint of improving collision safety of automobiles and reducing vehicle weight. As this high-strength steel sheet, a steel sheet having a tensile strength of about 440 MPa is usually used in order to maintain good press formability, but recently, a 590 MPa material having improved press formability (ductility) is also used. It is getting.
[0003]
However, in the case of the 590 MPa material, although the press formability is improved, the strength (hardness) is extremely high, so that it is necessary to increase the pressing ability, and there are production problems such as springback. Further, since the mold life is shortened with the conventional cast iron, tool steel must be used, and further, its surface must be subjected to surface treatment such as TiC treatment to improve wear resistance. In addition, when a steel sheet having higher strength is used, it is difficult to form a complicated shape, and it is necessary to increase the strength of the mold and surface treatment, which requires a large capital investment.
[0004]
Therefore, as shown in Patent Literatures 1 and 2, for example, a mild steel sheet for nitriding treatment (extensive tensile strength of about 280 MPa) excellent in formability is press-formed and then subjected to nitriding treatment to obtain high press-formability and high press formability. It is conceivable to achieve both strength and strength. That is, when the mild steel sheet is subjected to the nitriding treatment in this manner, the hardness of the steel sheet becomes a high hardness of Hv200 to Hv400 in Vickers hardness, thereby making it possible to increase the tensile strength to about 600 to 1000 MPa.
[0005]
Patent Document 3 discloses a method of manufacturing a body member of an automobile, in which a steel sheet is press-formed, nitrided, and then the steel sheet is welded to another sheet material, and then painted and dried. Is disclosed.
[0006]
[Patent Document 1]
JP-A-11-279686 [Patent Document 2]
JP 2002-20853 A [Patent Document 3]
Japanese Patent Application Laid-Open No. 2002-18868
[Problems to be solved by the invention]
However, the above-mentioned nitrided steel sheet has a problem that cracks easily occur because the steel sheet becomes too hard and has relatively low ductility. In particular, as described in Patent Document 3, when a nitrided steel sheet is welded (spot welding), a portion of the nitrided steel sheet adjacent to a nugget (fused joint) is heated to about 700 ° C. or more by heat during welding. A heat-affected zone (hereinafter referred to as HAZ (Heat Affected Zone)), which has been strengthened by quenching at the temperature, is formed, and the outer periphery of the HAZ is 300 ° C. or higher and A _When the temperature reaches _one transformation point or lower, and the nitrogen is re-dissolved to form a solid solution, a re-heated portion having high strength is formed. However, the re-heated portion is very liable to crack.
[0008]
On the other hand, a body member of an automobile is coated after welding as in Patent Document 3 described above, and is usually about 140 ° C. to 190 ° C. (100 ° C. to 100 ° C. including sealer drying) in order to dry a coating film of the coating. (About 190 ° C.). If this heating is used, the nitriding-treated steel sheet is tempered, and in particular, in the above-mentioned reheated portion and the HAZ portion and portions other than the reheated portion, FeN is softened by precipitation and ductility is increased. Cracks can be suppressed.
[0009]
However, in the method of tempering a nitrided steel sheet using the heat of painting and drying as described above, the reheating portion is easily broken until the body member is painted and dried. Cracks may occur during transportation to the process. Moreover, since the coating and drying of the vehicle body member are usually performed in a state where the vehicle body is assembled, the heating temperature differs depending on the position on the vehicle body where the vehicle body member is placed. The strength varies. That is, the hardness of the nitrided steel sheet decreases as the heating temperature increases when the heating temperature is in the range of 100 ° C. to 200 ° C. Therefore, when the heating temperature is different, the hardness of the nitrided steel sheet varies, It becomes difficult to secure stable strength. As a result, when the vehicle body member controls a deformation mode such as a collision member (frame member), it becomes difficult to achieve a desired deformation mode.
[0010]
Therefore, after the welding step and before the coating step, tempering may be performed on the nitrided steel sheet at a constant temperature of 100C to 190C. In this way, cracks in the reheating section before the coating step or the drying step can be suppressed, and the hardness of the nitrided steel sheet, that is, the strength, can be stabilized.
