JP2007321182A - Automobile member having high collision absorbing performance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an automobile member having excellent collision absorbing performance by using a stainless steel sheet having high strength as the stock, and controlling the shape of a nugget formed by spot welding. <P>SOLUTION: The automobile member is assembled by subjecting a stainless steel sheet containing, by mass, ≤0.15% C, ≤1.0% Si, ≤2.0% Mn, 6.0 to 11.0% Ni, 16.0 to 22.0% Cr and ≤0.20% N to spot welding. The nugget formed by the spot welding has a thickness D<SB>1</SB>; D<SB>1</SB>=(0.6 to 1.6)×t and a diameter D<SB>2</SB>; D<SB>2</SB>=(3.0 to 5.5)×t in the relation with the sheet thickness t of the stainless steel sheet. The weld zone has a metal structure comprising an austenitic phase preferably in the ratio of ≥30 vol.%. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an automobile member having a large impact absorption capacity, such as a vehicle body, a structural member, and a reinforcing member of an automobile including a truck, which is assembled by spot welding of stainless steel.

In the field of automobiles, studies are being actively conducted on energy saving and weight reduction considering environmental measures while ensuring the safety of passengers in the event of a collision. In terms of safety, in order to reduce the impact on the human body, a structure that absorbs shock by buckling deformation of some of the parts incorporated in the vehicle body is studied, and the material has the characteristics necessary for that structure. Development is desired.
An example of the impact absorbing member is a crash box. The crash box requires performance that can reduce the number of repairs and ensure the safety of passengers by preventing damage to the vehicle body including the side members during minor collisions and absorbing high energy during high-speed collisions. Is done. In order to develop a shock absorbing capacity that satisfies the required characteristics, it is necessary to maintain a high average load during the crushing process of the crash box, and an excellent shock absorbing capacity is desired for the material itself.

Spot welding is often used in the car body assembly line, and assuming that the crash box is manufactured by spot welding of steel plates, the strength of the spot welded part is absorbed by the energy absorbed by the crush box (shock absorbing member) crushing. Can also be said to have a big impact. In this respect, it is important that not only the base metal but also the spot welded portion is excellent in strength and shock absorption capability.
Conventionally, high-strength steel has been used for automobile bodies and reinforcing members, and high-strength materials with a tensile strength of 780 N / mm ² or more are applied to enable thin and light weight while ensuring high shock absorption capacity. Has also been considered. The use of austenitic stainless steel for shock absorbing members is also introduced in Patent Document 1.
JP 2002-20843 A

  When high-strength and high-strength steel is spot-welded, martensite is generated in part or all of the weld. Although the welded part may be strengthened by martensite, the welded part may become brittle due to martensitic transformation depending on the welding conditions. In particular, the influence of embrittlement is likely to appear under the condition that the stress to be peeled off in the plate thickness direction is applied to the welded portion from the outside, and the welded portion may break at a considerably lower stress than the base metal. On the other hand, in order to obtain a shock-absorbing member that can absorb a larger amount of impact energy and is lighter, development of a material that has higher strength and superior shock-absorbing ability than conventional high-strength steel is desired.

  The present invention has been devised to meet such demands, using austenitic high-strength steel as a raw material, and controlling the nugget shape formed by spot welding, thereby making spot welding using ordinary high-strength steel as a raw material. An object of the present invention is to provide an automobile member having a shock absorbing ability far superior to that of the case.

The automobile member of the present invention has C: 0.15% by mass or less, Si: 1.0% by mass or less, Mn: 2.0% by mass or less, Ni: 6.0 to 11.0% by mass, Cr: 16. A stainless steel plate having a composition containing 0 to 22.0 mass%, N: 0.20 mass% or less, and the balance including Fe and inevitable impurities, is assembled by spot welding.
Nugget formed by spot welding, 0.6 to 1.6 times the thickness D ₁ is the thickness t, it is preferable diameter D ₂ is 3.0 to 5.5 times the plate thickness t. The nugget and the weld heat affected zone (hereinafter collectively referred to as “welded zone”) preferably contain 30 volume% or more of austenite.

