JPH05247586A - Hot dip galvanized steel sheet having high strength and high ductility and excellent in adhesive strength of plating - Google Patents

Abstract

PURPOSE:To improve the adhesive strength of plating without deteriorating the superior balance between strength and ductility, which is the feature of a complex structure steel sheet containing residual austenite, by specifying respective contents of C, Mn, Si, Al, etc., in the composition of a steel sheet. CONSTITUTION:The steel sheet for hot dipping has a composition consisting of, by weight, 0.05-0.30% C, <=0.5% Si, 0.8-3.0% Mn, <=0.02% P, <=0.01% S, 0.5-1.5% Al, <=0.008% N, and the balance Fe with inevitable impurities and further containing, as necessary, one or >=2 kinds selected from 0.005-0.05% Ti, 0.005-0.05% Nb, 0.0005-0.003% B, 0.2-1.0% Cu, 0.2-1.0% Ni, 0.2-1.0% Cr, and 0.2-1.0% Mo. A cold rolled steel sheet of this composition, passes through a continuous hot dip galvanizing line and subjected to annealing and to the formation of a galvanized layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength and high-ductility hot-dip galvanized steel sheet having excellent plating adhesion, without impairing the excellent strength / ductility balance which is a characteristic of a composite structure steel sheet containing retained austenite The present invention provides a hot-dip galvanized steel sheet having excellent plating adhesion.

[0002]

2. Description of the Related Art In recent years, there has been a demand for improvement in fuel efficiency of automobiles from the viewpoint of global environmental protection, and there is an active movement to reduce the weight of vehicle bodies for the purpose of achieving the improvement. That is, for this reason, the weight of the vehicle body is reduced by thinning the steel sheet used for automobiles, and the decrease in the vehicle body strength due to the thinning is compensated by the higher strength of the steel sheet. The demand for higher ductility is becoming more and more severe, and a material that has both high strength and high ductility is expected.

In order to meet such requirements, the tensile strength of 8 can be obtained by utilizing the work-induced transformation of retained austenite.
A steel sheet having a breaking elongation of about 30% at 0 to 100 kgf / mm ² is proposed in JP-A-60-43430. Such a steel sheet is produced by subjecting a steel sheet containing C, Si, and Mn as basic components to austenitizing, followed by quenching in the bainite transformation temperature region and holding at an isothermal temperature, that is, a so-called austempering treatment.

This austempering treatment concentrates C in austenite to produce retained austenite, but Mn is necessary for stabilizing austenite, and Si is necessary to stabilize C in austenite. It is considered to be an essential element for promoting thickening, and is characterized in that a large amount of C, Si, and Mn is added as compared with other types of steel sheets. Recently, materials and processes, vol.2
(1989), 1846, it has been found that Al has a similar effect to Si on the formation of retained austenite.

[0005]

While steel sheets for automobiles are required to have higher strength and ductility, steel sheets that have been electroplated or hot-dipped to improve the corrosion resistance of automobile bodies are There is an increasing demand for hot-dip galvanizing, which is required and is particularly advantageous in terms of cost because it enables thick plating. However, since a large amount of Si is added to the above steel sheet, it is difficult to perform hot dip galvanizing, and although it has an excellent balance of strength and ductility, it has a problem in terms of corrosion resistance.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted extensive research in order to solve the above-mentioned problems, while making use of the excellent strength / ductility balance of a composite structure steel sheet containing retained austenite, Various studies were conducted on chemical components for improving plating adhesion.

That is, as an example, C: 0.11%, M
Manufacture various cold rolled steel sheets containing n: 1.82 wt%, P: 0.009%, S: 0.005% and different Si and Al contents,
A galvanized steel sheet having a basis weight of 60/60 g / m ² was obtained on a continuous hot-dip galvanizing line. TS (tensile strength) x E as an index of strength / ductility balance of the obtained plated material
The l (elongation at break) was investigated by a tensile test. Further, the amount of plating peeling as an index of plating adhesion was investigated by a draw bead test. The results are as shown in FIG.

That is, Si> 0.5% or Al> 1.5%
In the area of, the amount of plating peeling is increasing, while Al <0.
Since TS × El is low in the 5% area,
Si that can balance plating adhesion and strength / ductility balance,
It can be seen from FIG. 1 that the amount of Al added is limited.

The materials and processes described above, vol. 2
(1989), 1846, the effect of the addition of Si and Al on the formation of retained austenite is described, but no mention is made of the plating adhesion, and the addition amounts of Si and Al are also related to the plating adhesion. Since it is not preferable, it is clearly different from the idea of the present invention aimed at achieving both plating adhesion and strength / ductility balance.

As a result of further research on chemical components such as C, Mn, and Ti, the present inventors found for the first time that a high-strength, high-ductility hot-dip galvanized steel sheet excellent in plating adhesion was obtained. The invention has been completed and is as follows.

