JPH0693329A - Production of cold rolled steel sheet excellent in baking hardenability - Google Patents

Abstract

PURPOSE:To improve the baking hardenability of a steel sheet, at the time of subjecting a cold rolled steel sheet of low carbon or medium carbon to decarburizing annealing by open coil annealing equipment, by executing the decarburizing annealing for specified time in accordance with the C content, thickness, surface are or the like of the cold rolled steel sheet. CONSTITUTION:At the time of subjecting the cold rolled steel sheet having a compsn. contg., by weight, 0.03 to 0.30% C, <0.8% Si, <0.40% Mn, <0.05% P, <0.020% S, <0.008% N and 0.020 to 0.100% Al to decarburizing annealing by open coil annealing equipment, when the sheet thickness of the cold rolled steel sheet is defined as Gmm as well as the surface area as Smm<2> and the carbon content as solid solution in the steel sheet is regulated to specified extra low one by the decarburizing annealing, the annealing is executed for (t) time in the formula I in the case of S<=1.5X10<9>mm<2> and in the formula II in the case of S>1.5X10<9>mm<2>, by which the C in the steel sheet is removed into 50 to 200ppm to improve its baking hardenability. Furthermore, in the formulae I and II, the ranges of 0.40 to 7.00 C1, 0.70 to 1.00 C2, 0.10 to 0.50 C3, 0 to 0.30 C4, 0.80 to 2.00 C5 and 1.00 to 1.50 C6 are regulated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled steel sheet to which the necessary bake hardenability has been imparted by decarburizing annealing.

[0002]

2. Description of the Related Art Cold-rolled steel sheets used as automobile steel sheets are required to have so-called bake hardenability, in which the yield strength is increased by baking coating after forming into a predetermined shape such as a panel. The bake hardenability is determined by the content of solute carbon in ferrite, as shown in FIG. Then, in consideration of occurrence of stretcher strain due to recovery of elongation at yield in steel sheet products, 3 to 6 kgf / mm ²
Is said to have appropriate bake hardenability. In order to keep the bake hardenability within this range, it is necessary to adjust the amount of solute carbon to 4 to 11 ppm from the relationship shown in FIG.

The amount of solid solution carbon in ferrite is 4 to 11 pp.
As a method for maintaining the m level, there are a method using continuous annealing equipment and a method using open coil annealing equipment. Since decarburization cannot be performed by continuous annealing, decarburization annealing using open coil annealing equipment is used. In the case of continuous annealing equipment, an appropriate amount of carbide forming elements such as Ti and Nb is added to an ultra-low carbon steel having a C content reduced to 20 to 50 ppm in the steelmaking stage, and C is stable until near the recrystallization temperature.
Carbides such as C and NbC are set. Then, the annealing temperature at the time of continuous annealing is set to the recrystallization temperature or higher, the carbides such as TiC and NbC are dissolved (decomposed), and then the dissolved C and T
To prevent carbides forming elements such as i and Nb from recombining, 20
By rapidly cooling at a cooling rate of not less than C / sec, the amount of dissolved C is adjusted to the range of 4 to 11 ppm. For example, in Japanese Unexamined Patent Publication No. 2-111841, C, N,
S, Ti, Nb, etc. are strictly controlled to produce an ultra-low carbon steel, and the added Ti, Nb, etc. fix excess C existing in the steel as a carbide, which is a solid solution effective for bake hardenability. It is introduced to maintain the carbon content at the target value.

In the case of the open coil annealing equipment, the cold rolled steel sheet of ultra-low carbon steel whose C content is reduced to 50 to 200 ppm in the steel making stage is heated to a recrystallization temperature or higher in the open coil annealing equipment, and then 60 to 100 ° C. It is cooling at a cooling rate of / hour. In this method, since the amount of C is as low as 50 to 200 ppm, there are few carbides as precipitation sites, and carbides such as Fe ₃ C do not sufficiently precipitate during the cooling process. Therefore, the solid solution carbon can be left in an appropriate amount of 4 to 11 ppm.

