JPH05171354A - High strength hot-dip zincing steel sheet excellent in plating peeling resistance and having baking hardenability and its production - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a high-strength galvannealed steel sheet with high strength in which the plating does not easily peel off even when processed. [Structure] C: 0.005% or less, Si: 0.5% or less, Mn: 0.80 to 1.50%, P: 0.080% or less, Al: 0.100% or less, N: 0. 0050% or less, B: 0.0002 to 0.0050% or less, Cr:
0.05-1.50%, Nb: 2xC (%)-7.5x
C (%), Ti: 0.003 to 0.030% is contained,
A steel sheet having the balance of iron and unavoidable impurities is subjected to galvannealing and the Fe concentration in the plated layer is adjusted to 5 to 15
%. As a result, it is possible to obtain a high-strength galvannealed steel sheet which is a plating layer and does not easily peel off even when subjected to processing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bake hardenable high-strength galvannealed steel sheet having excellent resistance to delamination and a method for producing the same, and in particular, the coating weight is 40 g.
The present invention relates to a high-strength hot-dip galvanized steel sheet with less peeling of the plating layer due to plastic deformation during press working even in thick plating of 1 / m ² or more (one surface) and a method for producing the same.

[0002]

2. Description of the Related Art Recently, steel sheets for automobiles have high strength as strength members and outer panel parts for weight reduction of a vehicle body, aiming to drastically reduce fuel consumption in light of global environmental problems. There is a movement to positively adopt the thin steel sheet. However, the application of high-strength steel sheets to automobiles is going to proceed in a different way from the conventional one. That is, surface-treated steel sheets are being rapidly adopted in order to deal with the problem of extending the vehicle body life of automobiles. Then, in order to further improve the corrosion resistance, there is a demand for a surface-treated steel sheet having a plating film thicker than before.

However, the thicker the film plated on the surface of the steel sheet, the more easily it is peeled off by press working, and there is a problem that the anticorrosive effect of the vehicle body is impaired. This problem becomes more apparent when a large amount of alloying elements are contained in order to secure the strength, especially in the case of a high strength steel plate. The peeling phenomenon of the plating film is called powdering property or flaking property depending on the peeling form. As means for solving these defective phenomena, for example,
A method for improving the composition of a galvanizing bath by adding Al to the galvanizing bath as disclosed in JP-A-52-131934, or an alloy as disclosed in JP-A-61-276962. There has been proposed a method of controlling the Fe concentration in a galvanized layer within a specific range to improve the plating composition.

[0004]

Although it is possible to reduce the peeling of the plating to some extent by adopting the above method, it is possible to obtain a high-strength galvannealed steel sheet with a high basis weight. In the case of manufacturing, the fact is that the above-mentioned methods have not yet been able to solve the problems of proper alloying and forming.

The present invention provides a high-strength, galvannealed steel sheet having good formability and good bake hardenability, in which peeling of a plating film, which is a problem due to an increase in coating weight and a higher strength, is minimized. It is intended to provide a manufacturing method thereof.

[0006]

As a result of various studies to achieve the above object, the inventors of the present invention added a large amount of Mn and Cr and B in combination and, at the same time, performed galvanization in an alloying galvanizing treatment. By controlling the Fe concentration in the layer to 5 to 15%, P which inhibits the galvannealing alloying
Content is significantly eased, and even if the coating amount is increased, the peeling of the plating film is small and the moldability is 35 kg.
It has been found that an alloyed hot-dip galvanized steel sheet having a high strength of not less than / mm ² can be manufactured.

