JP2007146201A - Hot-dip aluminum-plated steel sheet having excellent spot weldability to aluminum material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-dip aluminum-plated steel sheet which has excellent spot weldability to an aluminum material, and has a strength as high as a 490 N grade or higher. <P>SOLUTION: The hot-dip aluminum-plated steel sheet uses a steel material comprising 0.05-0.18% C, 1.50% or less Si, 0.8-2.5% Mn, 0.05% or less P, 0.02% or less S, 0.001-0.02% Mg, 0.015% or less sol. Al and 0.0060-0.0150% N as a base material: and has an AlN-enriched layer formed in an interface between the base material and a plated layer. The base material may include one or more elements selected from the group consisting of 0.02-0.1% Ti and/or Nb, 0.3% or less Mo and 0.0030% or less B. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a hot-dip galvanized steel sheet that is suitable as a reinforcing member for automobiles, has excellent spot weldability with aluminum materials such as aluminum and aluminum alloys, and has a strength of 490 N / mm ² or more.

  Aluminum materials such as aluminum and aluminum alloys are used for bonnets, roofs, fenders, etc. in order to reduce the weight of automobile bodies. Aluminum material is joined to the body made of steel parts by mechanical fastening such as bolting and rivets, but mechanical fastening requires extra man-hours such as drilling, which makes the assembly process expensive. Prone.

As joining methods replacing mechanical fixing, various proposals have been made to apply spot welding with excellent productivity to joining of iron-based materials / aluminum materials. However, in ordinary spot welding, a hard and brittle intermetallic compound is generated at the interface between the aluminum material and the iron-based material, and the joint strength is significantly reduced. Therefore, a method of spot welding an aluminum or aluminum alloy thin plate and a mild steel plate with an aluminum-based insert material interposed therebetween (Patent Document 1), after coating a metal or alloy having a melting point lower than the melting point of the aluminum material, the steel material is made of aluminum. Improvement of joint strength by a method of spot welding with a material (Patent Document 2) or the like has been studied.
JP-A-56-117888 Japanese Patent Laid-Open No. 4-251676

In the method using the insert material, the process becomes complicated and the manufacturing cost increases.
Even if an iron-based material is pre-coated with a coating layer such as Al, Al-Zn-Al-Si, or Zn, it is inevitable that a hard and brittle intermetallic compound will be produced by welding with an aluminum material, resulting in a decrease in bonding strength. Tend to.
On the other hand, it is strongly desired to provide a high-strength material excellent in spot weldability with an aluminum material in response to the recent reduction in weight of materials for automobiles. However, most conventional iron-based materials have a tensile strength of 440 N class or less, and the joint strength of joints formed by spot welding is not sufficient.

  The present invention has been devised to solve such problems, and by concentrating AlN that suppresses Fe diffusion from the base material to the surface layer at the interface of the base material / plating layer, it is possible to perform spot welding. An object of the present invention is to provide a hot-dip galvanized steel sheet that prevents the formation of intermetallic compounds and has a tensile strength of 490 N grade or higher.

The present invention has a mass ratio of C: 0.05 to 0.18%, Si: 1.50% or less, Mn: 0.8 to 2.5%, P: 0.05% or less, S: 0.02 %, Sol.Al: 0.015% or less, and solid solution N: 0.0006 to 0.0150%. The base material may contain Ti and / or Nb: 0.02 to 0.1%, Mo: 0.3% or less, and B: 0.0003% or less as necessary.
AlN effective for suppressing the diffusion of Fe is concentrated at the interface between the substrate and the molten aluminum plating layer.

Effects and embodiments of the invention

When spot welding the aluminum material and the iron-based material, if the surface of the iron-based material is aluminized, the affinity with the aluminum material is increased and the spot weldability is expected to be improved. However, simply providing an aluminum plating layer produces a hard and brittle intermetallic compound at the joint interface exposed to high temperatures during spot welding, and high weld strength cannot be obtained.
As a result of various investigations and examinations on intermetallic compounds that adversely affect spot weldability, the present inventors have reduced the amount of brittle intermetallic compounds produced when it is possible to suppress Fe diffusion from the base material to the plating layer during spot welding. And found that the welding strength increases. Then, it was clarified that Fe diffusion to the substrate / plating layer interface can be suppressed by modifying the surface layer with concentrated AlN as the barrier layer.

In order to make the AlN concentrated surface layer, it is necessary to react Al in the molten aluminum plating layer with N in the steel. Therefore, an alloy design is adopted in which a sufficient amount of N is dissolved in steel and sol.Al is regulated to be low so that the solid solution state of N is maintained.
Hereinafter, alloy components and contents contained in the base material will be described.
[C: 0.05-0.18%]
It is an alloy component effective for improving the strength, and at least 0.05% or more of C is necessary to obtain the 490 grade. However, if an excessive amount of C exceeding 0.18% is included, the hardness of the spot welded portion may rapidly increase and weld cracking may occur. Therefore, the C content is selected from 0.05 to 0.18% (preferably 0.05 to 0.14%).