Steel materials for automobile members include annealed materials and plate materials subjected to temper rolling and processing. It is not necessary that all the materials constituting the automobile member are the above stainless steel plates, and other stainless steels, ordinary steels, non-ferrous metals, etc. may be used in part. Spot welding may be either the same material welding of the stainless steel plate or a different material welding with another metal.
Bicycle bodies are also used under the same environment as automobile parts, and are required to have excellent shock absorption. From this point of view, in this specification, the term “automobile member” is used to include a bicycle body.

Effects and embodiments of the invention

  The inventors of the present invention have made various studies on materials suitable for members that require high collision safety performance in a spot welded state among automobile members. As a result, it has been found that it is effective to produce an automobile member by spot welding using a stainless steel plate having both high strength and high ductility as a material. In particular, when the diameter along the plate thickness direction and plate surface direction of the nugget is regulated in relation to the plate thickness, and further when the austenite of the welded portion is quantitatively regulated, the shock absorbing ability is further improved. I found.

Hereinafter, the components and contents of the stainless steel plate used in the present invention will be described individually.
(Ingredient design)
C: 0.15% by mass or less C is an alloy component that is important for increasing strength, and as the strength increases, the impact absorbing ability also improves. In this respect, it is preferable that a large amount of C is contained, but an excessive amount of C adversely affects the formability and the corrosion resistance of the welded portion, so the upper limit was made 0.15% by mass. Preferably, the C content is selected in the range of 0.02 to 0.1% by mass.

-Si: 1.0 mass% or less Although it is a component added as a deoxidizer, it also has the effect | action which strengthens a solid solution. However, excessive content adversely affects moldability, so 1.0 mass% is the upper limit. Preferably, the Si content is selected in the range of 0.2 to 0.8 mass%.
Mn: 2.0% by mass or less Although there is an effect of suppressing the formation of δ ferrite in a high temperature range, excessive addition is not preferable for corrosion resistance, so 2.0% by mass was made the upper limit. Preferably, the Mn content is selected in the range of 0.2 to 1.5 mass%.

Ni: 6.0 to 11.0% by mass
It is an alloy component necessary for austenite formation and contributes to improvement of corrosion resistance. In order to obtain the effect of adding Ni in this component system, 6.0 mass% or more (preferably 6.5 mass% or more) Ni is required. However, it is an expensive element, and even if it is added in excess of 11.0% by mass, the effect of improving corrosion resistance is saturated, which is economically disadvantageous.

・ Cr: 16.0 to 22.0 mass%
It is an element indispensable for improving corrosion resistance, and depending on the place where the impact absorbing member is used, it is possible to omit plating normally employed in high-tensile steel. In order to ensure the corrosion resistance necessary for the impact absorbing member, the Cr content is set to 16.0% by mass or more. However, excess Cr exceeding 22.0% by mass causes the δ ferrite generated in the high temperature region to remain, causing deterioration of workability. Moreover, since it is necessary to increase the amount of Ni added in accordance with the increase of Cr in order to maintain the austenite structure, it is economically disadvantageous. Preferably, the Cr content is selected in the range of 16.5 to 19.0 mass%.

N: 0.20% by mass or less Like C, it is an effective component for increasing the strength of steel and improving the shock absorption capacity. By substituting a part of C with N to suppress the addition of a large amount of C, it is possible to avoid the sensitization of the weld that tends to occur in the cooling process after spot welding. However, excess N adversely affects ductility and formability, so 0.20% by mass was made the upper limit. Preferably, the N content is selected in the range of 0.01 to 0.15% by mass.
In addition, it may contain elements such as Cu, Mo, Nb, V, Ti, Mg, and Al that are inevitably mixed in the steelmaking stage, but the upper limit for Cu and Mo is 1.0% by mass. The upper limit is preferably 0.1% by mass.

