JPH0681121A - Baking hardening type hot-dip galvanized steel sheet having delayed aging characteristic at ordinary temperature - Google Patents

Abstract

PURPOSE:To provide a baking hardening type hot-dip galvanized steel sheet combining BH characteristic with delayed aging characteristic at ordinary temp. by specifying the average solid-solution C content between the ferrite surface of a steel sheet and a position at a specific depth and also over the whole sheet thickness. CONSTITUTION:A hot-dip galvanizing layer is provided on the surface of a steel sheet for forming. The average solid-solution C content in the grain boundaries between the ferrite surface of this steel sheet and a position at least at a depth one-tenth the sheet thickness is regulated to >=5ppm. Further, the average solid-solution C content in the grain boundaries over the whole sheet thickness of ferrite is regulated to <=10ppm. The steel sheet prepared has a composition consisting of, by weight, <=0.05% C, <=2% Si, <=3% Mn, <=0.3% P, <=0.05% S, 0.002-0.1% Al, <=0.01% N, and the balance Fe with inevitable impurities. Further, one or more elements among Ti, Nb, Zr, V, B, Ni, Cr, Cu, and Mo are incorporated. By this method, secondary working brittleness resistance can be improved.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a bake hardening type molten zinc having a room temperature delayed aging property which is suitable for use in applications where good workability and high strength are required in combination, such as an automobile exterior plate. The present invention relates to a plated steel sheet.

[0002]

2. Description of the Related Art As a steel sheet having both good workability and high strength, a so-called BH bake hardening type steel sheet which improves strength by fixing solid solution C to dislocations generated by the processing after forming and coating and baking. (BH type steel plate) is known (JP-A-57-89432, JP-A-57-70258 and JP-A-57-89432).
57-76131 publications, etc.). However, regarding such BH type steel sheet, in order to improve the bake hardenability, solid solution C in the steel
When the amount was increased, there was a problem in that room temperature aging was accompanied with it.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to obtain a bake hardening type having a room temperature slow aging property which does not lead to room temperature aging even if the above problems are advantageously overcome and the bake hardening property is improved. We are proposing a hot-dip galvanized steel sheet.

[0004]

Means for Solving the Problems Both the bake hardenability (BH) property and the aging at room temperature are aging phenomena, which occur when solid solution C is fixed to dislocations, especially dislocations in crystal grains, and movement of dislocations is hindered. Is. Therefore, the solid solution C in the crystal grains
As the amount increases, the BH property increases accordingly and the room temperature aging also progresses. However, according to the research conducted by the inventors regarding this point, although the solid solution C existing in the crystal grains and the solid solution C existing in the grain boundaries are involved in the aging phenomenon in the same manner, their behavior Turned out to be different.

That is, of the solid solution C, the solid solution C existing in the crystal grains is fixed to the dislocations with time. On the other hand, C existing in the grain boundaries diffuses from the grain boundaries into the grains and is fixed to dislocations in the grains only after aging at a high temperature such as BH. It does not diffuse into the grains by low temperature aging. Therefore, while the solid solution C existing in the grains before the aging treatment is involved in both the BH property and the room temperature aging, the solid solution C existing in the grain boundaries is
It was found that it was not involved in room temperature aging, but only in BH property.

The present invention is based on the above findings, and the gist of the invention resides in the following configuration. A thin steel sheet for forming, which has a hot-dip galvanized layer on its surface, wherein the average solid solution C within the grain boundaries between the base steel surface of this steel sheet and at least 1/10 the depth of the sheet thickness
The amount of 5 ppm or more, and the average amount of solute C in the crystal grains over the entire thickness of the base iron is 10 ppm or less, which is a bake hardening type hot-dip galvanized steel sheet having room temperature slow aging (first invention). ).

In the first aspect of the invention, the composition of the steel sheet is C: 0.05 wt% (hereinafter simply referred to as%) or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.3% or less, S: 0.05. % Or less, Al: 0.002 to 0.1% and N: 0.01% or less, and a bake hardening type hot-dip galvanized steel sheet (second invention) having the composition of balance Fe and unavoidable impurities.

In the first invention, the composition of the steel sheet is C: 0.05% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.3% or less, S: 0.05% or less, Al: 0.002 to 0.1%. And bake hardening type hot-dip galvanized steel sheet containing N: 0.01% or less, Ti: 0.3% or less and Nb: 0.2% or less, and a composition of the balance Fe and unavoidable impurities. (Third invention).