The present invention is constructed based on these findings,
The gist is, in terms of weight%, C: 0.005% or less, Si: 0.5% or less, Mn: 0.80 to 1.50%, P: 0.080% or less, Al: 0. 100% or less, N: 0.0050% or less, B: 0.0002 to 0.0050%, Cr: 0.05 to 1.5%, Nb: 2 x C (%) to 7.5 x C (% ), Ti: 0.003 to 0.030%, and the balance consisting of iron and unavoidable impurities on a steel sheet coated with an alloyed galvanized layer having a Fe concentration in the galvanized layer of 5 to 15%. It is a bake hardenable high strength galvannealed steel sheet with excellent peelability. Further, a steel slab comprising the above components is hot-rolled at a finishing temperature of 750 ° C. or higher and a coiling temperature of 800 ° C. or lower, cold-rolled at a rolling reduction of 60% or more, and then recrystallized in a continuous hot dip galvanizing line. Then, after cooling, hot dip galvanizing is performed at 420 to 500 ° C., and subsequently alloying treatment is performed at 400 to 600 ° C. to form an alloyed galvanizing layer having a Fe concentration of 5 to 15% in the plating layer. A method for producing a bake hardenable high-strength hot-dip galvanized steel sheet having excellent peelability.

Here, the new knowledge which is the basis of the present invention will be described. C: 0.004%, Si: 0.02%,
Mn: 0.3%, S: 0.01%, Al: 0.050
%, N: 0.0025%, Nb: 0.025%, Ti:
0.015% as a base component and P content of 0.015%,
Steels containing 0.03%, 0.05%, 0.07% and Mn content increased to 1.0%, Cr added to 0.8% and B added to 10 ppm in the composition shown on the left. It was melted in a vacuum and hot-rolled and cold-rolled by a conventional method to obtain a 0.8 mm cold-rolled sheet. After that, after recrystallization annealing in a continuous hot-dip galvanizing pilot line, a basis weight of 60 g / m
^{In 2} , the Fe concentration in the plating layer was controlled to 8 to 10%, and a plate subjected to galvannealing was formed into a rectangular tube, and the state of plating delamination before and after forming (powdering property) was investigated. The result is shown in FIG.

As is clear from FIG. 1, the Mn content is 0.3.
%, The amount of P peeling significantly increases when the P content is 0.05% or more. On the other hand, in the steel in which the Mn amount was increased to 1.0% and Cr and B were also added, the P amount was increased to 0.07% and it was found that the degree of plating peeling was light.

Next, among the above-mentioned samples, high-Mn steels containing Cr and B added together and containing 0.07% of P were Fe in the plating layer depending on the alloying treatment conditions after hot dip galvanization.
The concentration was changed. Then, the relationship between the formability (limit drawing ratio) and the amount of plating peeling and the Fe concentration in the plating layer was investigated. The result is shown in FIG. From Fig. 2, F in the plating layer
It was clarified that the moldability and plating peeling were not problems when the e concentration was in the range of 5% to 15%.

The reasons for limiting the components and production conditions of the present invention will be described in detail below. C is limited to 0.005% or less in order to obtain a high r value and elongation required for drawing. It is preferably 0.0030% or less for applications where processing is particularly severe. Si is effective as a strengthening element of the steel sheet, but if it is too much, it causes non-plating, so it is made 0.5% or less. In the case of low strength, 0.10% or less is preferable. Mn is an element effective in improving the resistance to peeling of plating, and is contained in an amount of more than 0.8% in order to sufficiently obtain the effect. However, if it is too large, the r value and the elongation are deteriorated, so the content is made 1.5% or less. P is the most effective element for increasing the strength, but if it is too much, the alloying characteristics are significantly deteriorated, so P is limited to 0.080% or less. Although the lower limit of P is not specified, it is preferably 0.010% or more from the viewpoint of strength.