[0011]
However, if the tempering of the nitriding steel sheet is performed at a lower temperature than during the coating drying, the strength changes during the coating drying. Also, even if tempering is performed at a higher temperature than during coating and drying, a certain degree of temperature variation occurs, and as described above, when the heating temperature of the nitrided steel sheet is in the range of 100 ° C to 200 ° C, Since the amount of change in hardness with respect to a temperature change is relatively large, the strength changes due to the temperature variation.
[0012]
The present invention has been made in view of the above points, and an object of the present invention is to manufacture a car body member using a nitriding steel sheet as described above, and to perform a nitriding steel sheet before a painting step or a drying step. In addition to suppressing cracking of the steel sheet, it is intended to stabilize the strength quality of the nitrided steel sheet after the drying step.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, in the present invention, after the welding step and before the painting step, the nitriding portion of the nitriding steel sheet is tempered at a temperature of more than 200 ° C and 250 ° C or less.
[0014]
Specifically, in the invention of claim 1, a plastic working step of plastic working a steel sheet, a nitriding treatment step of nitriding at least a part of the steel sheet subjected to the plastic working, and a nitriding treatment part of the steel sheet and the like. Welding a plate member to form a body member constituting at least a part of the vehicle body; a painting step of painting at least a part of the body member; And a drying step of drying the coating film of the painted portion by heating to the following temperature.
[0015]
After the welding step and before the coating step, a tempering step of tempering the nitrided portion of the steel sheet at a temperature of more than 200 ° C. and 250 ° C. or less is provided.
[0016]
This can prevent the nitriding steel sheet from cracking before the coating step or the drying step, and can stabilize the strength quality of the nitriding steel sheet after the drying step. That is, the hardness of the nitrided steel sheet increases as the heating temperature increases in a range where the heating temperature is higher than 200 ° C. and equal to or lower than 250 ° C. Since the tempering is performed within this temperature range, the nitriding portion is sufficiently softened and ductility increases, and cracking in the reheating portion can be suppressed. Even if the tempering temperature fluctuates to some extent, the variation in hardness of the nitrided portion becomes relatively small. Further, even after the tempering, even if the nitriding steel sheet is heated by painting and drying, the drying temperature is lower than the tempering temperature, so that it does not affect the hardness of the nitriding portion at all, and the Hardness is maintained. As a result, a stable strength can be obtained after the drying step.
[0017]
According to a second aspect of the present invention, in the first aspect of the invention, in the tempering step, only a part of the nitriding portion of the steel sheet is tempered.
[0018]
According to a third aspect of the present invention, in the second aspect of the invention, the welding step is a step of spot-welding the nitrided portion of the steel sheet to another sheet material to produce a vehicle body member. Only the above-mentioned spot weld or its surroundings in the nitrided portion of the above are tempered.
[0019]
According to these inventions, it is possible to reliably perform the tempering of the reheated portion, in which cracks are particularly likely to occur, and it is possible to accurately heat the reheated portion to the set temperature, and to obtain more stable strength. Become like
[0020]
According to a fourth aspect of the present invention, in any one of the first to third aspects, the vehicle body member is a vehicle body frame member.
[0021]
That is, since the deformation mode needs to be controlled in the frame member, it is difficult to achieve a desired deformation mode if the strength varies. However, according to the present invention, since a stable strength is obtained, the intended deformation mode can be reliably achieved, and the effects of the inventions of claims 1 to 3 can be effectively exhibited.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0023]
FIG. 1 is a process diagram showing a method of manufacturing a vehicle body member according to an embodiment of the present invention. In the first step S1, a steel plate (in this embodiment, a mild steel plate having a tensile strength of 280 MPa) is plastically worked by pressing. (Plastic working process).
[0024]
In the next step S2, a part (a predetermined part requiring strength) of the steel sheet subjected to the plastic working is subjected to a nitriding treatment (nitriding step). This nitriding treatment is preferably nitrocarburizing (for example, gas nitrocarburizing, salt bath nitriding, etc.) with a relatively short treatment time, but is not limited to this nitrocarburizing. Further, the entire steel sheet subjected to the plastic working may be subjected to nitriding treatment.