(1) wt%, C: 0.05 to 0.30%,
Si: 0.5% or less, Mn: 0.8 to 3.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.5 to 1.5%, N: 0.0. 00
A high-strength, high-ductile hot-dip galvanized steel sheet having excellent plating adhesion, containing 8% or less, and the balance being Fe and inevitable impurities.

(2) wt%, Ti: 0.005 to 0.05
%, Nb: 0.005 to 0.05%, B: 0.0005 to 0.00
3%, Cu: 0.2 to 1.0%, Ni: 0.2 to 1.0%, Cr: 0.2
~ 1.0%, Mo: 0.2-1.0% selected from 1 or 2
High-strength, high-ductility molten zinc excellent in plating adhesion according to the above item (1), characterized by containing at least one element in a range such that the sum of Ti, Nb, and B is 1.0% or less. Plated steel sheet.

[0013]

In the present invention as described above, the reasons for limiting the content range of alloying elements are as follows. C: 0.05 to 0.30% C is concentrated in the austenite and stabilizes the austenite, whereby residual austenite is generated. The presence of this retained austenite improves the strength / ductility balance, but it is necessary to add C in an amount of 0.05 wt% or more to obtain retained austenite sufficient to exert its effect. On the other hand, if the content exceeds 0.3 wt%, the amount of retained austenite increases, but the weldability and local ductility are significantly deteriorated, so the upper limit is made 0.3%.

Si: 0.5% or less Si has the function of promoting the concentration of C in austenite and facilitating the formation of retained austenite, but it is concentrated as an oxide in the surface layer of the steel sheet and forms a mixture with molten zinc. Since it deteriorates the wettability, it is an extremely harmful element in terms of plating adhesion. Therefore, the upper limit of the added amount is 0.5%. In addition,
If you want to secure stable plating adhesion,
It is preferably 0.3% or less.

Mn: 0.8-3.0% Mn, like C, is an austenite stabilizing element,
Furthermore, since the nose of ferrite-pearlite transformation shifts to the long side, it is an inexpensive element necessary for the formation of retained austenite by bainite transformation. In particular, when rapid cooling cannot be performed after soaking as in a continuous hot-dip galvanizing line, the addition of Mn is very effective in securing the desired strength / ductility balance. In order to exert such effects, it is necessary to add 0.8% or more. However, if added excessively, the plating adhesion will deteriorate, so the upper limit is made 3.0%.

P: 0.02% or less P is preferably low in terms of workability and plating adhesion, and is limited to 0.02% or less in the present invention.

S: 0.01% or less Since S remarkably deteriorates the ductility of steel, it is desirable that S is as small as possible. Therefore, it is limited to 0.01% or less.

Al: 0.5-1.5% Al has the function of promoting the concentration of C in austenite and facilitating the formation of residual austenite, and it is necessary to add 0.5% or more. To do. However, excessive addition deteriorates the plating adhesion, so the upper limit is made 1.5%.

N: 0.008% or less N is preferable because it deteriorates the ductility of steel, so N is preferably less than 0.008%.

Although the basic elements in the present invention are as described above, it is possible to further improve the strength / ductility balance by adding the following elements.

Ti and Nb are both strong carbonitride forming elements, and the structure is refined by fine carbonitrides, and B is a solid solution in steel to refine the structure, so that the balance between strength and ductility is obtained. Is improved. To obtain this effect, Ti, Nb
Is 0.005% or more, B is 0.0005%
The above addition is required. However, if added excessively, Ti
A large amount of carbonitrides such as C and BN are precipitated, and these precipitates deteriorate the ductility of steel. Therefore, Ti and Nb are 0.0
5% or less, and B is limited to 0.003% or less.

Although Cr and Mo are ferrite stabilizing elements, they have the effects of improving hardenability and leaving austenite remaining. The effect is obtained by addition of 0.2% or more, but if it is added over 1.0%, stable carbides are generated and conversely the retained austenite is decreased. Therefore, the added amounts of Cr and Mo are set to 0.2 to 1.0%.

Cu and Ni are austenite-stabilizing elements and are effective in increasing strength as well as allowing austenite to remain. This effect cannot be obtained with the addition of less than 0.2%, and the addition of more than 1.0% reduces the ductility of the steel sheet. Therefore, the addition amount of Cu and Ni is set to 0.2 to 1.0%.

When two or more of these elements are added, the addition amount of each element is within the above range and at the same time Ti, N
b, B total 0.05% or less, Cu, Ni, Cr, Mo total
It is necessary to add it in the range of 1.0% or less. This is because if the total amount of addition exceeds the upper limit, ductility is reduced.