In the first invention, the composition of the steel sheet is C: 0.05% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.3% or less, S: 0.05% or less, Al: 0.002 to 0.1%. And N: 0.01% or less and Zr: 0.1% or less, V: 0.1% or less, B: 0.01% or less, Ni: 3.0% or less, Cr: 5.0% or less, Cu: 3.0% or less and Mo: 1.0% or less. Contains one or more selected from the following, with the balance Fe
And a bake-hardened hot-dip galvanized steel sheet having a composition of unavoidable impurities (fourth invention).

In the first invention, the composition of the steel sheet is C: 0.05% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.3% or less, S: 0.05% or less, Al: 0.002 to 0.1%. And N: 0.01% or less, Ti: 0.3% or less and Nb: 0.2% or less, and one or two kinds selected, and Zr: 0.1% or less, V: 0.1% or less, B: 0.01 % Or less, Ni: 3.0% or less, Cr: 5.0% or less, Cu: 3.0% or less, and Mo: 1.0% or less, and the balance Fe
And a bake-hardened hot-dip galvanized steel sheet having the composition of unavoidable impurities (fifth invention).

[0011]

As described above, among the solid solution C, C existing at the grain boundaries does not diffuse into the grains during low temperature aging such as normal temperature aging, and does not leave the grain boundaries after high temperature aging such as BH. Diffuse into the grain. Therefore, the solid solution C existing at the grain boundary is only involved in the BH property. Therefore, in the present invention, the amount of BH is secured by increasing the amount of solid solution C in the grain boundary,
By suppressing the normal temperature aging by limiting the amount of solid solution C in the grains, both high BH property and good normal temperature delayed aging were realized.

From the viewpoint of securing the BH property, the amount of solute C in the crystal grain boundaries is required to be at least 5 ppm. Below this value, a satisfactory improvement in the BH property cannot be expected. The upper limit of the grain boundary C is not particularly limited, but if it is too large, it may adversely affect the room temperature delayed aging. Therefore, it is desirable to set it to about 100 ppm or less.
On the other hand, from the viewpoint of room temperature slow aging, it is important to suppress the amount of solid solution C in the crystal grains to 10 ppm or less.

Next, in the present invention, the range in which the amount of grain boundary solid-soluted C should be controlled is at least 1/10 of the thickness of the steel plate from the surface of the base metal.
The limit to the depth is defined from the practical point of view that it is sufficient to limit the amount of solid solution C to at least 1/10 the depth of the plate thickness. It does not deny control.

In the present invention, the average amount of solid solution C in the crystal grains over the entire plate thickness of the base steel and the average crystal grain boundary solid solution C of the steel plate from the surface of the base iron to at least 1/10 of the plate thickness, that is, in the surface layer portion. The amount was determined as follows. The average amount of solid solution C in the grain over the entire thickness of the base steel was determined by the internal friction method. The average grain boundary solid-soluted C amount in the surface layer portion of the base metal is defined here as follows. (Average grain boundary solid-soluted C amount in surface layer portion) = (total C in surface layer portion)
Amount)-(Average amount of solid solution C in the surface layer portion)-(Amount of precipitated C in the surface layer portion) Here, the total C amount in the surface layer portion was determined by C analysis. Further, the average amount of solid solution C in the surface layer portion was determined by an internal friction method after preparing a surface layer portion sample. Furthermore, precipitation C on the surface layer
The amount was determined by determining the amount of precipitated Ti and the amount of precipitated Nb by the electrolytic extraction method, and then subtracting the amount of oxides, sulfides, and nitrides from them.

Next, regarding the steel types applied to the first invention,
The range of the component composition will be described. C: 0.05% or less; The amount of C in steel must be 0.05% or less in terms of workability, particularly deep drawability. In the present invention, in order to achieve the intended purpose, the amount of solid solution C is particularly important, and the average amount of solid solution C at the grain boundary from the surface of the base metal to at least 1/10 depth is 5 ppm or more. As described above, it is necessary to suppress the average amount of solute C in the crystal grains over the entire thickness of the base steel to 10 ppm or less. In addition, the above-mentioned grain boundary average solid solution C amount of 5 ppm or more is a ratio with respect to the whole base iron from the base iron surface to a depth of 1/10.

Si: 2.0% or less, Mn: 3.0% or less, P: 0.
3% or less; Si, Mn, and P are elements useful for improving the strength of steel by their solid solution strengthening action,
Since excessive addition causes deterioration of ductility, Si ≤ 2.0
%, Mn ≦ 3.0%, P ≦ 0.3%.