[0025]
In the next step S3, a part of the body of the automobile is formed by spot welding the nitriding part of the nitriding steel sheet and another sheet material (for example, a mild steel sheet (may be another nitriding steel sheet)). (For example, a sub-assembly such as an automobile body frame member) to be manufactured (welding process). Note that, at this stage, the vehicle body member may constitute the entire vehicle body. Further, not limited to spot welding, laser welding or the like may be performed.
[0026]
As shown in FIG. 2, in the nitrided steel sheet 1, the portion adjacent to the nugget 2, which is the fusion joint, is heated to about 700 ° C. or more by the heat at the time of welding, and is quenched by the spot welding. The HAZ portion 3 having increased strength is formed at the temperature of 300 ° C. or more and the A ₁ transformation point or less at the outer periphery of the HAZ portion 3, and nitrogen is re-dissolved to increase the strength. The reheating unit 4 is formed. In FIG. 2, reference numeral 7 denotes another sheet material (the mild steel sheet in FIG. 2) welded to the nitrided steel sheet 1.
[0027]
In the next step S4, the nitriding portion of the nitriding steel sheet is tempered at a temperature exceeding 200 ° C. and 250 ° C. or less (tempering step). This tempering may be performed on the entire nitriding portion by heating the entire nitriding steel plate in a furnace, but only a part of the nitriding portion, specifically, the nitriding process of the nitriding steel plate is performed. It is preferable to perform the process only on the spot welded portion or the periphery thereof (at least the portion including the reheated portion). In this way, it is possible to reliably perform tempering of the reheating portion, which is particularly susceptible to cracking, and to accurately heat the reheating portion to the set temperature. In the case where only a part of the nitriding portion is tempered in this way, for example, high-frequency heating or the like may be performed.
[0028]
In the next step S5, the entire body member is painted (painting step). In the present embodiment, this painting is performed on the entire vehicle body after assembling a plurality of vehicle body members (sub-assemblies) that have undergone the steps S1 to S4 to complete the vehicle body. That is, electrodeposition coating is performed by immersing the entire vehicle body in the electrodeposition liquid.
[0029]
In the next step S6, the electrodeposited coating film is dried by heating the entire vehicle body to a temperature of 190 ° C. or lower (drying step).
[0030]
Although the step is omitted in FIG. 1 after the drying step, a vehicle body sealer is applied to the vehicle body, the vehicle body sealer is dried, and then the intermediate coating is performed, and the coating film of the intermediate coating is dried. Then, a top coat is applied, and the coated film of the top coat is dried. At the time of drying each of the electrodeposition coating, the vehicle body sealer, the intermediate coating, and the topcoating, the vehicle body is heated to 100 ° C or more and 190 ° C or less (excluding drying of the body sealer, which is 140 ° C or more and 190 ° C or less). .
[0031]
The painting and drying need not necessarily be performed on the entire vehicle body, but may be performed on a vehicle body member in a subassembly state. At that time, the entire body member may be painted and dried, and a part of the body member may be painted and at least the painted portion (in order to exert the effect of the present invention, the nitrided portion of the nitrided steel sheet ( In particular, it is desirable to include a spot weld or its surroundings).
[0032]
Here, the relationship between the heating temperature and the hardness (that is, the strength) of the nitriding steel sheet is shown in FIG. This nitriding steel sheet is an ultra-low carbon steel sheet containing 0.07% of Ti and subjected to salt bath nitriding treatment (tuffering: oil cooling at 580 ° C. × 1.5 h).