[Nugget formed by spot welding]
Automobile parts are assembled by spot welding stainless steel plates adjusted to a predetermined composition, but if the shape of the nugget formed by spot welding is regulated in relation to the thickness of the stainless steel plate, welding that utilizes the strength of the base metal A joint is formed. Specifically, assuming that the thickness of the stainless steel plate is t, the thickness of the nugget is D ₁ , and the diameter is D ₂ , D ₁ : (0.6 to 1.6) × t, D ₂ : (3.0 (5.5) × t is established (FIG. 1).
In spot welding, nuggets are formed at the joint interface, but the vicinity of the plate surface remains the base material. The remaining base material is greatly different in form from welds produced by fusion welding such as TIG, MIG, etc. The strength of the entire weld is greatly affected by the strength of the nugget, the base material, and the size of the nugget. . Thickness D ₁ is an important factor for stabilizing welding strength, and diameter D ₂ is an important factor for increasing the strength of the weld. Welding with the required strength with D ₁ ≧ 0.6 × t and D ₂ ≧ 3.0 × t, respectively. Although a joint can be obtained stably, if D> 1.6 × t and D ₂ > 5.5 × t, the proportion of the base material in the welded section becomes too small, and the high tensile properties of the stainless steel plate are impaired.

The weld strength improves as the thickness D ₁ increases, but the thickness D ₁ exceeding 1.6 × t promotes the generation of dust, and depending on the alloy component, it may cause carbide to precipitate at the grain boundaries of the weld heat affected zone. Also become. When the weld heat-affected zone where carbides are deposited is exposed on the steel sheet surface, the corrosion resistance deteriorates. Even with a large-diameter nugget exceeding D ₂ > 5.5 × t, dust is likely to be generated and the welding conditions become unstable.

[Austenite content of nugget and weld heat affected zone]
When a stainless steel plate is spot-welded, a martensite phase or a ferrite phase may be generated in a part of the weld depending on the chemical composition of the counterpart material. When a martensite phase containing excessive C is generated, embrittlement due to martensite occurs in the weld. Such a weld has a reduced impact absorption capability, and may be broken even when the stress in the plate thickness direction applied to the spot weld is considerably low.

  The embrittlement caused by martensite can be alleviated by setting the amount of austenite in the nugget and the weld heat affected zone to 30 volume% or more (preferably 50 volume% or more). The amount of austenite: 30% by volume or more depends on the components of the stainless steel to be spot welded and the counterpart material, but is easily achieved by using stainless steel that satisfies the component conditions of the present invention. Furthermore, it is more desirable to adjust the Ni amount higher and the Cr amount lower. Even in the weld zone where the ferrite phase is generated, if the austenite phase is present at a ratio of 50% by volume or more, the strength and toughness are maintained at a high level, and an excellent shock absorbing ability can be obtained.

  100 kg of a steel material having the chemical composition shown in Table 1 was melted in a vacuum melting furnace, and a hot-rolled sheet having a thickness of 2.5 to 4.0 mm was manufactured through casting and hot rolling. In the table, A1 to A6 are steels that satisfy the component conditions specified in the present invention, B1 to B3 are comparative steels, B1 is equivalent to 590N grade ordinary high strength steel, and B2 is equivalent to 980N grade ordinary high strength steel. , B3 is a steel type containing an excessive amount of C.

Each hot-rolled sheet is subjected to annealing at 1100 ° C. × soaking 60 seconds → furnace cooling, cold-rolled after pickling, and then subjected to 1080 ° C. × soaking 60 seconds → air-cooling finish annealing to a thickness of 1.0 mm. A fire annealed sheet was obtained. About a part of steel type A1, it was set as the tempered rolled board of 1.0 mm in thickness which performed temper rolling of 15% of reduction ratio. For even steels B1, B2, annealing, repeated cold rolling, plate thickness: tensile 1.0mm strength was adjusted production conditions so that the ^{700N / mm 2, 1050N / mm} 2 , respectively.
Table 2 shows the mechanical properties of stainless steel as a base material.

Each steel plate was spot welded and the welding strength was investigated.
In the spot welding, a dome radius type electrode was used, the welding pressure was changed between 300 to 500 kgf and the welding current between 4000 to 6500 kA, and weld joints with different nugget thickness D ₁ and diameter D ₂ were formed.

A test piece specified in JIS Z3136 was cut out from the welded stainless steel plate, and the welding strength at room temperature was measured by a test method specified in JIS Z3137.
Further, after polishing the cross section of the welded portion, the metal structure was revealed by etching using a mixed solution of hydrofluoric acid / nitric acid / glycerin, and the thickness D ₁ and the diameter D ₂ of the nugget were measured by microscopic observation.
Furthermore, after confirming the metal structure of the welded part, the sample of the nugget and the weld heat affected zone is cut out, the cutting distortion introduced at the time of cutting is removed by electropolishing, and the amount of magnetization is then measured using a vibrating sample magnetometer did. The amount of ferrite and martensite were calculated by comparing the measured value with the amount of saturation magnetization, and the balance was determined as the amount of austenite.

