JPH0441658A - Galvannealed steel sheet excellent in powdering resistance and having baking hardenability and high strength and its production - Google Patents

Abstract

PURPOSE:To reduce the peeling of a plated film under the conditions of thick plating and high strengthening and to obtain a galvannealed steel sheet excellent in powdering resistance and having baking hardenability and high strength by rolling a steel slab with limited components under specific conditions, applying hot-dip galvanizing to the resulting steel sheet, and then applying specific alloying treatment to the above. CONSTITUTION:A slab of a steel having a composition consisting of, by weight, <=0.010% C, <=0.5% Si, 0.15-0.80% Mn, <=0.030% P, <=0.03% s, <=0.100% Al, <=0.0050% N, 0.0005-0.0050% B, (2-7.5)XC(%) Nb, further one or more kinds among 0.05-0.80% Sn, 0.005-0.080% Sb, and 0.020-1.50% Cr, and the balance iron with inevitable impurities is prepared. This slab is hot-rolled at >=750 deg.C finishing temp. at <=800 deg.C coiling temp., cold-rolled at >=60% reduction of area, and subjected, by means of a continuous hot-dip galvanizing line, to recrystallization annealing, to forced cooling, and to hot-dip galvanizing at 420-500 deg.C. Successively, alloying treatment is carried out at 400-600 deg.C and an galvannealed layer having 6 to <15% Fe concentration in the galvanized layer can be applied to the steel sheet.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a high-strength alloyed hot-dip galvanized steel sheet for bake-hardenable press working and a method for manufacturing the same, and particularly relates to a high-strength alloyed galvanized steel sheet for use in bake-hardenable press working and a method for manufacturing the same. The present invention relates to a bake-hardenable high-strength hot-dip galvanized steel sheet with excellent powdering resistance and a method for manufacturing the same, with a thick coating area on one side (one side) and less peeling of the coating layer during plastic deformation during press working.

(Conventional techniques*) In the automobile industry, the use of high-strength cold-rolled steel sheets has been actively promoted mainly for strength components, etc., with the aim of reducing the weight of car bodies from the viewpoint of resource and energy conservation. High-strength steel sheets are now being actively used for parts called exterior panels.In particular, for exterior panels, the paint baked on after press forming is treated to increase its strength in order to ensure punt resistance. The application of hardenable paint baking is increasing.In general, baking is required to have an increase in yield point of 3 kg/mu" or more due to treatment.

On the other hand, recently, the issue of extending the lifespan of automobile bodies has begun to attract attention, and the automobile industry is applying surface-treated steel sheets with thicker plating than before to many parts, including inner and outer panels. is being advanced. Particularly for exterior panels, the use of thick, high-strength, surface-treated steel sheets with excellent hardening properties for paint baking is being considered. However, the thicker the film plated on the surface of the steel sheet, the more easily it peels off during press working.

This has the problem of impairing the rust prevention effect of the car body and deteriorating the working environment. Furthermore, this problem becomes even more apparent when a large amount of alloying elements are contained in order to ensure strength, such as in high-strength steel sheets. The peeling phenomenon of plating films is called powdering or flaking.
) a method of containing Al in a hot-dip galvanizing bath as disclosed in JP-A-52-131934; 2) a method of containing Al in a galvannealed layer as disclosed in JP-A-61-276962; How to control the concentration within a specific range,
Up to now, methods mainly based on improving plating operating conditions have been studied. It is certainly possible to reduce the peeling of coating to some extent by adopting these methods, but especially in the case of high-strength alloyed hot-dip galvanized steel sheets with a large basis weight, the problem cannot be solved. The reality is that we have not yet reached that point.

(Problems to be Solved by the Invention) The present invention aims to reduce the problem of peeling of the plating film as much as possible, especially by increasing the thickness and increasing the strength. The purpose of the present invention is to provide a galvanized steel sheet and a method for manufacturing the same.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the present inventors optimized the plating operating conditions and conducted various studies on the composition of steel sheets. The alloyed hot-dip galvanized steel sheet has very little peeling even when the coating is increased, has hardenability when baking at a temperature of about 3 kg/m2 or more, and has a high strength of 35 kg/+am2 or more.1) Ultra-low carbon steel As a base, without actively adding P as in the past, combined addition of Nb in an amount equal to or less than C and B in a solid solution state,
2) It was newly discovered that the product is produced by the synergistic effect of alloyed galvanizing treatment in which the Fe concentration in the galvanized layer is controlled to be less than 5% to 15%.

The gist is: C: 0.010% or less, Si: 0.5
% or less 1Mn: 0.15-0.80%, P: 0
．． 0.030% or less, S: 0.03% or less, AQ: 0
．． 100% or less, N: 0.0050% or less, B
: 0.0005-0.0050%, Nb:2×C(%
) to 7.5×C (%), and Sn: 0.05 to 0
．． 80%, Sb: 0.005-0.080%, Cr
: An alloyed galvanized layer with an Fe concentration of 5 to 15% on a steel sheet containing one or more of 0.02 to 1.50%, the balance being iron and unavoidable impurities. This is a bake-hardenable high-strength alloyed hot-dip galvanized steel sheet with excellent powdering resistance.

