JPS6047886B2 - Manufacturing method of high-strength thin steel plate for processing by continuous annealing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides high-strength cold-rolled steel sheets with a tensile strength of 40 to 60 kg, which have a low yield ratio, and high-strength steel sheets for processing, which are used as surface-treated steel sheets. This invention relates to a manufacturing method using continuous annealing of thin steel sheets.

Recently, 9L-class high-strength cold-rolled steel sheets have been rapidly adopted for automobile steel sheets for the purpose of protecting the safety of passengers and reducing fuel consumption. In particular, there is an increasing need to use hot-dip galvanized high-strength surface-treated steel sheets to improve the lifespan and durability of vehicle bodies. Conventional high-strength cold-rolled steel sheets and surface-treated steel sheets achieve high strength through solid solution strengthening and precipitation strengthening methods, but this inevitably results in a higher descent point and an increased amount of springback during press forming. However, it has the disadvantage of poor shape fixability and the occurrence of wrinkle defects known as surface distortion. An example of a high-strength cold-rolled steel sheet that has been proposed in the past is JP-A-54-83924.

This involves manufacturing high-strength cold-rolled steel sheets by containing a relatively large amount of Si (0.7 to 1.5%) to increase strength, and by including B to control the cooling rate after annealing. However, the cold-rolled steel sheet obtained by this manufacturing method has a very high descending point of 50 km or more, as shown in the examples, and is problematic as a steel sheet for press working. For these reasons, the present inventors have proposed that by adding an appropriate amount of B to the C-Mn system, a high-strength cold-rolled steel plate with a low descending point while having high strength as an alternative to conventional high-strength steel sheets. The present inventors have already confirmed that a steel plate can be obtained and have previously filed an application.

That is, it involves adding an appropriate amount of B to the C-Mn system, controlling the existence form of B to a solid solution state, and reducing the solid solution B content to 0.0.
003 to 0.0070% and annealed in the α+γ temperature range, it exhibits a composite structure and exhibits high strength at a low depression point, which significantly improves the drawbacks of conventional high-strength cold rolled steel sheets. can.

In the above C-Mn solid solution B system, the cooling rate is certainly 10~
Even by continuous annealing at 15°C Isec, a high-strength cold-rolled copper plate with a composite structure with a low drop point and a tensile strength of 40 to 50 kg can be manufactured.
It is sometimes difficult to stably produce kilo-grade, high-strength cold-rolled steel sheets with a low drop-off ratio.

This tendency is especially seen when the cooling rate after continuous annealing is slow. Furthermore, when applied to hot-dip galvanized steel sheets, it has the disadvantage that it is somewhat difficult to form a composite structure. In particular, the reason why it is difficult to form a composite structure in hot-dip galvanized steel sheets is that hot-dip galvanized copper sheets are most commonly manufactured using a continuous hot-dip galvanizing line using an in-line annealing method, such as the Sendzimer method. This in-line annealing method (a) has a particularly short soaking time, which is disadvantageous for concentrating C and Mn necessary for forming a composite structure; The speed is usually 1
Since it is very slow at ~80C1 sec, austenite transforms into ferrite and pearlite, making it difficult to obtain the martensitic structure necessary for lowering the drop state ratio.

(c) By performing the hot-dip plating treatment at approximately 450°C within 1 to 2 degrees, even the generated martensite is tempered, the tensile strength decreases, the descent point becomes high, and the descent ratio increases. It rises, and elongation of the descent point also occurs. In view of the above-mentioned current situation, the inventors of the present invention have found that even when the cooling rate is fast or slow, continuous annealing or hot-dip galvanizing lines can easily form a complex structure, high-strength steel sheets are more stable than low-falling steel sheets. As a result of studying methods that can produce C-Mn-B based. In terms of ingredients, B<! :． It was clarified that this objective could be achieved by adjusting the components in consideration of the interaction between the three elements, N and C, and by combining hot rolling conditions and annealing conditions.

That is, the gist of the present invention is that C: 0.02 to 0.20%, Si:
0.8% or less, Mn: 0.8-2.0%, acid soluble A1
(hereinafter referred to as SOIN): 0.005 to 0.060%,
B: B/C is 0.04 or more, and B-0.7N is 0.0003% or more and 0.0050% or less, N: 0.00
When hot rolling steel consisting of 60% or less, the balance being iron and unavoidable impurities, it is finished at 3 or more points of Ar, and then 30%
Cooled at a cooling rate of ~150'Clsec, rolled at 680°C or less, then cold-rolled at a reduction rate of 65% or more, then heated at 720-800°C for ~5 minutes after soaking at 1°C Ise.
A method for producing a high-strength thin steel sheet for processing by continuous annealing in which the process is performed by cooling at a temperature of c or more.

The present invention will be explained in detail below.

C is required to be 0.02% or more in order to obtain a martensitic structure in the cooling process from the α+γ two-phase temperature range.

