JPH07116581B2 - High strength alloyed hot dip galvanized steel sheet with less plating peeling due to processing and excellent bake hardenability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a high-strength galvannealed steel sheet for press working, which is excellent in bake hardenability, and particularly has a galvanized layer of 40 g / m ² or more (one side). The present invention relates to a high-strength galvannealed steel sheet with high peeling of the plating layer even if the coating weight is thick even if the plastic deformation occurs during press working.

(Prior Art) In the automobile industry, from the viewpoint of resource saving and energy saving,
High-strength cold-rolled steel sheets have been actively adopted mainly for strength members for the purpose of reducing the weight of the vehicle body, but recently, high-strength steel sheets have been positively applied to so-called outer panels. It's starting to happen. Particularly for outer panels, in order to secure dent resistance, a type having a paint bake-hardening property, in which strength is increased by a paint baking treatment after press working, has been increasingly used. Generally, it is required that the yield point should be increased by 3kg / mm ² or more by baking.

On the other hand, recently, the issue of extending the vehicle body life of automobiles has begun to be greatly highlighted, and in the automobile industry, surface-treated steel sheets with a thicker plating film than before are applied to considerable parts including inner and outer plates. Things are going on. Especially for outer panels, the use of high-strength, high-strength surface-treated steel sheets with excellent coating bake hardenability is being considered.

However, the thicker the plated film on the surface of the steel sheet, the more easily it is peeled off by press working, which impairs the rust preventive effect of the vehicle body and deteriorates the working environment. Furthermore, this problem becomes more apparent when a large amount of alloying elements are contained in order to secure the strength as in a high strength steel plate.

The peeling phenomenon of the plating film is called powdering property or flaking property. As means for solving these defective phenomena, 1) hot dip galvanizing bath as disclosed in JP-A-52-131934. Method of Incorporating Al in 2)
Methods such as a method of controlling the Fe concentration in the alloyed zinc plating layer within a specific range, as disclosed in JP-A-61-276962, have mainly been studied so far by methods for improving plating operation conditions.

(Problems to be Solved by the Invention) Although it is possible to surely reduce the peeling of the plating to some extent by adopting the method described above, a high-strength alloyed hot-dip galvanized steel sheet having a particularly large basis weight is provided. In the case of, the fact is that the problem has not been resolved, and this solution is desired.

The present invention provides a high-strength hot-dip galvanized steel sheet with excellent bake hardenability, which minimizes the peeling of the plating film, which is a problem particularly due to thickening and high strength, in order to satisfy the above demands. With the goal.

(Means for Solving the Problems) In order to achieve the above-mentioned object, the present inventors have optimized the plating operation conditions and conducted various studies on the component composition of the steel sheet. As a result, the alloyed hot-dip galvanized steel sheet that has a large basis weight and very little plating peeling, has a bake hardenability of about 3 kg / mm ² or more, and has a high strength of 35 kg / mm ² or more 1) Based on ultra-low carbon steel, avoiding the positive addition of P, and combining the contents of C and Nb in an equivalent amount or less and B in a solid solution state (that is, conventional strengthening For this reason, the idea was completely opposite to the technical idea of adding P.) 2) Synergistic effect of performing galvannealing treatment in which the Fe concentration in the zinc plating layer was controlled to 5 to less than 15% It was newly discovered that it is manufactured by.

The present invention is based on these findings, and the gist thereof is that C: 0.010% or less, Si: 0.5% or less, Mn: more than 0.30 1.00%, P: 0.030% or less, S: 0.03% Below, Al: 0.100% or less, N: 0.0050% or less, B: 0.0005 to 0.0050% or less, Nb: 2 x C (%) to 7.5 x C (%), and the balance being steel and inevitable impurities, A high-strength hot-dip galvannealed steel sheet having excellent galvannealing properties with less peeling of the plating due to the processing of the galvannealed galvanized layer having a Fe concentration in the galvanized layer of 5 to less than 15%.