Also, C: 0.010% or less, Si: 0.5% or less, Mn: 0.30 to 0.80%, P: 0.030
% or less, S: 0.03% or less, AΩ: 0.10
0% or less, N: 0.0050% or less.

B: 0.0005 to 0.005o%, Nb: 2×C
(%) to 7.5×C (%), and further contains Sn
: 0.05-0.80%, Sb2 0.005-0.0
A steel billet containing one or more of the following: 80%, Cr: 0,020 to 1.50%, with the balance consisting of iron and unavoidable impurities, is processed at a finishing temperature of 750°C or higher and a coiling temperature of 800°C or lower. After hot rolling at a rolling reduction rate of 60% or more, recrystallization annealing at a continuous hot-dip galvanizing line,
After forced cooling, hot-dip galvanizing was performed at 420 to 500°C, followed by alloying treatment at 400 to 600°C to form an alloyed galvanized layer in which the Fe concentration in the plating layer was less than 5 to 15%. This is a method for producing a bake-hardenable high-strength alloyed hot-dip galvanized steel sheet with excellent powdering resistance.

The present invention will be explained in detail below.

C is limited to 0.010% or less in order to obtain the high r value and elongation required in press working. In particular, when high workability is required, the content is preferably 0.0050% or less.

Although Si is effective in improving the strength of steel sheets, too much Si impairs the adhesion of plating, so it should be kept at 0.5% or less. If high strength is not required, the content is preferably 0.10% or less.

Mn is a component added to increase the strength of steel sheets.
Further, in order to prevent hot embrittlement, the content should be 0.15% or more. On the other hand, if it is too large, the r value will deteriorate, so it should be set at 0.80% or less.

P is the most effective element for increasing strength, but it causes the plating film to peel off significantly during press working, so P is 0.03
Limited to 0% or less. Although the lower limit of P is not specified, it is preferably 0.010% or more from the viewpoint of strength.

S is effective in generating fine sulfides and increasing strength, but too much S deteriorates ductility, so 0.030%
Do the following. Usually, it may be 0.010% or more.

80 is contained as an effective component for fixing N. To this end, Al/N is added in an amount of 2 or more, but especially in the present invention, in order to effectively exhibit the effect of B, which will be described later, it is preferable to add 0.040% or more.
However, too much will only increase the alloy cost, so 0.
10% or less.

N is a harmful component in the present invention, and if its content is too large, it will cause deterioration of the r value and ductility, and B
Since the content effect is inhibited, the content should be 0.0050% or less.

Until now, B has been added mainly for the purpose of 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 this effect, it is necessary to contain 0.0005% or more, but on the other hand, if the content exceeds 0.0050%, the effect will be saturated and the r value will decrease.
% or less.

Nb fixes C in solid solution and requires a N b /C (wt% ratio) of 2 or more in order to achieve both non-aging properties and bake hardenability, but if it exceeds 7.5, burn hardenability occurs. Since this cannot be expected, it is set to 7.5 or less.

The present invention further provides one or two of Sn, Sb, and Cr.
Contain more than seeds. These elements are added to increase the strength without adversely affecting the peeling of the plating, and for that purpose, Sn is 0.05%, Sb is 0.005%, and Cr is 0.02%. Otherwise, no effect will be obtained. On the other hand, too much content impairs deep drawability, so the upper limits are set at 0°80% for Sn, 0.080% for Sb, and 1.50% for Cr.

Molten steel having the above chemical composition is melted in a conventional converter or electric furnace, made into steel slabs by continuous casting or ingot-blending, and then hot rolled. The steel slabs may be directly hot-rolled while they are hot, or they may be charged into a reheating furnace in the hot state and then hot-rolled, and the temperature of the reheating furnace is usually 1000 to 1
The temperature shall be 300°C.

The finishing temperature during hot rolling is set at 750'C or higher since surface defects called striping occur at temperatures below 750'C.

Although the upper limit of the finishing temperature does not need to be regulated from the viewpoint of workability, it is preferably 950° C. or lower from the viewpoint of roll wear. If the winding temperature after finishing is too high, it will be difficult to ensure strength, so the temperature should be 800°C or less. Although the lower limit of the winding temperature is not particularly specified, it is usually preferably 300° C. or higher. In addition,
If the r value is important, it is recommended to wind the film at a high temperature of 700 or higher.

Next, cold rolling is performed at a reduction rate of 60% or more. If the cold rolling rate is too low, the recrystallization temperature will rise and it will be difficult to secure a high r value, so it is set to 60% or more. The upper limit is 8 to ensure strength.
00℃. Usually, cold rolling is preferably carried out at a cold rolling rate of 75 to 85%.