Al is contained as an effective component for fixing N and precipitating fine AlN to increase the strength. For that purpose, 2 or more is added as Al / N, but in particular in the present invention, it is preferably 0.030% or more in order to effectively exhibit the action of B described later. However, if it is too large, it only increases the alloy cost, so 0.10
% Or less. If the content of N is too large, it becomes a factor that deteriorates the r value and the ductility, so N is made 0.0050% or less. Up to now, B has been mainly added for fixing N, but in the present invention, B is added based on the new finding that solid solution B suppresses peeling of plating. To obtain the effect, 0.0002% or more must be added,
If it exceeds 0.0050%, the effect is saturated and the r value is lowered, so the content is made 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, the bake hardenability is reduced and the resistance to peeling of the plating is reduced, so it is set to 7.5 or less. Ti is S
Is an element effective for fixing N and securing the solid solution state of B described above. To obtain these effects, at least 0.003% Ti is required. On the other hand, if it is too much, the peeling of the plating is promoted, so the upper limit of Ti is made 0.030%. Cr is an element effective for increasing the strength without peeling the plating film,
To obtain the effect, 0.05% or more is necessary.
However, if it is too large, the manufacturing cost increases, so the content is made 1.5% or less.

Molten steel having the above composition is melted in a converter or an electric furnace, and is continuously cast or ingot-segmented into steel pieces, which are then hot-rolled. The steel slab may be hot-rolled directly while it is hot, or may be hot-rolled after it is charged into the reheating furnace in the state of hot slab. In case of reheating, the furnace temperature is usually 1
000 to 1300 ° C. The finishing temperature during hot rolling is 750 ° C. or higher because surface defects called ridging occur when the temperature falls below 750 ° C. The upper limit of the finishing temperature does not need to be restricted in terms of workability, but it is 95% from the viewpoint of roll wear.
It is preferably 0 ° C or lower. If the winding temperature after finishing is too high, it becomes difficult to secure the strength, so the temperature is set to 800 ° C or lower.
Although the lower limit of the coiling temperature is not particularly specified, it is usually preferable to set it to 300 ° C. or higher. If the r value is important, 700
It is recommended to wind at a high temperature of ℃ or more.

Next, cold rolling is performed at a rolling reduction of 60% or more. If the cold rolling rate is low, the recrystallization temperature rises, and it becomes difficult to secure a high r value. Conversely, if the rolling reduction is too high, the r value decreases, so the upper limit is made 95% or less.
Usually, it is good to carry out at a cold rolling rate of 75 to 85%.

After cold rolling, hot dip galvanizing and alloying treatment are carried out in a continuous hot dip galvanizing line. In order to prevent the peeling of the plating film due to working, in addition to the above-mentioned component composition, these treatment conditions are also applied. It is also important. First, a recrystallization annealing is performed by passing a non-oxidizing furnace / reducing furnace through a conventional method, and the annealing temperature is usually 700 ° C. or higher.
After that, it is forcibly cooled to the temperature of the plating bath, 420 to 50
Hot-dip galvanizing at 0 ° C. The plating temperature range is limited to the above range because the plating property is not good if it is too low or too high. In addition, Al is 0.08 in the plating bath.
It is effective to control the alloying by making it contain at a concentration of 0.30%.

Next, the alloying treatment condition is one of the important factors that control the peelability of the plating by pressing.
Alloying treatment is performed at a temperature of 400 to 600 ° C. to control the Fe temperature in the plated layer to 5 to 15%. If the alloying temperature is too low, the alloying reaction slows down, and if it is too high, the alloying progresses too much, which makes it difficult to control the Fe concentration in the plating layer to an appropriate range. Is specified to be 400 to 600 ° C. If the Fe concentration in the plating layer is too low, it will not be alloyed and the corrosion resistance and weldability after coating will be poor, so 5% or more is necessary, but if it is too high, plating peeling will be a problem, so it should be 15% or less. In addition, after performing the alloying zinc plating treatment, further performing upper layer plating such as Fe—Zn system is effective for further improving the plating peeling resistance. Finally, a yield rate of 0.3% or more is applied by a skin pass or tension leveler to eliminate the yield point elongation. In addition to the alloyed galvanized steel sheet, the present invention is applicable to Z
It can be applied to zinc-based plated steel sheets such as n-Sn and can enjoy the effects of the invention.

