JPH07216459A - Production of cold rolled steel sheet excellent in aging characteristic and workability - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel sheet excellent in aging characteristic and workability by increasing heating velocity at the time of subjecting an as-cold-rolled strip of low carbon steel with specific composition to uncoiling and to continuous annealing. CONSTITUTION:A steel, which has a composition consisting of, by weight, 0.01-0.05% C, 0.01-1.0% Si, 0.05-0.5% Mn, <=0.10% P, <=0.01%S, 0.005-0.1% Al, <=0.005% N, and the balance Fe and containing, if necessary, 0.0003-0.004% B, is continuously cast into a slab. After reheating or directly after casting, finish rolling is completed at a temp. not lower than the Ar3 transformation point. After coiling at 650-800 deg.C, acid pickling is done and then cold rolling is performed by the ordinary method. Subsequently, at the time of continuous annealing, heating rate is regulated to (100 to 3000) deg.C/sec and the cold rolled sheet is heated to 750-910 deg.C, and, without holding or after holding for <=30sec, cooling is done at >=70 deg.C/sec cooling rate, followed by temper rolling.

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 having excellent workability and aging property, and more particularly, to rapid and high temperature winding in hot rolling and rapid heating and holding in continuous annealing. The present invention relates to a method for producing a cold-rolled steel sheet having excellent aging and workability by heat treatment for a short time.

[0002]

2. Description of the Related Art The conventional heating method in continuous annealing is direct heating with a gas burner or radiant heating with a radiant tube. Therefore, the heating rate was about 10 ° C./s. Therefore, in the continuous annealing until now, it was inevitable to remelt the carbide agglomerated and coarsened in the hot rolled sheet during heating and soaking. Therefore, non-aging in low carbon steel has been achieved by efficiently performing precipitation of carbides in the grains by cooling after soaking and overaging. However, since the carbides precipitated in the grains are fine, n
The value had to be degraded.

On the other hand, application of electric heating to continuous annealing is also disclosed, for example, JP-A-2-166234 and JP-B-5-57333. In each case, a heating method by induction heating is adopted as an example, the former is to heat the high temperature region of the heating zone by induction heating, and the latter is a rapid heating to a temperature higher than the soaking temperature in the soaking section. It heats up. Therefore, this is completely different from the present invention in which rapid heating is performed in the entire heating portion. In particular, the former is completely different from the present invention in that it does not feature the material of the steel sheet to be obtained. In addition, there is no regulation on the heating rate in any of them, and it is considered that the heating rate is lower than the heating rate prescribed in the present invention inferred from the heating method and the examples. Especially in the latter case T
The object is extremely low carbon steel to which i is added, which is completely different from the present invention which is directed to low carbon steel.

[0004]

A conventional cold-rolled steel sheet having excellent workability made of a low carbon steel has an aging property by finely and uniformly precipitating carbides in the grains by cooling and overaging treatment in continuous annealing. The workability was improved, especially the deep drawability. However, fine precipitation within the grain of carbide is n
The value is lowered, and there is a problem in overhanging property. The present invention has been made for the purpose of providing a method for producing a cold-rolled steel sheet excellent in aging and workability by solving the problems in producing such low carbon steel in continuous annealing.

[0005]

Means for Solving the Problems As a result of intensive studies in view of the above circumstances, the inventors of the present invention rewinded the steel strip as cold rolled to accelerate the heating rate when performing continuous annealing, It has been found that the dissolution of carbides during annealing is suppressed, and as a result, the precipitation of carbides in the grains is reduced, and the present invention has been completed. The gist thereof is (1) C: 0.01 to 0. 05wt%, Si: 0.01
~ 1.0 wt%, Mn: 0.05-0.5 wt%, P:
0.10 wt% or less, S: 0.01 wt% or less, Al:
Steel consisting of 0.005 to 0.1 wt%, N: 0.005 wt% or less, the balance Fe and unavoidable impurity elements is made into a slab by continuous casting, and then reheated or immediately after casting, at a temperature of Ar ₃ transformation point or higher. After finishing hot rolling with 650
After winding in a temperature range of 800 ° C., pickling, and cold rolling in a usual method, continuous annealing is performed at a heating rate of 100 to 3000.
C./s, heating up to a temperature range of 750 to 910.degree. C., without holding after holding, or holding for 30 seconds or less, followed by cooling at a cooling rate of 70.degree. C./s or more, and further temper rolling. A method of manufacturing a cold rolled steel sheet having excellent workability.