[Si: 1.50% or less]
Although it is an alloy component that improves the strength by dissolving in the matrix, it has an adverse effect on the plating properties and weldability, so 1.50% (preferably 0.08%) was made the upper limit. In the steel type to which Si is added, plating defects caused by Si can be prevented by pre-plating Fe, Fe-B, etc. prior to continuous molten aluminum plating.
[Mn: 0.8 to 2.5%]
It is an alloy component effective for improving the strength, and an effect of addition is seen at 0.8% or more, but addition of an excessive amount exceeding 2.5% causes deterioration of ductility and spot weldability. Preferably, the Mn content is determined in the range of 0.8 to 2.1%.

[P: 0.05% or less]
Although it is an alloy component contributing to strength improvement, an excessive amount of P exceeding 0.05% adversely affects low temperature toughness.
[S: 0.02% or less]
It is an impurity element, and the lower the S content, the better the workability. However, the content of 0.02% or less does not impair the gist of the present invention.

[Sol.Al: 0.015% or less]
Although it is a component added as a deoxidizer, it reacts with N in steel to produce AlN, and consumes solid solution N necessary for forming an AlN concentrated layer having an effect of inhibiting Fe diffusion. If sol.Al exceeds 0.015%, the amount of N necessary for forming the AlN layer tends to be insufficient at the time of spot welding, and the effect of suppressing Fe diffusion is not sufficiently exhibited, resulting in a decrease in welding strength. Therefore, it is preferable to set the Al addition amount as low as possible, and the upper limit of the Al content is set to 0.015% (preferably 0.010%).

[Solution N: 0.0006 to 0.0150%]
It is a component necessary for forming an AlN concentrated layer having an effect of inhibiting Fe diffusion, and the solid solution N amount is set to 0.0006% or more in order to form a sufficient AlN concentrated layer. However, if an excessive amount of solute N exceeding 0.0150% is contained, the steel sheet itself becomes hard and the moldability deteriorates. Excess N is also a cause of pinhole formation on the surface of the slab during the steel making process, and the production conditions become unstable. In applications where deterioration of workability is regarded as a problem, the upper limit of the amount of solute N is set to 0.0100%.
In addition, the solid solution N amount as used in this specification means the amount of N dissolved in the matrix except for N fixed as a nitride or the like. The amount of solute N can be measured, for example, by the following method. (1) sol.N (N in a form soluble in hydrochloric acid, including N that is solid solution N or AlN) was quantified by the distillation neutralization titration method of JIS G1228, and (2) AlN was Br -Extracted with alcohol and quantified by distillation neutralization titration method. (3) The amount of solute N is calculated as the difference between sol.N value and AlN value.

[Ti and / or Nb: 0.02 to 0.1%]
It is an alloy component that is added as necessary, and contributes to increasing the strength of the steel material and also to reducing the structure of the nugget and suppressing the softening of the spot weld heat affected zone. The effect of fixing C in the steel and improving the ductility of the steel sheet is also exhibited. Such an effect becomes remarkable by adding 0.02% in total of one or two of Ti and Nb, but saturates at 0.1%. Even if over 0.1% is added, the property improvement effect commensurate with the increase cannot be obtained, which not only increases the material cost but also increases the recrystallization temperature and decreases the ductility. Therefore, the total amount of Ti and / or Nb is determined in the range of 0.02 to 0.1% (preferably 0.02 to 0.06%).

[Mo: 0.3% or less, B: 0.0003% or less]
Both are effective components for increasing the strength and are added as necessary. The effect of addition becomes remarkable when Mo is 0.05% or more and B is 0.0005% or more. However, the excessive addition of Mo exceeding 0.3% not only increases the steel material cost, but also leads to a decrease in the balance between strength and ductility, and in turn, workability. Moreover, addition of B exceeding 0.0003% inhibits the formation of a sufficient AlN concentrated layer because it consumes N in the steel in addition to an increase in steel material cost, and at the interface of the substrate / plating layer during spot welding. Promotes the reaction of forming Al—Fe intermetallic compounds. Therefore, when added, the upper limit of the content is set to Mo: 0.3% (preferably 0.2%) and B: 0.0030% (preferably 0.000020%), respectively.
Although the concentration and thickness of AlN are not necessarily clear, it is effective to suppress the diffusion of Fe that it is concentrated at least twice the solid solution N in the center of the material in the nano-order range from the interface with the aluminum alloy. it is conceivable that.

