JPH0633213A - Ultrahigh strength galvanized steel sheet free from generation of hydrogen embrittlement - Google Patents

Abstract

PURPOSE:To manufacture an ultrahigh strength galvanized steel sheet free from the generation of hydrogen embrittlement. CONSTITUTION:In the steel sheet obtd. by applying the surface and rear of the ultrahigh strength steel sheet having >=1180N/mm<2> tensile strength applied with pure zinc or zinc series plating, cracking connecting from the boundary between the steel sheet and the plated layer to the surface of the plating is allowed to exist in the range of one piece/mm<2> to 10<7> pieces/mm<2>. As the method for manufacturing the cracking, the regulation of the heat treating conditions in the plated layer or mechanical methods such as skinpass and stretching after the galvanizing or the like are adopted. It is suitable for the application requiring lightness in weight and high corrosion resistance such as an automotive bumper and the reinforcing member for a door as well as building scaffolding or the like.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-high strength galvanized steel sheet suitable for applications requiring lightweight and high corrosion resistance such as automobile bumpers, door reinforcing members, and construction scaffolding materials. is there.

[0002]

2. Description of the Related Art US CAFE (CorporateAverage
FuelEConomy) regulatory tightening proposal to reduce vehicle weight
Has progressed to 1180N / for bumpers, door reinforcements, etc.
mm²The above ultra-high-strength thin steel sheets have come to be adopted.
It was These steel sheets are non-plating materials and have problems with corrosion resistance.
Recently, zinc plating is being considered to improve its rust prevention.
It's starting to happen.

However, when galvanizing such an ultra-high strength steel sheet, for example, in electrogalvanizing, hydrogen generated by cathodic reaction in the pickling and plating bath before plating penetrates into the steel sheet, and hot dip galvanizing occurs. In plating, it is known that hydrogen in the heating atmosphere of the line penetrates into the steel sheet and causes hydrogen embrittlement in any case. It is known that such hydrogen embrittlement is likely to occur at a tensile strength of 1180 N / mm ² or more, similarly to delayed fracture.

It has been reported that hydrogen embrittlement of a steel sheet is caused by this diffusible hydrogen, and the hydrogen is reduced by heating at 200 ° C. for several hours (for example, “Metal surface technology” Vol. 39, No. .7, 1988, p.52). However, it is difficult to completely remove diffusible hydrogen in the steel even if the steel sheet is heat-treated in this way, and hydrogen embrittlement cannot be prevented. Further, if the steel sheet is heat-treated after galvanizing, the galvanized layer deteriorates and the desired corrosion resistance cannot be obtained.

[0005]

As described above, the prior art is as follows.
There was no galvanized steel sheet having good corrosion resistance and hydrogen embrittlement with a tensile strength of 1180 N / mm ² or more.

In view of the above-mentioned situation, the present invention has a tensile strength of 1180 N / mm that does not cause hydrogen embrittlement without applying reheating treatment for releasing hydrogen and still applying predetermined zinc or zinc-based plating. ^The purpose is to provide ^two or more steel plates.

[0007]

[Means for Solving the Problems] As a means for solving the above problems, the present invention provides a steel sheet having pure zinc or zinc-based plating on the front and back surfaces of an ultra high strength steel sheet having a tensile strength of 1180 N / mm ² or more. In 1st, the number of cracks connecting from the interface between the steel plate and the plating layer to the plating surface is 1 / mm ² or more, 10 ⁷ / mm ²
It is characterized by being present in the following range.

The present invention will be described in more detail below.

[0009]

[Action]

As a result of earnest studies on the cause of hydrogen embrittlement in ultrahigh-strength steel sheets and countermeasures, the inventors of the present invention have found that pure zinc or pure zinc is applied to the front and back surfaces of ultrahigh-strength steel sheets having a tensile strength of 1180 N / mm ² or more. In a steel sheet that has been plated with zinc,
It has been found that hydrogen embrittlement does not occur when cracks connecting from the interface between the steel plate and the plating layer to the plating surface are present in the range of 1 / mm ² or more and 10 / mm ² or less. The reason for this is not clear, but it is considered that the hydrogen contained in the steel plate is released to the outside of the steel plate through cracks during the plating process.