Hereinafter, the present invention will be described in detail.

First, the reasons for limiting the steel plate components of the present invention will be described.

C is limited to 0.010% or less in order to obtain the high r value and elongation required in press working. When especially high workability is required, 0.0050% or less is preferable.

Si is effective in improving the strength of the steel sheet, but if it is too much, it deteriorates the adhesion of the plating, so it is made 0.5% or less. In the case of low strength, 0.10% or less is preferable.

Mn is a component contained to increase the strength of the steel sheet 0.30%
To be super. On the other hand, if too much, the r value deteriorates, so 1.0
% Or less.

P is the most effective element for increasing the strength, but 0.03 because it significantly peels off the plating film by pressing.
Limited to 0% or less. Although the lower limit of P is not particularly specified, it is preferably 0.010% or more from the viewpoint of strength.

S is effective in generating fine sulfides and increasing the strength, but if it is large, it deteriorates the ductility, so it is made 0.030% or less. Usually, it may be 0.010% or more.

Al is contained as an effective component for fixing N. For that purpose, 2 or more is added as Al / N, but in particular in the present invention, it is preferably 0.040% or more in order to effectively exhibit the effect of B described later. However, if it is too large, it only increases the alloy cost, so the content is made 0.10% or less.

N is a harmful component in the present invention, and if its content is too large, it causes deterioration of r value and ductility, so 0.0050
% Or less.

Until now, B has been added mainly for fixing N, but
In the present invention, B is added based on the new finding that solid solution B suppresses peeling of plating. In order to obtain the effect, 0.0005% or more must be added, but if it exceeds 0.0050%, the effect is saturated and the r value is lowered, so 0.
0050% or less.

Nb needs to be 2 or more as Nb / C (wt% ratio) in order to fix solid solution C and achieve both non-aging property and bake hardenability.
If it exceeds 7.5, bake hardenability cannot be expected and the powdering property is deteriorated.

Next, the manufacturing method of the present invention will be described.

Molten steel of the steel having the above-described composition is melted in a conventional converter or electric furnace, and is continuously cast or ingot-cast into ingots and then hot-rolled. The slab may be hot-rolled directly as it is hot, or may be charged into the reheating furnace in the state of a hot piece and then hot-rolled.
And The finishing temperature during hot rolling is 950 to 750 ° C.
The upper limit of this temperature is specified from the problem of wear of the rolling rolls, and the lower limit is specified from the problem of surface defects such as ridging. If the coiling temperature after finishing is too high, it will be difficult to secure the strength. The lower limit is not specified, but is usually 300 ℃
The above is preferable. If the r value is important, 70
It is recommended to wind at a high temperature of 0 ° C or higher.

The hot-rolled steel strip thus obtained is then cold-rolled at a rolling reduction of 60% or more and 95% or less. If the cold rolling rate is too low, the recrystallization temperature will rise, making it difficult to secure a high r value.
% Or more. If the rolling reduction is too high, the r-value decreases, so 95% or less is necessary. Usually, it is recommended to carry out at a cold rolling rate of 75 to 85%.

After cold rolling, hot dip galvanizing and alloying treatment are performed in a continuous hot dip galvanizing line, but in order to prevent the peeling of the plating film due to processing, these treatment conditions as well as the above component composition is important.

First, the steel strip is recrystallized and annealed by passing it through a non-oxidizing furnace / reducing furnace by a usual method, and the annealing temperature is usually 700 ° C. to Ac ₃ points. After that, it is forcibly cooled to the temperature of the plating bath with a water cooler or gas jet, and hot dip galvanizing is performed at 420 to 500 ° C. If the plating temperature is too low or too high, the plating properties are not good, so it is specified at 420-500 ℃. Further, it is effective for controlling the alloying that the plating bath contains Al at a concentration of 0.08 to 0.30%.