[0005]

In the method of imparting the bake hardenability to the cold-rolled steel sheet which has been made into an extremely low carbon steel at the steelmaking stage by annealing in the continuous annealing equipment or the open coil annealing equipment, it takes time to decarburize at the steelmaking step. It causes a cost increase. In particular, when a carbide forming element such as Ti or Nb is added to an ultra-low carbon steel having a carbon content of 20 to 50 ppm to control the amount of solid solution carbon, Ti, Nb or the like is an element other than C, N,
Since it also binds to S and the like, in order to control the amount of solute carbon in the target range of 4 to 11 ppm, C, Ti, Nb,
It is necessary to strictly control the content of N, S, etc.,
Difficult operations are forced. The present invention has been devised to solve such a problem, in which low carbon or medium carbon cold-rolled steel sheet is subjected to decarburization annealing with an open coil annealing equipment to adjust the carbon content in the steel to 50 to 200 ppm. By doing
The purpose is to impart the necessary bake hardenability to a cold rolled steel sheet regardless of the carbon content in the steel before annealing.

[0006]

In order to achieve the object, the manufacturing method of the present invention has C: 0.03 to 0.30% by weight, Si: 0.80% by weight or less, Mn: 0.40% by weight. %
Below, P: 0.05% by weight or less, S: 0.020% by weight
Hereinafter, N: 0.008% by weight or less, Al: 0.020 to
For a cold-rolled steel sheet containing 0.100% by weight, when the decarburization time during annealing is t (hours), the thickness of the steel sheet is G (mm), and the surface area of the steel sheet is S (mm ² ), S When ≦ 1.5 × 10 ⁹ mm ² , t = C ₁ × (G × C ₂ + C ₃ ) ² + C ₄ (1) When S> 1.5 × 10 ⁹ mm ² , t = Substituting G and C _{1 to} C ₆ into the formula (1) or formula (2) represented by C ₁ × (G × C ₂ + C ₃ ) ² + C ₅ / G−C ₆ ... (2) It is characterized in that decarburization annealing is performed so that carbon in steel becomes 50 to 200 ppm by using an open coil annealing equipment at the decarburization time t obtained as described above.

Here, the coefficients and constants C _{1 to} C ₆ are values that have been grasped in advance from theory and many actual results. Coefficient C
₁ and the constant C ₃ are values determined according to the carbon concentration in the steel before decarburization annealing, and C ₁ is in the range of 0.40 to 7.00,
C ₃ is in the range of 0.10 to 0.50. The coefficient C ₂ , the constant C _4, and the constant C ₆ are values determined according to the carbon concentration in the target range steel after decarburization annealing, and C ₂ is 0.70 to 1.0.
0, C ₄ is in the range of 0 to 0.30, C ₆ is 1.
It is in the range of 00 to 1.50. The constant C ₅ is a value determined according to the decarburization annealing weight and is in the range of 0.80 to 2.00.

[0008]

[Operation] According to the study by the present inventors, it was found that the bake hardenability of the cold rolled steel sheet annealed by the open coil annealing equipment has the relationship shown in Fig. 2 with the carbon content. . In the case of FIG. 2, Si: 0.01 wt% or less, Mn:
0.10 to 0.20% by weight, P: 0.010 to 0.02
A 0% by weight cold rolled steel sheet was annealed at a temperature of 680 to 710 ° C. in an open coil annealing equipment, and a cooling rate was 80 to 1
Cooled at 00 ° C / hr. The bake hardenability in FIG. 2 is represented by the difference in yield stress in the tensile test before and after the heat treatment of holding at 170 ° C. for 20 minutes. This carbon content is
Unlike the measurement of the solid solution carbon content, it can be determined relatively easily. Then, the carbon content can be easily adjusted to the target value by changing the conditions such as the annealing time, the decarburizing temperature, the decarburizing atmosphere during the decarburizing annealing, and obtaining the target bake hardenability. Is possible. In contrast,
In order to obtain the target bake hardenability by adjusting the composition in the steelmaking stage, as described above, in addition to the C content, T
The contents of i, Nb, S, N, etc. must be strictly controlled. As a result, it is difficult to operate.

50 to 20 with which a predetermined bake hardenability can be obtained
Reducing the carbon content to 0 ppm is fully possible by open coil annealing. The carbon content can be adjusted to any value within the range of 50 to 200 pm by changing the decarburizing conditions such as decarburizing time, decarburizing temperature and decarburizing atmosphere. The steel sheet after annealing whose carbon content is adjusted to this range changes the bake hardenability in relation to the carbon content. The decarburization annealing is performed by using an open coil annealing equipment and heating the cold rolled steel sheet to a high temperature of 600 ° C or higher. As the decarburizing atmosphere, AX gas mixed with water vapor is used. The amount of water vapor mixed is preferably adjusted so that the dew point of the atmosphere is maintained in the range of 30 to 60 ° C. in order to smoothly carry out the decarburization reaction. Dew point is 30 ℃
In a lower dry atmosphere, the decarburization reaction rate of C + H ₂ O → CO + H ₂ is small, and long-time high temperature heating is required. Further, when the dew point exceeds 60 ° C., the H ₂ O / H ₂ ratio is high and an oxidizing atmosphere is created, so that the steel sheet surface is easily oxidized.