When the number of cracks is less than 1 / mm ² , hydrogen release in the steel sheet is delayed and hydrogen cracking occurs. On the other hand, if it exceeds 10 ⁷ lines / mm ² , peeling of the plating layer is likely to occur and corrosion resistance and the like are deteriorated, which is not preferable. Note that the width of the crack is not particularly limited as long as hydrogen can diffuse. Also, the method of making a crack is not particularly limited,
It may be introduced by adjusting the heat treatment conditions of the plating layer or by a mechanical method such as skin pass or tension after zinc plating.

The type and method of galvanizing are not particularly limited. Pure zinc plating includes electrogalvanizing and hot dip galvanizing. Examples of the zinc-based plating include zinc alloys such as Zn-Ni and Zn-Mn, and galvannealing alloys. Whether the plating layer of these galvanized steel sheets is a single layer or a composite layer composed of two or more kinds of plating, the surface of the plating layer is further subjected to various surface treatments, that is, chromate treatment or phosphoric acid. The effect of the present invention does not change even if salt treatment or the like is performed.

The type of the steel sheet to be plated is not particularly limited, either a cold rolled high strength steel sheet or a hot rolled high strength steel sheet may be used. It may be a coil material or a sheet material.

Next, examples of the present invention will be described.

[0015]

[Example] Cold-rolled steel sheet with transformation structure strengthened by continuous annealing furnace
(1.6 mm thickness) and various hot-rolled steel sheets (2.4 mm thickness) utilizing the transformation structure strengthening and precipitation strengthening, various galvanized steel sheets shown in Table 1 are 70 mm long and 15 mm wide. Cut the test piece by various methods, give it the same strain as the coil by 4-point bending, and keep it at room temperature in the air for 1 week to crack (hydrogen embrittlement)
The occurrence of cracks and the number of cracks in the plating layer were examined. The results are also shown in Table 1.

[0016]

[Table 1]

As a method of making cracks in the plating layer, for CG-A of steel No. 3 and No. 8 to No. 10, the Fe concentration in the zinc plating layer was increased to about 11 mass% and then the V leveler was applied. Steel No. 4 to No. 7 adopted the method by V leveler and skin pass, and the number of cracks was adjusted by changing the rolling condition. Hydrogen embrittlement was evaluated by immersing a sample in which strain was imparted by four-point bending and immersing it in hydrochloric acid, and checking for breakage. The powdering property was evaluated by the amount of zinc peeling at the V-bent portion at 45 °.

In Table 1, since steel No. 1 has low strength, hydrogen embrittlement does not occur even after shearing on both sides. Steel N
Both No. 2 and No. 8 are conventional methods, and hydrogen embrittlement occurs because the number of cracks in the plating layer is not appropriate.
On the other hand, Steel Nos. 3, No. 7, No. 9 and No. 10 are all examples of the present invention, the plating layer has an appropriate number of cracks, and hydrogen embrittlement does not occur.

[0018]

As described in detail above, according to the present invention,
Even with a steel plate having a tensile strength of 1180 N / mm ² or more and pure zinc or a zinc-based plating, the occurrence of hydrogen embrittlement can be prevented.

Claims (1)

[Claims] 1. An ultrahigh strength steel sheet having a tensile strength of 1180 N / mm ² or more, which has pure zinc or a zinc-based plating on its front and back surfaces, and has one crack connected from the interface between the steel sheet and the plating layer to the plating surface. / Mm ² or more and 10 ⁷ lines / mm ² or less, an ultrahigh-strength galvanized steel sheet that does not cause hydrogen embrittlement.