JP2876966B2 - Cold rolled steel sheet with excellent low temperature bake hardenability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold rolled steel sheet which is excellent in bake hardenability at a low paint baking temperature and is suitable for, for example, a car body outer panel.

[0002]

2. Description of the Related Art In recent years, global environmental problems have attracted attention,
For automobiles, reducing the amount of harmful exhaust gas has become an important issue. As one of the measures to reduce the amount of harmful exhaust gas, by increasing the strength of the outer panel of the automobile, etc., the outer panel is thinned, the body weight is reduced, and the fuel efficiency is improved. That is being done. However, if the dents occur in the outer panel, the appearance is significantly impaired. Therefore, when the outer panel is made thinner, it is necessary to take care that the dent of the outer panel does not occur. Dent resistance is a property indicating the dent resistance of steel sheets. Dent resistance is proportional to the square of the thickness of the steel sheet,
And it is proportional to the yield point of the steel sheet. In order to effectively maintain excellent dent resistance even after the outer panel is thinned, the property that the yield point increases in the baking coating process after press molding (hereinafter referred to as “bake hardening”). It is particularly necessary that the steel sheet be provided.

[0003] From the above-mentioned viewpoints, studies for improving the bake hardenability of steel sheets have been conventionally conducted. In recent years, in particular, a technique for improving the bake hardenability of a steel sheet by adding a carbonitride forming element to an ultra-low carbon steel having excellent workability has been developed. As such a steel plate, for example, Japanese Patent Publication No. Sho 63-4
No. 899 discloses that an extremely low carbon steel with a reduced content of sulfur (S) and nitrogen (N) is annealed at a high temperature with a steel sheet added with titanium (Ti), and then cooled at a high cooling rate. A method of improving the bake hardenability of a steel sheet by leaving solid solution carbon (C) in steel (hereinafter referred to as Prior Art 1) is disclosed.

[0004] The bake hardenability of steel is a phenomenon in which the yield strength of steel is increased by strain aging of C and N dissolved in steel during bake coating after press forming. Therefore, conventionally, it has been considered that the bake hardenability of a steel sheet can be improved only by increasing the amount of solute C or solute N in steel.

In addition, Japanese Patent Publication No. 60-47328 discloses that when boron (B) is added to steel, the bake hardenability of the steel sheet is improved even if the steel contains C at the same level as in the prior art. (Hereinafter referred to as Prior Art 2) is disclosed.

Further, Japanese Patent Application Laid-Open No. 4-136123 discloses a method for producing a steel sheet having excellent bake hardenability by adding boron to steel and specifying a cooling rate within a specific range ( Hereinafter, this is referred to as Prior Art 3).

On the other hand, the coating baking temperature is usually about 170 ° C., but from the viewpoint of energy saving, it is required that the coating baking temperature be lower than the above-mentioned temperature. Therefore, development of a steel sheet excellent in bake hardenability at a low paint bake temperature (hereinafter, referred to as “low temperature bake hardenability”) has become important. As a technique to improve low-temperature bake hardenability, for example,
JP-A-3-211228 discloses improving the low-temperature bake hardenability of a steel sheet by adding boron to steel (hereinafter referred to as prior art 4).

[0008]

In prior art 1, since the amount of dissolved carbon in the steel is large, the bake hardenability is high, but at the same time, the aging deterioration of the material at normal temperature is remarkable. There's a problem. Prior arts 2 to 4 still have a problem that the low-temperature bake hardenability of the steel sheet is inferior. That is, for example, in Prior Art 4, the low temperature bake hardenability of a steel sheet at 120 ° C. is as low as about 3 kgf / mm ² (29 MPa). Further, in Prior Art 4, the difference in bake hardenability between the low temperature bake hardenability of a steel sheet at 170 ° C. and the low temperature bake hardenability of a steel sheet at 120 ° C. is about 1.5 kgf / mm ² (15 MPa). ), And the baking curability is reduced by lowering the baking temperature. The same can be said for prior arts 2 and 3 as for prior art 4.

[0009] For these reasons, there is a demand for the development of a cold rolled steel sheet excellent in low-temperature baking hardenability without deterioration of the material, but such a steel sheet has not yet been proposed.

[0010] Accordingly, an object of the present invention is to solve the above-mentioned problems and to provide a cold-rolled steel sheet excellent in low-temperature baking hardenability without deteriorating the material.

[0011]

In order to obtain excellent low-temperature bake hardenability, the low-temperature bake hardenability of a steel sheet at 100 ° C. must be improved.
It is necessary that the difference in bake hardenability between 35 MPa or more and the low temperature bake hardenability of the steel sheet at 170 ° C. is 5 MPa or less. For that purpose, it is indispensable to optimize the contents of oxygen and boron, which are important elements for improving the low-temperature bake hardenability of the cold-rolled steel sheet.