Next, the alloying treatment condition is one of the important factors that control the peelability of the plating by pressing, and the alloying treatment is performed at a temperature of 400 to 600 ° C. 15
Control to%. If the alloying temperature is too low, the alloying reaction will slow down, and if it is too high, the alloying will proceed too much and it will be difficult to control the Fe concentration in the plating layer to an appropriate range. Is specified at 400 to 600 ° C. If the Fe concentration in the plating layer is too low, the corrosion resistance and weldability after coating will be poor, so 5% or more is necessary, but if it is too high, the peeling of the plating will be a problem, so less than 15%. Fe concentration is usually 8
The range of about 12% is preferable.

Although the galvannealing is applied as described above, the amount of the galvannealed coating is 40 g / m ² or more on one side in order to satisfy severe corrosion resistance. Heavy weight is 70
Even at about g / m ² , the effect of the present invention is not impaired.

After the alloying zinc plating treatment, Fe-
Applying a Zn-based upper layer plating is effective in improving the plating peel resistance.

Finally, the skin pass and the tension leveler give the elongation
Eliminate the yield point elongation by giving 0.5% or more.

INDUSTRIAL APPLICABILITY The present invention can be applied to zinc-based plated steel sheets such as Zn-Sn other than the alloyed galvanized steel sheets, and can enjoy the effects of the invention.

(Examples) The steels shown in Table 1 were melted and subjected to the hot rolling conditions shown in the table.
A hot-rolled sheet of 4.0 mm was formed, which was then cold-rolled to 0.8 mm, and then hot-dip galvanized and alloyed under the conditions shown in Table 2. The amount of zinc adhered was 60 g / m ^{2 on} one side, and the weight was made thick. Tensile test, r-value, BH property, and peeling state of the plating layer due to working were investigated for the steel sheet obtained after applying Sumipass 1.0%. Regarding the releasability of the plating layer, drawing was performed with a square tube punch, and a cellophane tape was attached to the corner portion of the square tube, and then the tape was peeled off to measure the amount of plating attached to the tape. The amount of adhesion is large, medium, small,
The strippability of the plating was evaluated by classifying it into four minimum levels. The results of various measurements are shown in Table 2.

As is clear from Table 2, those satisfying all of the composition of the present invention, hot rolling conditions, hot dip galvanizing conditions and alloying conditions all have excellent plating peeling resistance, BH property,
It has tensile strength and r-value.

(Effects of the Invention) As described above, the galvannealed steel sheet according to the present invention has a bake hardenability in which strength is increased by coating baking while ensuring high strength, and also has a high r value, so It has sufficient drawability, and even if it is subjected to severe processing, the peeling of the thick coating layer is extremely small, and the performance can be improved significantly compared to the past, so dent resistance and severe workability and conventional It can be applied to automobile parts requiring extended body life as described above, and has industrially very useful effects such as reduction of defective rate of molded products and improvement of working environment.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Kenichi Asagawa Inventor Kenichi Asagawa 1-1-1 Edamitsu, Yawatahigashi-ku, Kitakyushu, Fukuoka (56) References, JPN 62-260046 (JP) , A) JP-A-61-56245 (JP, A)

Claims (1)

[Claims] [Claim 1] C: 0.010% or less, Si: 0.5% or less, Mn: over 0.30 to 1.00%, P: 0.030% or less, S: 0.03% or less, Al: 0.100% or less, N: 0.0050 % Or less, B: 0.0005 to 0.0050%, Nb: 2 x C (%) to 7.5 x C (%), the balance of iron in the galvanized layer is 5 to 15% in the steel sheet with the balance being iron and unavoidable impurities. less than in, 35 kg / mm ² or more high strength alloyed plating peeling due to processing, characterized in that subjected to galvannealing layer excellent in less bake hardenability which has one side 40 g / m ² or more thick mass per unit area Hot-dip galvanized steel sheet.