The steel containing the above-mentioned chemical components is usually hot-rolled or cold-rolled into a steel sheet having a desired thickness by a conventional method. Then, the obtained cold rolled steel sheet is passed through a continuous hot dip galvanizing line to anneal and form a galvanized layer. Here, the heating temperature during annealing is Ac ₁ to Ac ₃ , the holding time after heating is 20 seconds to 3 minutes, the cooling rate after that is 5 ° C./sec or more, and the temperature of the hot dip galvanizing bath is 480 ° C. or less. Is most preferable from the viewpoint of strength / ductility balance. In addition, when performing an alloying process as needed, it is desirable to complete an alloying process in the shortest time possible. Further, the effect of the present invention is not impaired even when Fe-rich Fe-Zn alloy electroplating is applied as the upper layer plating.

[0026]

EXAMPLES Specific examples of the present invention will be described below. That is, first, the chemical compositions of representative steels according to the present invention and the comparative examples specifically adopted by the present inventors are as shown in Tables 1 and 2 below.

[0027]

[Table 1]

[0028]

[Table 2]

The steels of Tables 1 and 2 as described above are melted and cast, and the heating temperature is 1200 ° C. and the finishing temperature is 900 ° C.
Hot rolling was performed at a winding temperature of 600 ° C. to obtain a steel plate having a thickness of 3.2 mm, followed by pickling and cold rolling to obtain a cold rolled steel sheet having a thickness of 1.2 mm. After that, in the continuous hot dip galvanizing line,
After heating and holding at 825 ° C. for 1 minute, it was cooled at a cooling rate of 10 ° C./sec and plated in a zinc plating bath at 460 ° C. to obtain a hot dip galvanized steel sheet having a basis weight of 60/60 g / m ² . ..

Incidentally, some of the steel sheets were subjected to an alloying treatment at 500 ° C. and an upper layer electroplating treatment of an 80% Fe--Zn alloy with 3 g / m ² per side. The obtained plated material was temper-rolled by 1% (however, the upper-layer plated material was temper-rolled before the electroplating treatment), and then subjected to a tensile test using JIS No. 5 test piece to obtain TS (tensile strength). ), E
1 (total elongation) and TS × El were investigated. In addition, a bending test was conducted to examine Rmin (minimum bending radius). Furthermore, a draw bead test was performed to evaluate the adhesion of the plating, and the amount of plating peeling was measured. The results are as shown in Tables 3, 4 and 5 below, respectively:
Sample Nos. 47 to 49 are alloyed, Sample N
o. 50 to 52 are those subjected to upper layer electroplating.

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

[Table 5]

According to the results of Tables 3 to 5 as described above, the sample Nos. 2 to 8, 11, 12, 14 according to the present invention were used.
~ 19, 22, 23, 27, 28, 30-37, 39-
41, 43, 44, 47 to 52 have TS × El of 2300
As described above, Rmin is 0.5 or less, and the amount of plating peeling is less than 5 g / m ² , and it is clear that the strength / ductility balance, bendability, and plating adhesion are excellent. In particular, Ti, Nb, B, Cu,
Sample No. 5-7, 16 with Ni, Cr, Mo added in appropriate ranges
~ 19, 22, 23, 33-37, 43, 44, 48,
Regarding 49, 51 and 52, the balance of strength and ductility is further improved.

On the other hand, sample Nos. 1, 9, 10, 13, 20, 21, 24 whose chemical composition is outside the scope of the present invention.
About 26 to 29, 38 and 42, TS × E1 is low or the amount of plating peeling is large, so that the plating adhesion and strength
It is clear that the compatibility of the ductility balance is impossible and the object of the present invention cannot be achieved. Further, it can be seen that in Sample Nos. 45 and 46 in which C is excessively added, Rmin is large, and the local ductility is deteriorated.

[0036]

Industrial Applicability According to the present invention described above, a hot-dip galvanized steel sheet having high strength, high ductility and excellent plating adhesion can be obtained, so that it has a great industrial utility value, especially for automobile bodies. The invention is extremely useful for weight reduction and rust prevention and has a large industrial effect.

[Brief description of drawings]

FIG. 1 is a chart summarizing the relationship between the amounts of Si and Al added, TS × El, and the amount of plating removal.

Front page continuation (72) Inventor Junichi Inagaki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. Wt%, C: 0.05 to 0.30%, Si: 0.
5% or less, Mn: 0.8 to 3.0%, P: 0.02% or less, S:
High strength with excellent plating adhesion characterized by containing 0.01% or less, Al: 0.5 to 1.5%, N: 0.008% or less, the balance being Fe and inevitable impurities. Highly ductile hot-dip galvanized steel sheet. 2. In wt%, Ti: 0.005 to 0.05%, Nb:
0.005 to 0.05%, B: 0.0005 to 0.003%, C
u: 0.2-1.0%, Ni: 0.2-1.0%, Cr: 0.2-1.0
%, Mo: One or more elements selected from 0.2 to 1.0% are contained in a range such that the total of Ti, Nb and B is 1.0% or less. Item 1. A high-strength, high-ductility hot-dip galvanized steel sheet having excellent plating adhesion according to Item 1.