From the above viewpoints, the present inventors have intensively studied to develop a cold-rolled steel sheet which is excellent in low-temperature baking hardenability without deterioration of the material. As a result, 0.0003
~ 0.0015wt.% Boron and 0.0005 ~ 0.003wt.%
When the amount of boron and the amount of oxygen satisfy the relationship of B ≧ 0.5 × O−0.0005,
It was found that a cold-rolled steel sheet having excellent low-temperature bake hardenability was obtained.

The present invention has been made based on the above-mentioned findings, and a cold-rolled steel sheet having excellent low-temperature bake hardenability according to the present invention comprises the following. Carbon (C): 0.001 to 0.005 wt.%, Silicon (Si): 0.3 wt.% Or less, Manganese (Mn): 0.1 to 2 wt.%, Phosphorus (P): 0.1 wt.% Or less, Sulfur (S) : 0.015 wt.% Or less, Soluble aluminum (Sol.Al): 0.005 to 0.1 wt.%, Nitrogen (N): 0.004 wt.% Or less, Titanium (Ti): from (48/14) N to (48/14) N + (48/32) S + 16
C, boron (B): 0.0003 to 0.0015 wt.%, Oxygen (O): 0.0005 to 0.003 wt.%, But B ≧ 0.5 × O−0.0005, balance: iron (Fe) and unavoidable impurities.

[0014]

The reasons why the chemical composition of the cold-rolled steel sheet of the present invention is limited to the above-described range will be described below. (1) Carbon (C): Carbon has a function of improving bake hardenability. However, when the carbon content is less than 0.001 wt.%, The function of improving bake hardenability cannot be obtained. on the other hand,
When the carbon content exceeds 0.005 wt.%, The amount of carbide increases and the ductility decreases. Therefore, the carbon content is 0.0
It should be limited to the range of 01-0.005 wt.%.

(2) Silicon (Si): Silicon is a solid solution strengthening element and has a function of increasing the strength of a steel sheet. Therefore, it is added when it is necessary to increase the strength of the steel sheet.
However, when the silicon content exceeds 0.3 wt.%, A strong oxide layer is formed during hot rolling, and the surface properties deteriorate. Therefore, the silicon content should be limited to 0.3 wt.% Or less.

(3) Manganese (Mn): Manganese is a solid solution strengthening element and has a function of increasing the strength of a steel sheet. When the manganese content is less than 0.1 wt.%, Hot embrittlement occurs. On the other hand, when the manganese content exceeds 2 wt.%, The ductility decreases and the formability deteriorates. Therefore, the manganese content should be limited to the range of 0.1 to 2 wt.%.

(4) Phosphorus (P): Phosphorus, like silicon and manganese, has a function of increasing the strength of a steel sheet. Therefore, if necessary, phosphorus is contained. However, when the phosphorus content exceeds 0.1 wt.%, The ductility of the steel sheet is reduced, the formability is deteriorated, and the secondary working brittleness may be caused at the time of forming. . Therefore, the phosphorus content is 0.1 wt.
It should be limited to%.

(5) Sulfur (S): Sulfur is contained in TiS and Ti ₄ C in steel.
₂ Form sulfides such as S ₂ and MnS. In particular, when Ti ₄ C ₂ S ₂ is formed in steel, the amount of solute C required for bake hardening decreases. Further, when the sulfur content exceeds 0.015 wt.%, The sulfide increases and the formability deteriorates. Therefore, the sulfur content should be limited to 0.015 wt.% Or less.

(6) Soluble aluminum (Sol. Al): Aluminum is an element necessary for deoxidizing steel. However, when the content of soluble aluminum exceeds 0.1 wt.%, The amount of inclusions in the steel increases, ductility decreases, and formability deteriorates. Therefore, the content of soluble aluminum should be limited to the range of 0.005 to 0.1 wt.%.

(7) Nitrogen (N): Like solute carbon, nitrogen has the function of improving bake hardenability. Nitrogen is
At room temperature, the diffusion rate is higher than that of carbon, and the material is significantly deteriorated at room temperature by aging. Therefore, it is necessary to contain Ti and fix nitrogen as TiN. Nitrogen content is 0.0
If the content exceeds 04 wt.%, The content of Ti for fixing as TiN increases, and the ductility of the steel deteriorates.
Therefore, the nitrogen content should be limited to 0.004 wt.% Or less.

(8) Titanium (Ti): Titanium has a function to fix nitrogen as TiN and to fix sulfur and carbon appropriately to improve the material. The reason for fixing nitrogen as TiN with titanium is to suppress aging due to nitrogen by fixing nitrogen and to suppress formation of BN as much as possible by fixing nitrogen as TiN. That is, it is necessary to fix nitrogen with titanium in order to maintain boron, which is important for improving low-temperature bake hardenability, in a solid solution state.