The decarburization temperature is set to a decarburization reaction initiation temperature of 600 ° C or higher, preferably 700 to 800 ° C. When the decarburization temperature is lower than 600 ° C., the decarburization reaction does not proceed or long-time decarburization is required. Also, 800 ℃
If the decarburization temperature exceeds, it becomes difficult to predict the progress of the decarburization reaction, and it becomes difficult to control the carbon content with high accuracy. The time for decarburization to a carbon content of 50 to 200 ppm depends on the initial carbon amount in the steel sheet before decarburization,
It depends on the plate thickness, surface area, weight, etc. For example, a cold-rolled steel sheet having an initial carbon content of 0.03% by weight, a sheet thickness of 0.7 mm, and a coil weight of 7 tons is degassed while introducing a decarburizing annealing atmosphere gas with a dew point of 50 ° C. at a supply rate of 68 Nm ³ / hour. When decarburizing and annealing under the condition that the target carbon content after charcoal annealing is set to 200 ppm, the steel sheet surface area is smaller than 1.5 × 10 ⁹ mm ² , so the decarburizing time t is obtained according to the following equation. t = C ₁ × (G × C ₂ + C ₃ ) ² + C ₄ = 0.43 × (0.7 × 0.77 + 0.49) ² = 0.46 (hours) ...

The initial carbon content is 0.30% by weight,
When the coil weight is changed to 15 tons and the target carbon content is changed to 50 ppm, the steel sheet surface area becomes larger than 1.5 × 10 ⁹ mm ² , so the decarburization annealing time t is obtained according to the following equation. _{t = C 1 × (G ×} C 2 + C 3) 2 + C 5 / G-C 6 = 6.62 × (0.7 × 1.0 + 0.11) 2 + 1.87 / 0.7-1.11 = 5.90 (hours)

Even if decarburization annealing is carried out for any time period obtained by any of the formulas or, the target carbon content is 200 pp.
It is possible to decarburize and anneal up to m, 50 ppm. The decarburized and annealed steel sheet exhibits excellent workability due to the reduced carbon content. For example, when press-forming a steel sheet after decarburization annealing, it is well suited to the press die,
Since there is no springback when removed from the press mold, the product has good shape characteristics. High yield strength can be obtained in a state where baking is applied after press molding. Moreover, since it has a predetermined bake hardenability provided by decarburization annealing regardless of the type of steel, it is a highly versatile manufacturing method.

In the cold-rolled steel sheet used in the present invention, it is not necessary to strictly control the contents of C, Ti, Nb, N, S, etc. in the steelmaking stage, and decarburization annealing is performed in the same manner as in the conventional process. It can be performed in a coil annealing facility. Therefore, the manufacturing cost can be significantly reduced as compared with the conventional one. C is an element that can be adjusted to a range of 50 to 200 ppm by decarburization annealing. However, if the initial carbon content in the steel sheet before decarburization is too high, long-term decarburization is required, and further, the long-term decarburization oxidizes selective oxidizing elements such as Si and Mn, and oxides are concentrated. The formed surface layer is easily formed. On the other hand, the use of a steel sheet having a low C content as a starting material reduces the merit of decarburization annealing according to the present invention, and requires long-term steelmaking decarburization. At this point, the C content of the cold rolled steel sheet before decarburization annealing is 0.03 to
It is set to 0.30% by weight.

Si is an element effective in improving the bake hardenability and the strength of the steel. But,
Selective oxidation is likely to occur during decarburization annealing, which causes discoloration of the steel sheet surface due to the formation of an oxide film. Therefore, the upper limit of the Si content is defined as 0.80% by weight. Mn is an element effective in improving the strength of steel, but tends to decrease the bake hardenability of low carbon steel as the amount of addition increases. Further, like Si, it causes discoloration of the steel sheet surface due to the formation of an oxide film. Therefore, the upper limit of the Mn content is defined as 0.40% by weight.