On the other hand, if the content is too large, workability and weldability will deteriorate significantly, so the upper limit is set at 0.20%. Preferably 0
．． 04 to 0.10%. Si is a preferable element that causes the solid solution C in ferrite to be discharged to the grain boundaries and has an auxiliary effect on the formation of a composite structure. Further, since it is a favorable element for increasing strength, it is included up to 0.8% in the present invention. 0.8%
When applied to a surface-treated steel sheet, such as a hot-dip galvanized steel sheet, it causes plating defects, and when surface painting a cold-rolled steel sheet, corrosion resistance can be significantly improved by cationic electrodeposition, but the Si content is still 0.8%. If it becomes too thick, a problem will arise in corrosion resistance. Preferably it is 0.6% or less. Mn
is an element that stabilizes the γ phase and facilitates the formation of a composite structure during the cold rolling process.
% or more is required.

On the other hand, if too much Mn is present, steel manufacturing operations become difficult and weldability deteriorates, and in the case of hot-dip galvanized steel sheets, plating performance deteriorates, so the upper limit of Mn is set at 2.0%. A1 is to fully demonstrate the effect of B, which will be described later.
It is an element necessary as a deoxidizing agent, and the acid-soluble N is 0.0
0.5% or more is required.

On the other hand, too much content will cause deterioration of surface properties and deterioration of workability due to inclusions, so the upper limit is set at 0.060%. B is an important element in the present invention. The forms of B present in steel are considered to be nitrides, carbides, oxides, and solid solution B, but in order to obtain a composite structure high strength cold rolled steel sheet with a low drop ratio, which is the object of the present invention, Among the above-mentioned forms of existence of B, it is important to make it exist as solid solution B. B easily reacts with N in the γ region temperature, forming B nitride (BN
) is unavoidable. Therefore, the solid solution B is the amount obtained by subtracting the amount of B that reacts with N from the total amount of B, that is, B - 0.7 × N
In order to achieve the object of the present invention, B
-0.7×N is required to be 0.0003% or more; on the other hand, if it is too much, there is a risk of surface cracking of the slag, so B
The upper limit of −0.7×N is set to 0.0070%. In order to control B in a solid solution state, first, as mentioned above, B is added after sufficiently deoxidizing the steel with A1 during melting.
It is necessary to prevent the formation of oxides.

Next, in order to suppress the formation of B carbides as much as possible, the temperature at the inlet of the hot finishing rolling mill should be set to 950°C or higher, preferably 1°C.
It is preferable to set the finishing temperature to 000°C or higher, the finishing outlet temperature to Ar'3 points or higher, and the winding temperature to 680'C or lower.

On the other hand, it is difficult to completely eliminate the formation of B carbides, and various studies are needed to ensure the amount of solid solution B even when the C content is relatively high and to create a high-strength copper plate with a composite structure with a low precipitation ratio. As a result, as shown in Figure 1, from the interaction of B (5C)
It has been found that the weight percent ratio B/C of C and C can be increased to 0.04 J:).

When B/C is below 0.0, the drop ratio becomes large or the drop point elongates, which is not preferable. In the figure, the mark 0 indicates the drop ratio <0.6 and the drop point elongation <0.5%, and the x mark indicates outside the above range. The base components of the test material in FIG. 1 are C: 0.05-0.18%, Mn: 1.60-1.
65%, N: 0.0020-0.0030%, B: 0.
The continuous annealing conditions after cooling were soaking at 760'C for 90 seconds and cooling at 5C Isec.

From the above, B! 0.0003% as 1B-0.7×N
Satisfying 0.0050% and B/C of 0.04 or more are essential conditions for achieving the object of the present invention.

B has generally been used as a shape element to improve hardenability.

And, its use has been made in such a way as to define the total amount of B. In contrast, the present inventors have found that in order to fully demonstrate the essential effects of B, it is necessary to consider the amount of B based on the balance between the amount of N and the amount of C, rather than regulating the total amount of B, and the need to consider the amount of B based on the balance between the amount of N and the amount of C. This is a new discovery of the need to regulate the spreading conditions within a specific range, and the technical content is greatly different from the conventional simple addition of B.

Since N generates BN by reaction with B, which is not preferable for controlling solid solution B, the upper limit is set to 0.0060%. Preferably it is 0.0040% or less. S as an unavoidable impurity is unfavorable for press workability, and is preferably 0.015% or less. On the other hand, the effect of the present invention is not lost even if P is contained as a solid solution strengthening element in an amount of 0.08% or less in order to increase strength, but from the viewpoint of press workability, a smaller amount is preferable. In addition to the above elements, Cr. ．． One or more elements that facilitate the production of martensite such as MO 0.2 to 1
．． Adding 0% is effective.

In addition, in order to improve elongation and flangeability, Ca. ．． R
E.M. ．． Addition of an element such as Zr that controls the form of sulfide is also effective. Next, the reason for limiting the manufacturing conditions will be described.

Steel within the above-mentioned composition range is melted in an electric furnace, converter, etc., and is made into a slab by ingot formation or continuous casting.