S: 0.05% or less; S is a harmful element which should be reduced as much as possible from the viewpoint of corrosion resistance, but S is allowable within the range of 0.05% or less.

Al: 0.002-0.1%; Al is deoxidized and N
Is an effective element for the precipitation and fixation of, and for that purpose, at least 0.002% is required, but if it exceeds 0.1%, the surface properties will be deteriorated, so the range was limited to 0.002 to 0.1%.

N: 0.01% or less; N advances the normal temperature aging and adversely affects the deep drawability, so it is limited to 0.01% or less.

Although the component composition range of the steel sheet for working has been described as a representative, the following elements may be added to further improve the deep drawability. Ti: 0.3% or less, Nb: 0.2% or less Both Ti and Nb are carbonitride forming elements and contribute very effectively to the improvement of deep drawability.
%, 0.20%, the workability and surface properties are deteriorated. Therefore, in either case of single addition or combination addition, the content should be within the above range.

All of B, Cu, Ni, Zr, V, Cr and Mo described below are equivalent as effective components for strengthening, but they also have the following effects. B: 0.01% or less B is also effective in increasing the strength and improving the secondary work embrittlement resistance, but if it exceeds 0.01%, the deep drawability is greatly deteriorated. Cu: 3.0% or less Cu is also effective for strengthening, but if it exceeds 3.0%, the ductility deteriorates significantly. Ni: 3.0% or less Ni is an element effective for promoting Cu precipitates and preventing the deterioration of the surface properties due to Cu.
If it exceeds, the deep drawability is greatly deteriorated. Zr: 0.1% or less, V: 0.1% or less, Cr: 5.0% or less, M
o: 1.0% or less; Zr, V, Cr and Mo all contribute effectively to the increase in strength, but even if added in excess of the respective upper limits, the effect reaches saturation, rather the workability deteriorates and the cost decreases. It becomes high.

Next, a method for increasing the amount of solid solution C in the grain boundary of the surface layer of the base metal will be described. Higher grain boundary solid solution C than ordinary steel
Since it is difficult to add C during molten steel to achieve the ratio, the present invention adopts a method of introducing C into the steel sheet.
By the way, in order to preferentially introduce C into the crystal grain boundaries of the surface layer of the base steel, it is preferable to carry out carburizing treatment after recrystallization annealing in the hot dip galvanizing line. This is because the grain boundaries are still moving during recrystallization annealing, so if a carburizing treatment is performed at this time, a large amount of carbon will be incorporated into the grains, but carburizing after recrystallization annealing If this is done, carbon is more likely to diffuse in the grain boundaries than in the grains, so the distribution of carbon after carburization is such that there are less carbon in the grains and more carbon in the grain boundaries. Therefore, it is possible to have both the slow aging at room temperature and the bake hardenability. The carburizing treatment after the recrystallization annealing is not limited to gas, and a method of introducing carbon by bringing a liquid or solid carbon source into contact with the surface of the base steel may be used. Further, as long as the grain boundary solid solution C ratio can be increased, any method may be used without being limited to the above method.

In the present invention, the hot dip galvanizing treatment formed on the surface of the base steel plate is not particularly limited, and a conventionally known method can be applied. Of course, there is no problem even if the hot-dip galvanized film is alloyed. The plating thickness of 30 to 90 g / mm, which is commonly used, is sufficient.

[0024]

Example After heating each slab having the composition shown in Table 1 to 1200 ° C., hot rolling was performed under the conditions of finishing temperature: 880 ° C. and winding temperature: 620 ° C. to obtain a 3.2 mm hot rolled sheet. Then, after descaling, cold rolling was performed to obtain a 0.7 to 0.8 mm cold rolled sheet. Next, after performing recrystallization annealing on each cold-rolled sheet at the maximum heating temperature shown in Tables 2 and 3 in the hot dip galvanizing line,
_{H 2: 3.2%, CO:} 0.5 ~20%, CO 2: 0.03%, in an atmosphere gas of the remaining N _2, subjected to a carburizing treatment for 20 seconds at a temperature shown in Table 2, to further 460 ° C. 30 After cooling at a rate of ° C / s, hot dip galvanizing was performed. In this hot-dip galvanizing, steel types other than F and O were alloyed at 500 ° C., and then 1% skin pass was performed on all steel types.