[0017]

EXAMPLES The steels shown in Table 1 were melted to form hot-rolled sheets of 4.5 mm under the hot rolling conditions shown in the same table, and then cold rolled to 0.
After the thickness was set to 8 mm, hot dip galvanizing and alloying treatment were performed under the conditions shown in Table 2. The amount of zinc deposited is 60 g / m per side
^{I made} it thick with ² . With respect to the steel sheet obtained after applying the skin pass of 1.0%, the tensile test, the r value, the BH property, and the peeling state of the plating layer and the forming limit drawing ratio as processing were investigated. For the peeling property of the plating layer, draw with a square cylinder punch of 70 mm side, apply cellophane tape to the corner of the square cylinder, and forcibly peel the plating. Was measured. The peelability of the plating (powdering property) was judged to be good when the peeling amount was 80 mg or less. Table 2 shows the results of various measurements.

[0018]

[Table 1]

[0019]

[Table 2]

As is apparent from Table 2, Samples A to D satisfying all the composition of components, hot rolling conditions, hot dip galvanizing conditions and alloying conditions of the present invention have excellent alloying properties and plating peeling resistance. It is evaluated as a high-strength galvannealed steel sheet with high deep drawability and bake hardenability. On the other hand, the sample E containing a large amount of P in the low Mn steel outside the scope of the present invention is not alloyed and has poor formability. Also, Nb and T
Sample F containing a large amount of i is too alloyed and the Fe concentration in the plating layer is too high, resulting in significant exfoliation.

[0021]

The galvannealed steel sheet according to the present invention has a bake hardenability in which the strength is increased by coating baking while ensuring a high strength, and also has a deep r drawability because it has a high r value. In addition, even if it is subjected to severe processing, peeling of the plating layer with a thick weight is extremely small, and the performance can be improved significantly compared to the past, so dent resistance, severe workability, and a longer vehicle life than before. It can be applied to automobile parts that need to be extended, and brings extremely useful effects in industry such as reducing the defective rate of molded products and improving the working environment.

[Brief description of drawings]

FIG. 1 is a diagram showing an influence of components on a plating peeling amount (powdering property) due to processing.

FIG. 2 is a diagram showing the relationship between the Fe concentration in the plating layer, the forming limit drawing ratio, and the plating peeling amount (powdering property).

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kosaku Ushioda 20-1 Shintomi, Futtsu City, Chiba Nippon Steel Corporation Corporate Technology Development Division

Claims (2)

[Claims] 1. As weight%, C: 0.005% or less, Si: 0.5% or less, Mn: 0.80 to 1.50%, P: 0.080% or less, Al: 0.100%. Below, N: 0.0050% or less, B: 0.0002 to 0.0050% or less, Cr: 0.05 to 1.5%, Nb: 2 x C (%) to 7.5 x C (%) , Ti: 0.003 to 0.030%, and the remainder being iron and unavoidable impurities, a galvanized layer having an Fe concentration in the galvanized layer of 5 to 15% is applied to a steel sheet. Bake hardenable high strength alloyed galvanized steel sheet with excellent resistance to stripping. 2. As weight%, C: 0.005% or less, Si: 0.5% or less, Mn: 0.80 to 1.50%, P: 0.080% or less, Al: 0.100% Below, N: 0.0050% or less, B: 0.0002 to 0.0050% or less, Cr: 0.05 to 1.5%, Nb: 2 x C (%) to 7.5 x C (%) , Ti: 0.003 to 0.030%, with the balance being iron and unavoidable impurities, a steel slab is hot-rolled at a finishing temperature of 750 ° C or higher and a coiling temperature of 800 ° C or lower, and a rolling reduction of 60%. After cold rolling as above, recrystallization annealing is performed in a continuous hot dip galvanizing line, and after cooling 420
Hot dip galvanizing at ~ 500 ° C, then 400 ~ 6
Baking hardenable high-strength hot-dip galvanizing with excellent resistance to stripping, characterized by performing an alloying treatment at 00 ° C. and applying an alloyed zinc plating layer having a Fe concentration of 5 to 15% in the zinc plating layer. Steel plate manufacturing method.