In order to fix nitrogen by titanium,
It is necessary to contain stoichiometrically equivalent titanium or more, that is, (48/14) N or more titanium. However, when the titanium content is more than (48/14) N + (48/32) S + 16C, it becomes difficult to dissolve Ti-based carbides even after high-temperature annealing, and carbon re-dissolves in the steel. The amount is too small to obtain excellent bake hardenability. Therefore, the titanium content is (4
It should be limited to the range 8/14) N to (48/14) N + (48/32) S + 16C.

(8) Boron (B): Boron is an important element having a function of improving low-temperature bake hardenability. It is presumed that the reason why boron enhances low-temperature bake hardenability is that, due to segregation of solid-solution boron at grain boundaries, carbon segregating at grain boundaries that does not contribute to low-temperature bake hardenability is reduced. However, when the boron content is less than 0.0003 wt.%, The amount of boron segregated at the grain boundary becomes small, and the segregation of carbon at the grain boundary cannot be suppressed. On the other hand, when the boron content exceeds 0.0015 wt.%, Not only does the above-mentioned effect reach a saturated state, but also the ductility and the like deteriorate, and the material deteriorates. Therefore, the boron content is 0.0003 to 0.0015
It should be limited to the range of wt.%.

(9) Oxygen (O): Oxygen, together with boron, is an important element having a function of improving low-temperature bake hardenability.
Oxygen exists as an oxide in the steel, and as shown in FIG. 1 described below, if the oxygen content is too high or too low, the low-temperature bake hardenability decreases. When the oxygen content is too high, the oxide distorts the crystal lattice, and the resulting distortion of the crystal lattice suppresses the diffusion of solute carbon at low temperatures. Alternatively, it is considered that carbon is adsorbed on the surface of the oxide, and solute carbon that contributes to enhancing the low-temperature bake hardenability decreases. That is, the oxygen content is 0.003
When the content exceeds wt.%, the lowering of the low-temperature baking hardenability becomes remarkable.

On the other hand, when the oxygen content is extremely small, the oxide becomes finer and the number increases, the surface area of the oxide increases, the amount of adsorbed carbon increases, and the low-temperature bake hardenability decreases. That is, when the oxygen content is less than 0.0005 wt.%, The low-temperature bake hardenability decreases. Therefore, the oxygen content should be limited to the range of 0.0005 to 0.003 wt.%.

As described above, by including the specified amounts of oxygen and boron, excellent low-temperature bake hardenability can be obtained. However, when either one of oxygen and boron does not satisfy the specified amount, excellent low temperature bake hardenability cannot be obtained. Further, in the range where the oxygen content is relatively high, the low-temperature baking hardenability decreases as the oxygen content increases, so the boron content is increased and the solid solution is effective for improving the low-temperature bake hardenability. Need to increase carbon. Therefore, as shown in FIG. 2 described later, when the oxygen content is in the range of 0.0015 to 0.003 wt.%, The boron content is B ≧ 0.5 × O−0.
0005 must be satisfied.

The cold-rolled steel sheet of the present invention includes a surface-treated steel sheet and has excellent surface treatment properties, and is suitable for use after being subjected to various surface treatments. That is, treatments such as hot-dip metal plating (zinc, aluminum, and zinc-aluminum alloy), alloyed hot-dip galvanizing, electro-galvanizing, electro-zinc alloy plating, organic composite plating, and tin plating are used alone or in combination as appropriate. The surface treatment does not impair the excellent low temperature bake hardenability.

[0028]

Next, the present invention will be described with reference to examples and comparative examples. Example 1 Steels A and B having the chemical components shown in Table 1 were used as bases.
Pickling, cold rolling, and annealing in a continuous annealing line were performed to prepare a cold-rolled steel sheet. For these cold rolled steel sheets,
2% BH at 170 ° C and 100 ° C baking temperatures were measured. The result is shown in FIG.

[0029]

[Table 1]

For 2% BH, a JIS G test piece was used and JIS G
Measured according to 3135 Appendix. 2% BH at a baking temperature of 170 ° C and 2% BH at a baking temperature of 100 ° C
Is defined as ΔBH.

As is clear from FIG. 1, steel A and steel B
In both cases, when the oxygen content exceeds 0.003 wt.%,
When the 2% BH at the baking temperature of 100 ° C drops sharply, and when the oxygen content is 0.003 wt.% Or less, the 2% BH at the baking temperature of 100 ° C increases and ΔBH becomes 5%.
MPa or less. On the other hand, when the oxygen content of both steel A and steel B is less than 0.0005 wt.%, The 2% BH at 100 ° C. baking temperature drops sharply. Therefore, in order to obtain excellent low temperature bake hardenability, it is necessary to control the oxygen content within the range of 0.0005 to 0.003 wt.% Regardless of the type of steel.