  After melting the steel materials whose components and compositions are shown in Table 1, they were continuously cast into slabs. After heating the slab to 1250 ° C. for 40 minutes in a soaking furnace, a hot rolled sheet with a finishing thickness of 880 ° C., an average cooling rate of 25 ° C./second, a coiling temperature of 620 ° C. and a thickness of 2 mm. It was. The hot rolled sheet was pickled and cold rolled to a thickness of 1.0 mm at a cold rolling rate of 50%.

Each cold-rolled steel plate was used as a plating base plate, and was subjected to 3 g / m ² Fe plating and passed through a continuous molten aluminum plating line. In the continuous molten aluminum plating line, reduction annealing was performed at an annealing temperature of 830 ° C., and the resultant was fed into an Al-9% Si molten plating bath, and the basis weight of the plating was adjusted to 120 g / m ² by gas wiping immediately after the pulling. Then, the coil wound by temper rolling at a rolling reduction of 2% was subjected to low-temperature annealing at 450 ° C. for 10 hours.

The obtained hot-dip aluminized steel sheet was subjected to a mechanical test, and the tensile strength TS and total elongation T.El were measured. Further, spot welding was performed with an aluminum alloy under the following conditions, and subjected to a cross tension test according to JIS Z3137 (spot test method for spot welded joints), and the spot weldability was evaluated by the cross tensile strength at break. In the iron-based material / aluminum material joined body, it is determined that the iron-based material TS ≧ 490 N / mm ² , TS × T.El ≧ 14000, and cross tensile strength ≧ 1000 N are acceptable (◯).

As can be seen from the results of the investigation in Table 2, the test No. 1 had a low C content and thus a low strength, and because of high Al and low N, a satisfactory value for the cross tensile strength could not be obtained. Test No. 2 also had insufficient cross tensile strength because of its low N content. In Test No. 3, the elongation decreased due to the high Mn, and the strength-ductility balance was inferior.
On the other hand, in Test Nos. 4 to 12 according to the present invention, a strength of 490 N or more is obtained, a cross tensile strength of 1000 N or more is obtained, and a sufficient joint strength is obtained even when spot welding is performed with an aluminum alloy. It was.

  As described above, by forming an AlN concentrated layer at the substrate / plated layer interface, Fe diffusion from the substrate to the surface layer is suppressed even when heated at a high temperature during spot welding, and a hard and brittle alloy layer Therefore, the hot dip galvanized steel sheet is spot welded to the aluminum material with high joint strength. The obtained bonded aluminum plated steel sheet / aluminum material joint structure has a high strength of 490 N class or more while utilizing the light weight and corrosion resistance of the aluminum material, so that it is used in a wide range of fields including automobile structural members. .

Claims (4)

  C: 0.05-0.18%, Si: 1.50% or less, Mn: 0.8-2.5%, P: 0.05% or less, S: 0.02% or less, sol.Al: 0.015% or less, solid solution N: 0.0060 to 0.0150% included and the balance is Fe, except for impurities, is used as a base material, and AlN is concentrated at the interface between the hot-dip aluminum plating layer and the base material. A hot dip galvanized steel sheet excellent in spot weldability with an aluminum material, wherein a layer is formed.   C: 0.05-0.18%, Si: 1.50% or less, Mn: 0.8-2.5%, P: 0.05% or less, S: 0.02% or less, sol.Al: Base material of steel sheet containing 0.015% or less, solid solution N: 0.0006 to 0.0150%, Ti and / or Nb: 0.02 to 0.1%, the balance being Fe and excluding impurities A hot dip galvanized steel sheet excellent in spot weldability with an aluminum material, characterized in that an AlN concentrated layer is formed at the interface between the hot dip aluminum plating layer and the substrate.   C: 0.05-0.18%, Si: 1.50% or less, Mn: 0.8-2.5%, P: 0.05% or less, S: 0.02% or less, sol.Al: 0.015% or less, solute N: 0.0006 to 0.0150% included, Mo: 0.3% or less, B: 0.0003% or less at least one kind, with the balance being Fe except for impurities A hot-dip galvanized steel sheet excellent in spot weldability with an aluminum material, characterized in that a steel sheet having a composition is used as a base material, and an AlN concentrated layer is formed at the interface between the hot-dip aluminum plating layer and the base material.   C: 0.05-0.18%, Si: 1.50% or less, Mn: 0.8-2.5%, P: 0.05% or less, S: 0.02% or less, sol.Al: 0.015% or less, solid solution N: 0.0006 to 0.0150%, Ti and / or Nb: 0.02 to 0.1%, Mo: 0.3% or less, B: 0.0030 %, Wherein the base material is a steel sheet having the composition of Fe except for impurities, and an AlN-enriched layer is formed at the interface between the hot-dip aluminum plating layer and the base material. Hot-dip aluminized steel sheet with excellent spot weldability with materials.