Next, it is hot rolled, with a finish exit temperature of Ar 3 or higher, then cooled at a cooling rate of 30 to 150°C for 1 second, and rolled at a temperature of 680°C or lower. If the finishing temperature is less than 3 points Ar, it is difficult to obtain a composite structure, and if the cooling rate after hot rolling is too slow, a composite structure that provides high strength with a low drop ratio cannot be obtained, so the lower limit is set at 30°C for 1 sec. do.
On the other hand, if the cooling rate is too fast, a bainitic quenched structure and axial ferrite will be formed in the hot rolled sheet.
Since the descending ratio becomes high and the ductility is significantly deteriorated, the upper limit is set to 1500 C1 sec. Also, the winding temperature is 680℃
If it exceeds this amount, a large amount of B carbide will be produced, making it impossible to achieve the object of the present invention. The hot-rolled coil is then pickled and cold-rolled, but in order to form a composite structure even at a slow cooling rate such as in a continuous hot-dip galvanizing line, precipitates such as carbides generated during hot-rolling must be cooled. It is necessary to finely crush C by rolling and promote re-solution of C during heating during annealing to facilitate concentration of C at grain boundaries at temperatures in the α-10γ range. Therefore, the cold reduction rate is set to 65% or more. After cold rolling, the cold rolled coil is soaked at an annealing temperature of 720 to 850° C. for 20 seconds to 5 minutes, and then continuously annealed at a cooling rate of 17 C'Sec or more.

If the annealing temperature is less than 720°C, it is not possible to obtain a two-phase state of α + γ, so the lower limit is set at 720°C.
When the temperature exceeds 50° C., the volume fraction of the α phase decreases, and even if the structure has two phases, the drop point increases, making it impossible to obtain a low drop ratio. If the soaking time is less than 20 minutes, the formation of a two-phase structure of α+γ is insufficient, and if it exceeds 5 minutes, the γ phase is coarsely formed and the ductility deteriorates. The preferred annealing range is 730-7800C and 6
0-12 2 is good. Next, regarding the cooling rate, if it is within the limited range of the ingredients and manufacturing conditions mentioned above,
A martensitic structure is obtained at a cooling rate of 1 sec or more.

The faster the cooling rate, the greater the amount of martensite produced and the higher the strength. However, if the material is cooled too quickly, a large amount of martensite is generated along grain boundaries, which becomes a stress concentration source during plastic deformation and deteriorates ductility. In order to achieve a good balance between strength and ductility, there is an appropriate cooling rate range, and for the steel of the present invention, a cooling rate of 10 to 5000 C1 sec is preferable. Next, examples of the present invention will be shown.

Example 1 Steel having the components shown in Table 1 was melted and subjected to hot rolling, cold rolling, and continuous annealing under the conditions shown in Table 1.

The mechanical properties after annealing are shown in Table 2.

From this, it can be seen that those that satisfy the ingredients and post-processing conditions within the scope of the present invention have high tensile strength but have a low descent point.
The descending ratio is 0.6 or less, and there is no occurrence of descending point or elongation, making it a cold-rolled steel sheet with excellent workability. Example 2 A cold-rolled steel plate made with the steel composition and manufacturing conditions shown in Table 3 was heated from room temperature to 600'C for 25 seconds, then heated to 770°C for 25 seconds. Immediately heat 6 times to 680°C (cooling rate 1.5°C 1 sec), 680°C
Cool from 450°C to 208°C (11 speed C1sec), maintain at 4500°C for 6 seconds,
Annealing was carried out to simulate the thermal cycle of a Sendzimir type galvanizing line, which involves air cooling to °C and then water rinsing.

The mechanical properties obtained are shown in Table 4. Steel plates B and C manufactured according to the present invention have a low i-drop ratio even in the annealing cycle with extremely slow cooling rate as described above, and exhibit characteristics unique to a composite structure with no drop point or elongation.

As explained above, according to the method of the present invention, in a continuous annealing method with a very slow cooling rate, such as in continuous hot-dip galvanizing equipment, a composite structure high-strength thin steel sheet with a low drop ratio can be produced with a relatively low alloy. It can be stably produced using the components of the system, and its industrial significance is great.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the influence of B/C on the drop ratio in the relationship between C content and B content.

Claims (1)

[Claims] 1. C: 0.02 to 0.20%, Si: 0.8% by weight
% or less, Mn: 0.8 to 2.0%, acid-soluble Al: 0.005 to 0.060%, B: B/C is 0.04 or more, and 0.0003 to B-0.7N.
0.0050%, N: 0.0060% or less, the balance consisting of iron and unavoidable impurities, finish rolling at Ar_3 points or higher during hot rolling, and then
Cooling at a cooling rate of 0 to 150°C/sec, rolling at 680°C or less, cold rolling at a reduction rate of 65% or more, and then
After soaking at 20℃ to 850℃ for 20 seconds to 5 minutes, 1℃/
A method for producing a high-strength thin steel plate for processing by continuous annealing, characterized by cooling at a rate of sec or more.