Example 2 FIG. 2 shows that a steel having a chemical composition in the range shown in Table 2 was cast into a slab by continuous casting, then hot-rolled, pickled, and cold-rolled, and then continuously annealed in a continuous hot-dip galvanizing line. ΔBH of hot-dip galvanized steel sheet prepared so that 2% BH at a baking temperature of 170 ° C. is in the range of 40 to 45 MPa, and the content of boron and oxygen content The result plotted on the matrix is shown.

[0033]

[Table 2]

The 2% BH was measured in the same manner as in Example 1. As is clear from FIG. 2, the boron content was set to 0.0003 wt.% Or more, and the oxygen content was set to 0.0005 wt.
Only when controlled within the range of ~ 0.003 wt.% Can 2% BH at 100 ° C baking temperature be comparable to 2% BH at 170 ° C baking temperature.

That is, when the oxygen content is greater than 0.003 wt.%, ΔBH is greater than 5 MPa, and when the oxygen content is less than 0.0005 wt.%, ΔBH is greater than 5 MPa. On the other hand, when the boron content is less than 0.0003, ΔBH
Is larger than 5 MPa, and it is not possible to obtain excellent low temperature bake hardenability. Further, as is apparent from FIG.
When the oxygen content is in the range of 0.0015 to 0.003 wt.%, Unless the boron content is increased with the increase in the oxygen content, excellent low-temperature baking hardenability cannot be secured, and the boron content is It must be greater than or equal to the value calculated by 0.5 × O −0.0005.

Example 3 Cold rolled steel sheets and surface-treated steel sheets were prepared using steels having the chemical components shown in Table 3. Table 4 shows the annealing temperature, 2% BH and the tensile properties of these steel sheets at the baking temperatures of 170 ° C and 100 ° C.

[0037]

[Table 3]

[0038]

[Table 4]

As is apparent from Table 4, in the steel sheets No. 1 to 13 within the scope of the present invention, the difference between 2% BH at a baking temperature of 100 ° C. and 2% BH at a baking temperature of 170 ° C. is 5 MPa or less. It is excellent in low-temperature bake hardenability, and the low-temperature bake hardenability is not impaired even as a surface-treated steel sheet.

On the other hand, in the comparative steel sheet No. 14 out of the range of the present invention, since the Ti content is out of the range of the present invention, the 2% BH at the baking temperature of 170 ° C. is low. In comparative steel sheet No. 15 out of the scope of the present invention, Ti
Since the content is small outside the range of the present invention, the amount of solid-dissolved boron is small, and 2% BH at a baking temperature of 100 ° C.
Is low. In comparative steel sheets No. 16 and No. 23, the oxygen content was small outside the scope of the present invention, and in comparative steel sheet No. 18, the oxygen content was large outside the scope of the present invention. Sufficient low temperature bake hardenability was not obtained.

In Comparative steel sheets No. 17 and No. 22,
Because the boron content is small outside the scope of the present invention,
Low temperature bake hardenability was low. In Comparative Steel Sheet No. 19, the low-temperature bake hardenability was low because both the oxygen content and the boron content were outside the scope of the present invention. In comparative steel sheets Nos. 20, 21 and 24, the boron content was
Low temperature bake hardenability was low because it was less than the value calculated by 0.5 × O −0.0005. From the above,
In a steel sheet having a chemical composition outside the scope of the present invention,
It is clear that sufficient low temperature bake hardenability cannot be obtained.

[0042]

As described above, according to the present invention,
A cold rolled steel sheet excellent in low-temperature bake hardenability can be obtained without deterioration of the material, and an industrially excellent effect can be obtained.

[Brief description of the drawings]

Figure 1: 2% BH at 170 ° C and 100 ° C baking temperatures
Fig. 3 is a graph showing the relationship between and oxygen content.

FIG. 2 is a diagram plotting ΔBH of a steel sheet on a matrix of boron content and oxygen content.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tomoyoshi Ohkita 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-56-93825 (JP, A) 3-257125 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) C22C 38/00 301 C22C 38/14

Claims (1)

(57) [Claims] 1. Carbon (C): 0.001 to 0.005 wt.%, Silicon (Si): 0.3 wt.% Or less, manganese (Mn): 0.1 to 2 wt.%, Phosphorus (P): 0.1 wt.% Or less , Sulfur (S): 0.015 wt.% Or less, Soluble aluminum (Sol. Al): 0.005 to 0.1 wt.%, Nitrogen (N): 0.004 wt.% Or less, Titanium (Ti): (48/14) N (48/14) N + (48/32) S + 16
C, boron (B): 0.0003 to 0.0015 wt.%, Oxygen (O): 0.0005 to 0.003 wt.%, But B ≧ 0.5 × O−0.0005, balance: iron (Fe) and unavoidable impurities Cold rolled steel sheet with excellent low temperature bake hardenability.