After cold rolling, hot-dip galvanizing and alloying are performed on a continuous hot-dip plating line, but in addition to the component composition mentioned above, these processing conditions are also important in order to prevent the plating film from peeling off during processing. be. First, the plate is passed through a non-oxidizing furnace and a reducing furnace to undergo recrystallization annealing using a conventional method, and the annealing temperature is usually 700° C. or higher. Thereafter, it is forcibly cooled down to the temperature of the plating bath using a water cooler or gas jet, and hot dip galvanized at 420 to 500°C. The plating temperature is set at 400 to 500°C since plating properties are not good if it is too low or too high. Furthermore, it is effective to include Al in the plating bath at a concentration of 0.08 to 0.30% for controlling alloying.

Next, alloying treatment conditions are one of the important factors governing the removability of plating by press working, and
Alloying treatment is carried out at a temperature of 0° C. to control the Fe concentration in the plating layer to less than 5 to IS%.

If the alloying temperature is too low, the alloying reaction will be slow;
If the temperature is too high, alloying progresses too much and it becomes difficult to control the Fe concentration in the plating layer within an appropriate range, so the alloying temperature is set 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 it should be 5% or more. On the other hand, if it is too high, peeling of the plating will become a problem, so it should be less than 15%.

Furthermore, after the alloyed galvanizing treatment, F
Applying an e-Zn-based upper layer plating is effective for further improving plating peeling resistance.

Finally, elongation of 0.5% or more is applied using a skin pass or tension leveler to eliminate elongation at yield point.

The present invention can be applied to zinc-based plated copper plates such as Zn-8n in addition to alloyed galvanized steel plates, and the effects of the invention can be enjoyed.

(Example) The steel shown in Table 1 was melted and rolled under the hot rolling conditions shown in Table 1.
．． A hot-rolled sheet with a thickness of 0 m+ was prepared, and then cold-rolled to a thickness of 0.8 m, followed by hot-dip galvanizing and alloying treatment under the conditions shown in Table 2. The amount of zinc deposited was 60 g/rrl per side. The steel plate obtained after applying 1.0% skin pass was subjected to a tensile test, and the r value, BH property, and peeling of the plating layer due to processing were investigated. Regarding the releasability of the plating layer, drawing was performed using a square tube punch, cellophane tape was pasted on a part of the corner of the square tube, the tape was peeled off, and the amount of stickiness adhering to the tape was measured.

The peelability of the plating was evaluated by classifying the amount of adhesion into four levels: large, medium, small, and extremely small. Table 2 shows various measurement results.

As is clear from Table 2, those that satisfy all of the component composition, hot rolling conditions, hot-dip galvanizing conditions, and alloying conditions of the present invention have excellent plating peeling resistance, bake hardenability, and high r value. It is evaluated as a high-strength alloyed hot-dip galvanized steel sheet.

(Effects of the Invention) The alloyed hot-dip galvanized steel sheet according to the present invention has bake hardenability that increases strength by baking the paint while ensuring high strength, and also has sufficient deep drawability due to its high r value. In addition, even when subjected to severe processing, there is extremely little peeling of the thick plating layer (performance is significantly improved compared to conventional methods, resulting in improved Punto resistance, tough processing, and longer car body lifespan than before). It can be applied to automobile parts that require extension, and has extremely useful industrial effects such as reducing the defective rate of molded products and improving the working environment.

Claims (2)

[Claims] (1) In weight%, C: 0.010% or less, Si: 0.5
% or less, Mn: 0.15-0.80%, P: 0.030
% or less, S: 0.03% or less, Al: 0.100% or less, N: 0.0050% or less, B: 0.0005 to 0.0
050%, Nb: 2×C (%) to 7.5×C (%), Sn: 0.05 to 0.80%, Sb: 0.005 to
0.080%, Cr: 0.020 to 1.50%, and the balance is iron and unavoidable impurities, and the Fe concentration in the galvanized layer is less than 5 to 15%. A bake-hardenable, high-strength alloyed galvanized steel sheet with excellent powdering resistance that is coated with a galvanized layer. (2) In weight%, C: 0.010% or less, Si: 0.5
% or less, Mn: 0.15-0.80%, P: 0.030
% or less, S: 0.03% or less, Al: 0.100% or less, N: 0.0050% or less, B: 0.0005 to 0.0
050%, Nb: 2×C (%) to 7.5×C (%), Sn: 0.05 to 0.80%, Sb: 0.005 to
0.080%, Cr: 0.020 to 1.50%, and the balance is iron and unavoidable impurities. After hot rolling and cold rolling at a reduction rate of 60% or more,
After recrystallization annealing on a continuous hot-dip galvanizing line and forced cooling, hot-dip galvanizing is performed at 420-500°C,
Subsequently, alloying treatment is performed at 400 to 600°C to form an alloyed galvanized layer in which the Fe concentration in the galvanized layer is less than 6 to 15%.High bake hardenability with excellent powdering resistance. A method for producing strength alloyed hot-dip galvanized steel sheets.