In each of the thus obtained hot-dip galvanized steel sheets, the average amount of grain boundary solid solution C in the surface layer of the base iron, the average amount of solid solution C in the crystal grains over the entire thickness of the base iron, BH property and room temperature aging property In addition, Tables 2 and 3 show the results of examining various mechanical properties. In addition, for comparison, Tables 2 and 3 also show the results of the investigation when the carburizing treatment was performed during the recrystallization annealing (enter only the heat treatment temperature in the atmosphere gas). The conventional steel is
In the steel sheet of baking hardening type Nb _{addition, H 2: 3.2%, CO} : 0%,
CO ₂ : 0%, the remaining N _{2 in} the atmosphere gas, after heat treatment for 30 seconds, it was cooled at a cooling rate of 25 ℃ / s.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

As is clear from Tables 2 and 3, the surface layer of the ground iron
When the amount of grain boundary C at a thickness of 10% and the amount of solid solution C in the entire plate of the base steel according to the present invention are in accordance with the present invention, BH is effectively increased without increasing Y-El.

[0030]

As described above, according to the present invention, the average amount of solid solution C in the crystal grains in the total thickness of the base iron and the average amount of solid solution C in the surface layer of the base iron within the appropriate range are set within appropriate ranges. By regulating,
It is possible to obtain a bake-hardened hot-dip galvanized steel sheet that has both the BH property and the room temperature delayed aging, which have been conventionally regarded as difficult.
Further, since the steel of the present invention has C at the grain boundary, it has an effect of improving the secondary work embrittlement resistance in the case of an extremely low C steel.

Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical indication location C22C 38/00 301 T 38/60 (72) Inventor Susumu Sato 1 Kawasaki-cho, Chuo-ku, Chiba-shi Chiba Prefecture Made by Kawasaki Iron Co., Ltd.Technical Research Headquarters (72) Inventor Masahiko Morita 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Headquarters (72) Inventor Nikohiko Nakagawa 1-chome, Mizushima Kawasaki-dori, Kurashiki City (No house number) Kawashima Steel Works Mizushima Works

Claims (5)

[Claims] 1. A thin steel sheet for forming, having a hot-dip galvanized layer on the surface, wherein the average solid solution in the grain boundaries between the base metal surface of the steel sheet and at least 1/10 of the plate thickness. A bake-hardening hot-dip galvanized steel sheet having room-temperature slow aging, characterized in that the amount of C is 5 ppm or more and the average amount of dissolved C in the crystal grains over the entire thickness of the base steel is 10 ppm or less. 2. The steel sheet according to claim 1, wherein the composition of the steel sheet is C: 0.05 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, P: 0.3 wt% or less, S: 0.05 wt% or less, A bake-hardening hot-dip galvanized steel sheet containing Al: 0.002 to 0.1 wt% and N: 0.01 wt% or less and having a composition of balance Fe and inevitable impurities. 3. The steel sheet according to claim 1, wherein the composition of the steel sheet is C: 0.05 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, P: 0.3 wt% or less, S: 0.05 wt% or less, Al: 0.002-0.1 wt% and N: 0.01 wt% or less, and one or two selected from Ti: 0.3 wt% or less and Nb: 0.2 wt% or less, and the balance Fe and unavoidable impurities. Bake hardening type hot-dip galvanized steel sheet that has the composition. 4. The steel sheet according to claim 1, wherein the composition of the steel sheet is C: 0.05 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, P: 0.3 wt% or less, S: 0.05 wt% or less, Al: 0.002-0.1 wt% and N: 0.01 wt% or less, and Zr: 0.1 wt% or less, V: 0.1 wt% or less, B: 0.01 wt% or less, Ni: 3.0 wt% or less, Cr: 5.0 wt% % Or less, Cu: 3.0 wt% or less and Mo: 1.0 wt% or less, and one or more selected, and the balance Fe
And a bake-hardened hot-dip galvanized steel sheet having a composition of unavoidable impurities. 5. The component composition of the steel sheet according to claim 1, wherein C: 0.05 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, P: 0.3 wt% or less, S: 0.05 wt% or less, Al: 0.002 to 0.1 wt% and N: 0.01 wt% or less, and one or two selected from Ti: 0.3 wt% or less and Nb: 0.2 wt% or less, and Zr: 0.1 wt% Or less, V: 0.1 wt% or less, B: 0.01 wt% or less, Ni: 3.0 wt% or less, Cr: 5.0 wt% or less, Cu: 3.0 wt% or less, and Mo: 1.0 wt% or less, or Contains two or more types, balance Fe
And a bake-hardened hot-dip galvanized steel sheet having a composition of unavoidable impurities.