(2) C: 0.01 to 0.05 wt%, S
i: 0.01 to 1.0 wt%, Mn: 0.05 to 0.5
wt%, P: 0.10 wt% or less, S: 0.01 wt%
Hereinafter, Al: 0.005 to 0.1 wt%, N: 0.00
5 wt% or less, B: 0.0003 to 0.004 wt%,
Steel made of the balance Fe and unavoidable impurity elements is continuously cast into a slab, and then reheated or immediately after casting, Ar.
Finish hot rolling at a temperature of ₃ transformation points or higher and 650-80
After winding in a temperature range of 0 ° C., pickling, and cold rolling by a usual method, continuous annealing is performed at a heating rate of 100 to 3000 ° C. /
s, heating to a temperature range of 750 to 910 ° C. without holding or holding for 30 seconds or less, cooling at a cooling rate of 70 ° C./s or more, and further temper rolling, and aging and workability. Of excellent cold rolled steel sheet.

(3) C: 0.01 to 0.05 wt%, S
i: 0.01 to 1.0 wt%, Mn: 0.05 to 0.5
wt%, P: 0.10 wt% or less, S: 0.01 wt%
Hereinafter, Al: 0.005 to 0.1 wt%, N: 0.00
Steel made up of 5 wt% or less and the balance Fe and unavoidable impurity elements is continuously cast into a slab, and immediately after reheating or casting, finishing hot rolling is completed at a temperature of Ar ₃ transformation point or higher, and 650 to 800 ° C. After winding in the temperature range, pickling and cold rolling in the usual way, continuous annealing at a heating rate of 100
To 3000 ° C./s and no heating after heating up to a temperature range of 750 to 910 ° C. or after holding for 30 seconds or less 70 ° C./s
A method for producing a cold-rolled steel sheet having excellent aging and workability, which comprises cooling at the above cooling rate, subsequently performing overaging treatment, and further temper rolling.

(4) C: 0.01 to 0.05 wt%, S
i: 0.01 to 1.0 wt%, Mn: 0.05 to 0.5
wt%, P: 0.10 wt% or less, S: 0.01 wt%
Hereinafter, Al: 0.005 to 0.1 wt%, N: 0.00
5 wt% or less, B: 0.0003 to 0.004 wt%,
Steel made of the balance Fe and unavoidable impurity elements is continuously cast into a slab, and then reheated or immediately after casting, Ar.
Finish hot rolling at a temperature of ₃ transformation points or higher and 650-80
After winding in a temperature range of 0 ° C., pickling, and cold rolling by a usual method, continuous annealing is performed at a heating rate of 100 to 3000 ° C. /
s, after heating to a temperature range of 750 to 910 ° C., without holding or after holding for 30 seconds or less, cooling at a cooling rate of 70 ° C./s or more, followed by overaging treatment, and further temper rolling. It is a method for producing a cold-rolled steel sheet which is characterized by excellent aging and workability.

[0009]

The present invention will be described in detail below with reference to the drawings. 1 and 2 show the experimental results leading to the establishment of the present invention. In this experiment, C: 0.015 wt%, Si: 0.0
1 wt%, Mn: 0.11 wt%, P: 0.01 wt
%, S: 0.01 wt%, Al: 0.035 wt%,
N: 0.0015 wt%, the balance Fe and the steel consisting of unavoidable impurity elements were used. This steel was hot-rolled by finishing at 900 ° C to obtain a 4 mm hot-rolled sheet, which was then cooled to 70
It was wound up at 0 ° C. After further pickling and cold rolling at 80%, a heat cycle as shown in FIG. 1 was performed, with the annealing temperature set to 800 ° C. and the heating rate varied in the range of 10 to 3000 ° C./sec. The material properties after aging treatment were investigated. The results are shown in FIG.

That is, FIG. 2 (A) shows the relationship between the heating rate and the aging index (Al: Aging Index),
FIG. 2B shows the relationship between the heating rate and the n value. According to this, it was found that by setting the heating rate to 100 ° C./s or more in continuous annealing, the aging index becomes 20 N / mm ² or less, and at the same time, the n value shows an overhanging property of 0.23 or more. It is presumed that this is because the cementite coarsened and agglomerated in the hot-rolled sheet stage has its remelting suppressed to some extent by increasing the heating rate during the annealing after cold rolling. That is, at a heating rate in normal continuous annealing of less than 100 ° C./s, cementite dissolves and finely precipitates in the grains due to overaging treatment, so the aging index is reduced, but the n value is at most 0.2. Stay in the degree.

First, the reasons for limiting the chemical components in the present invention will be described. C must be 0.05 wt% or less in order to secure aging property and workability. However, if it is less than 0.01 wt%, it becomes difficult to reduce the solid solution C without addition of the carbide forming element.
The lower limit is 0.01 wt%. Si is added to increase the strength of steel, but excessive addition deteriorates weldability. Further, in order to improve the adhesion of plating, it is preferable that the addition amount be small, and the upper limit was set to 1.0 wt%. However, if the amount is less than 0.01 wt%, the cost will increase significantly, so 0.01 wt% is the lower limit.