P is an effective element for improving the strength and bake hardenability of steel. However, as the P content increases, properties such as spot weldability and secondary work crack resistance tend to deteriorate. Therefore, the P content is 0.0
It was regulated to 5% by weight or less. S is an impurity element contained in steel and has an effective effect on bake hardenability. However, when the S content becomes large, hot embrittlement occurs,
The upper limit of the S content was set to 0.020% by weight. Al
Is an element used as a deoxidizing agent, and 0.020% by weight or more is necessary to perform sufficient deoxidizing. However, since a large amount of Al content adversely affects the surface properties of steel, its upper limit was set to 0.100% by weight. N is
It has a high equilibrium solubility in steel and effectively acts to improve bake hardenability. However, in a steel in which a large amount of N is dissolved, normal temperature aging is likely to occur, which causes defects such as reduction in ductility and stretcher strain. Therefore, the N content is 0.0
The amount is regulated to 08% by weight or less, and the whole amount is fixed as an AlN precipitate.

[0016]

【Example】

Example 1: Decarburization time when decarburizing and annealing a cold-rolled steel sheet having a plate thickness of 0.72 mm with different initial carbon content before decarburizing and annealing to different target carbon contents, in steel after decarburizing Table 1 shows the actual results of the carbon content, the coefficients in the equations (1) and (2), and the values of the constants C _{1 to} C ₆ . In the decarburization annealing, the decarburization temperature was 700 ° C., the gas composition was N ₂ : 22% by volume, H ₂ : 67% by volume and H ₂ O: 11% by volume at a supply amount of 68 Nm ³ / hour. It was carried out under the conditions of supply.

[0017]

[Table 1]

The coefficients and constants C _{1 to} C ₆ are values that are known in advance depending on the size of the steel material to be decarburized and decarburizing conditions. Substituting these coefficients and constants C _{1 to} C ₆ into the equations (1) and (2) to obtain the decarburization time t, and when performing decarburization annealing on the cold-rolled steel sheet for this decarburization time t, Table 1 As shown in the figure, it can be seen that the actual carbon content value matches the target carbon content with extremely high accuracy. For cold-rolled steel sheets whose carbon content in steel is controlled by decarburization annealing in an open coil annealing facility and cold-rolled steel sheets that have been annealed without decarburization, the relationship between the content of each component and bake hardenability is shown. 2 shows. As the decarburization annealing and the annealing not accompanied by the decarburization reaction, the annealing temperature is set to 700 ° C. and the cooling rate is 60 to 80.
The condition of cooling the annealed material at ° C / sec was adopted. The bake hardenability was represented by the difference in yield stress in the tensile test values before and after the heat treatment of holding at 170 ° C. for 20 minutes. As is clear from Table 2, when the carbon content of the cold-rolled steel sheet is decarburized and annealed by the open coil equipment to a range of 50 to 200 ppm, good bake hardenability is obtained.

[0019]

[Table 2]

[0020]

As described above, according to the present invention, regardless of the initial carbon content in the cold-rolled steel sheet before annealing, the bake hardenability that requires the carbon content in the steel by decarburization annealing is It can be controlled to the value obtained. Also,
The actual value of carbon content after decarburization annealing matches the target carbon content with high accuracy. Therefore, it is not necessary to strictly adjust the components in the steelmaking stage, and a product having excellent bake hardenability can be obtained at a low manufacturing cost.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a conventionally known amount of solute carbon and bake hardenability.

FIG. 2 is a graph showing the relationship between the carbon content and the bake hardenability of the annealed material found by the present inventors.

Claims (1)

[Claims] 1. C: 0.03 to 0.30% by weight, Si:
0.80 wt% or less, Mn: 0.40 wt% or less, P:
0.05% by weight or less, S: 0.020% by weight or less, N:
0.008% by weight or less, Al: 0.020 to 0.100
For a cold-rolled steel sheet containing wt.%, The decarburization time during annealing is t (hours), the thickness of the steel sheet is G (mm), and the surface area of the steel sheet is S.
(Mm ² ), when S ≦ 1.5 × 10 ⁹ mm ² , t = C ₁ × (G × C ₂ + C ₃ ) ² + C ₄ (1) S> 1.5 × In the case of 10 ⁹ mm ² , t = C ₁ × (G × C ₂ + C ₃ ) ² + C ₅ / G-C ₆ ... (2) In the decarburization time t obtained by substituting G and C _{1 to} C ₆ into the formula (1) or the formula (2), the carbon content in the steel becomes 50 to 200 ppm by using the open coil annealing equipment. A method for producing a cold-rolled steel sheet excellent in bake hardenability, which comprises performing decarburization annealing so that