  In order to investigate the influence of the welded part on the shock absorption capacity, a spot welded 70 mm square and 300 mm high rectangular tube member was subjected to a falling weight test. The spot welding conditions were the same as those when the spot welding strength was investigated except that the spot interval was set to 25 mm. A weight of 400 kg in the longitudinal direction of the rectangular tube member was collided by colliding it at a speed of 25 km / hour, and the absorbed energy in the buckling deformation process with a displacement of 0 to 100 mm was measured.

As can be seen from the investigation results in Table 3, D ₁ = (0.6 to 1.6) × t, D ₂ = (3.0 to 5.5) × t satisfying thickness D ₁ and diameter D ₂ All of the welded structures formed with nuggets have a high tensile strength with a cross tensile load of 5.4 kN or more, a shear load of 6.3 kN or more, and a shock absorption energy of 5.5 kJ or more by a falling weight test. In addition, the shock absorbing ability was excellent. In particular, D ₁ ≧ 0.6 × t, D ₂ ≧ 3.0 × t, and A1, A2, A4, and A5 with austenite content of weld zone ≧ 50% by volume, compared with other steel types. It can be understood that proper management of the nugget thickness D ₁ , diameter D ₂ , and austenite amount is effective in improving the shock absorption capacity. Moreover, it can be seen from the example using the temper rolled sheet of the steel type A1 that the excellent characteristics of the stainless steel sheet are maintained even if temper rolling is performed.

On the other hand, in the welded structure using the steel types B1 and B2, which are ordinary high-strength steels, both the cross tensile load and the impact absorption energy are smaller than those of the example of the present invention. Low cross-tension load and impact absorption energy are not high at the nugget interface when martensite is formed in spot welds in normal high-strength steel and axial stress is applied to the weld as in the cross-tension test. This suggests that the toughness was lowered by the above, and as a result, it broke at low load.
That is, in the falling weight test, axial stress is applied to the welded part in addition to the shear direction, so the spot welded part of ordinary high-strength steel where martensite is generated breaks at low stress, and the shock absorption energy is reduced. ing.

Steel grade B3 also shows a lower cross tensile load than that of the present invention example, but this includes an excessive amount of C, and it is surmised that excessive carbide was generated in the weld heat affected zone to reduce toughness. .
As is clear from the above comparison, it can be obtained by spot welding of conventional ordinary high-strength steel by specifying the composition and composition of the stainless steel plate as the material and controlling the form of the nugget formed by spot welding. Thus, an automobile member having an excellent shock absorbing ability compared with a welded portion is provided.

Cross-sectional view of welded portion showing nugget formed by spot welding D ₁ : Nugget thickness D ₂ : Nugget diameter t: Plate thickness of stainless steel plate

Claims (4)

  C: 0.15 mass% or less, Si: 1.0 mass% or less, Mn: 2.0 mass% or less, Ni: 6.0 to 11.0 mass%, Cr: 16.0 to 22.0 mass% , N: An automobile member having a high impact absorption capacity, characterized in that it is made of a stainless steel plate having a composition containing 0.20% by mass or less and the balance containing Fe and inevitable impurities, and is assembled by spot welding. When the thickness of the stainless steel plate is t and the thickness of the nugget formed by spot welding is D ₁ , the stainless steel plate is the counterpart of the weld having a nugget of thickness D ₁ = (0.6 to 1.6) × t. The automobile member according to claim 1, which is joined to the material. When the thickness of the stainless steel plate is t and the diameter of the nugget formed by spot welding is D ₂ , the stainless steel plate is the counterpart in the welded portion having a nugget with a diameter D ₂ = (3.0 to 5.5) × t. The automobile member according to claim 1, which is joined to the material.   The automobile member according to any one of claims 1 to 3, wherein the amount of austenite in the nugget and the weld heat affected zone is 30% by volume or more.

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