[0033]
As can be seen from the figure, when the nitrided steel sheet is heated (at 300 ° C. or less), nitrogen dissolved in the matrix precipitates as iron nitride (FeN) and thereby softens. Specifically, at temperatures lower than 100 ° C., it hardly softens. However, when the temperature exceeds 100 ° C., the hardness decreases with a relatively large gradient to around 200 ° C., and when it exceeds 200 ° C., the hardness decreases to 250 ° C. However, when the temperature exceeds 250 ° C., the temperature decreases again to 300 ° C. with a large gradient (a gradient substantially equal to the gradient from 100 ° C. to around 200 ° C.). When heated a plurality of times (300 ° C. or lower), the hardness becomes the maximum heating. That is, when the current heating temperature is higher than the previous heating temperature, the hardness is equivalent to the current heating temperature, and when the current heating temperature is lower than the previous heating temperature, the hardness corresponding to the previous heating temperature is maintained.
[0034]
On the other hand, when the temperature exceeds 300 ° C., the precipitated nitrogen forms a solid solution again, whereby the hardness increases as the heating temperature increases. It is considered that the reheating unit is in a state in which nitrogen has been re-dissolved. Then, when heated to 300 ° C. or lower from this state, nitrogen precipitates as iron nitride and has a hardness corresponding to the heating temperature.
[0035]
If the tempering step is omitted, the coating step and the drying step (including the drying of the body sealer) are performed after the welding step, so that the nitrided steel sheet is heated to 100 ° C or more and 190 ° C or less. In this drying step, the same tempering as in the above tempering step is performed, and the hardness of the nitrided steel sheet can be reduced. Here, FIG. 4 shows the relationship between the distance x (see FIG. 2) from the nugget end and the hardness inside the nitrided steel sheet before and after coating and drying when the tempering step is eliminated after the welding step. From this, it can be seen that the hardness of the reheated portion and the portions other than the HAZ portion and the reheated portion are reduced after the coating is dried.
[0036]
However, if the tempering step is eliminated, the nitriding steel sheet may be broken during the transportation to the coating step or the drying step, and in the drying step, the body member is located at any position of the vehicle body. The heating temperature is different depending on whether it is present, and when the heating temperature is in the range of 100 ° C. to 190 ° C., the hardness (that is, the strength) of the reheated part is extremely high, in particular, due to the relationship between the heating temperature and the hardness of the above-described nitrided steel sheet. It will vary greatly (see hardness after drying in FIG. 4). Such a variation in strength makes it difficult to achieve a desired deformation mode when the body member controls a deformation mode like a frame member (a front frame, a center pillar, or the like). .
[0037]
Further, it is conceivable that the tempering temperature in the tempering step is set at a constant temperature of 100 ° C. to 190 ° C. which is the same as the heating temperature in the drying step. However, even in this case, although it is possible to suppress cracking in the reheating unit before the coating step or the drying step, when tempering the nitrided steel sheet is performed at a temperature lower than that at the time of coating and drying, The strength changes during the drying process.
[0038]
Actually, as a result of performing tempering at 100 ° C. on a nitrided steel sheet having a hardness of Hv 330 after nitriding, the hardness became Hv 325, which decreased to Hv 300 after coating and drying at 140 ° C. Further, as a result of tempering the nitrided steel sheet at 140 ° C., the hardness became Hv300, which was Hv295 after the coating and drying at 140 ° C., but did not change much, but after the coating and drying at 180 ° C. Decreased to Hv265. Further, as a result of tempering the nitrided steel sheet at 140 ° C., the hardness became Hv265, and after the coating and drying at 180 ° C., it became Hv263, and hardly changed.
[0039]
And, even if tempering is performed at a higher temperature than during coating and drying, a certain degree of temperature variation occurs, and as described above, when the heating temperature of the nitrided steel sheet is in the range of 100 ° C to 200 ° C, Since the amount of change in hardness with respect to temperature change is relatively large, the strength changes due to the temperature variation.
[0040]
On the other hand, in the present embodiment, after the welding step and before the painting step, the nitrided portion of the nitrided steel sheet is tempered at a temperature of more than 200 ° C. and 250 ° C. or less. Cracking of the treated steel sheet can be suppressed, and the strength quality of the nitrided steel sheet after the drying step can be stabilized. That is, the tempering is performed at a higher temperature than during the drying step, and in this temperature range, the hardness change amount with respect to the temperature change is relatively small and stable, so even if the tempering temperature varies to some extent, the nitriding treatment is performed. The variation in hardness in the nitrided portion of the steel sheet is relatively small. Therefore, in the drying process after the tempering process, the hardness of the tempering process is not affected at all and the hardness of the tempering process is maintained, and as a result, a stable strength is obtained.
[0041]
Actually, as a result of performing tempering at 210 ° C. on the above-described nitrided steel sheet whose hardness after nitriding treatment is Hv 330, the hardness becomes Hv 264, which becomes Hv 263 after coating and drying at 140 ° C. Hv262 even after coating and drying at 180 ° C., and hardly changed. Further, as a result of tempering the nitrided steel sheet at 250 ° C., the hardness became Hv 275, which after coating and drying at 180 ° C., became Hv 277, and hardly changed.
[0042]
Therefore, in the above embodiment, after the welding step and before the coating step, the nitriding portion of the steel sheet is tempered at a temperature of more than 200 ° C. and 250 ° C. or less. Cracking of the steel sheet can be suppressed, and the strength quality of the nitrided steel sheet after the drying step can be stabilized.
[0043]
【The invention's effect】
As described above, according to the present invention, a plastic working step of plastic working a steel sheet, a nitriding treatment step of nitriding at least a part of the steel sheet, welding the nitriding portion of the steel sheet and another sheet material, A welding step of producing a body member constituting at least a part of the vehicle body, a coating step of painting at least a part of the vehicle body member, and at least a painted portion of the body member, by heating to a temperature of 190 ° C. or less, As a method of manufacturing a vehicle body member by sequentially performing a drying process of drying the coating film of the painted portion, after the welding process and before the painting process, the nitriding portion of the steel sheet is heated to a temperature exceeding 200 ° C and 250 ° C or less. By providing a tempering step of tempering at a temperature of, the cracking of the nitrided steel sheet before the coating step or the drying step can be suppressed, and the nitriding after the drying step can be suppressed. It is possible to stabilize the intensity quality management steel.
[Brief description of the drawings]
FIG. 1 is a process chart showing a method for manufacturing a vehicle body member according to an embodiment of the present invention.
FIG. 2 is a cross-sectional view showing a spot-welded portion of a nitriding steel sheet.
FIG. 3 is a graph showing the relationship between the heating temperature and the hardness of a nitrided steel sheet.
FIG. 4 is a graph showing the relationship between the distance x from the end of the nugget and the hardness inside the nitrided steel sheet before and after coating and drying when the tempering step is eliminated after the welding step.
[Explanation of symbols]
1 Nitrided steel sheet 2 Nugget 3 Heat affected zone (HAZ zone)
4 Reheating section

Claims (4)

A plastic working process for plastic working a steel plate;
A nitriding step of nitriding at least a part of the steel plate subjected to the plastic working, and welding the nitriding part of the steel sheet and another sheet material to produce a body member constituting at least a part of the vehicle body Process and
A coating step of coating at least a part of the vehicle body member,
A method of manufacturing a vehicle body member, comprising: drying at least a painted portion of the vehicle body member to a temperature of 190 ° C. or less, and drying a coating film of the painted portion.
A method for manufacturing a vehicle body member, comprising: a tempering step of tempering a nitrided portion of the steel sheet at a temperature higher than 200 ° C. and equal to or lower than 250 ° C. after the welding step and before the painting step. The method for manufacturing a vehicle body member according to claim 1,
A method for manufacturing a vehicle body member, characterized in that in the tempering step, only a part of the nitriding portion of the steel sheet is tempered. The method for manufacturing a vehicle body member according to claim 2,
The welding process is a process of producing a body member by spot welding a nitriding portion of a steel plate and another plate material,
A method for manufacturing a vehicle body member, characterized in that in the tempering step, only the spot welded portion or its periphery in the nitriding portion of the steel sheet is tempered. The method for manufacturing a vehicle body member according to any one of claims 1 to 3,
A method for manufacturing a vehicle body member, wherein the vehicle body member is a vehicle body frame member.

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