Mn is also effective for increasing the strength of steel, but excessive addition thereof may cause deterioration of ductility (El) and deep drawability (r value) due to hardening of steel. Therefore, 0.5 wt%
Was set as the upper limit. However, if it is less than 0.05 wt%, hot brittle cracking is caused during hot rolling, so the content is made 0.05 wt% or more. P is an element having a larger solid solution strengthening ability than Si and Mn, and is an element with less deterioration in ductility and deep drawability due to addition, so it is important for increasing the strength while securing the formability. It is an element. In the present invention as well, it is added for the purpose of increasing the strength, but excessive addition causes deterioration of secondary workability due to segregation of P at grain boundaries, so the upper limit was made 0.10 wt%.

If S exceeds 0.01 wt%, not only the ductility deteriorates but also hot cracking occurs, so 0.01 wt% is added.
% Or less. Al is 0.005w for deoxidizing steel
Although t% or more is required, an excessive addition increases cost and leaves inclusions in the steel, so the upper limit is made 0.1 wt%. Similar to C, N is 0.005 wt% or less in order to secure aging property and workability.

B is added if necessary, but especially N
It is added when the purpose of fixing is. The addition of 0.004 wt% or more hardens the steel and deteriorates the workability, so the upper limit is this. Further, if less than 0.0003 wt%, there is no effect, so 0.0003 wt% is the lower limit. Although not specified in the present invention, Ca, Zr, Ce
You may add rare earth elements, such as.

Next, the manufacturing method according to the present invention will be described. The steel having the above-mentioned chemical composition is obtained as a slab by ordinary continuous casting, but it may be produced by a thin slab continuous casting method. Then, hot rolling is performed with or without reheating. However, when the final hot rolling is performed at a temperature lower than the Ar ₃ transformation point, the crystal grains of the hot rolled sheet become coarse, so the r value after cold rolling / annealing And deteriorates ductility. Therefore, the finishing temperature in hot rolling is set to the Ar ₃ transformation point or higher. After that, it is wound by a usual method, but it is necessary to raise the temperature to 650 ° C. or higher because it is necessary to agglomerate and coarsen the carbides in the hot rolling plate stage. However, if the temperature exceeds 800 ° C., the pickling property deteriorates, so this is made the upper limit. Here, the cooling rate up to the winding is not particularly specified in the present invention, and the reduction rate in cold rolling is within the range that is usually carried out.

Next, the heating rate in continuous annealing, which is the most important factor in the present invention, needs to be such that carbides agglomerated and coarsened in the hot-rolled sheet stage are not redissolved during annealing. At a heating rate of less than 100 ° C / s, the carbides are redissolved during the temperature rise and finely precipitate in the grains due to the subsequent cooling and overaging treatment, which lowers the n value in the tensile test and deteriorates the workability. To do. Meanwhile, 3000 ° C / s
If the heating rate is higher than 1, not only the control becomes difficult, but also the electric power required for heating becomes remarkably large, and furthermore, the interval between the paths for carrying out energization becomes too long, which is not preferable.

When the annealing temperature is lower than 750 ° C., grain growth is insufficient, so that sufficient ductility cannot be obtained and workability is deteriorated. On the other hand, if the temperature exceeds 910 ° C., annealing occurs in the austenite region, and the texture is randomized so that the r value is lowered and the deep drawability is poor. The retention during annealing should not be preferable. Even if held, if over 30 seconds, re-dissolution of carbide becomes remarkable, and if there is no overaging treatment,
Solid solution carbon remains, which causes deterioration of aging, and
When the overaging treatment is performed, fine carbides are precipitated in the grains and the n value is deteriorated. Therefore, the upper limit is 30 seconds.

The cooling rate after heating is preferably as high as possible to maintain the state where the carbide is precipitated, and the cooling rate is 70 ° C./s or more. If the cooling rate is slower than this, redissolution of carbides will occur during cooling, and solid solution carbon will remain unless there is overaging treatment.
Deteriorates value and ductility. Further, even when overaging treatment is performed, fine precipitation of carbides occurs in the grains, so that n
The value deteriorates. Here, the heating and cooling methods are not particularly specified, and are not particularly limited as long as the above specified conditions are satisfied. For example, the heating method is energization heating, and the cooling method is steam cooling or the like. It is effective to do in.

[0019]

【Example】

Example 1 C: 0.017 wt%, Si: 0.02 wt%, Mn:
0.11 wt%, P: 0.005 wt%, S: 0.00
75 wt%, Al: 0.042 wt%, N: 0.001
Steel was taken out of the converter from 5 wt%, the balance Fe and unavoidable impurities, and continuously cast into a slab. Hot rolling was carried out by heating at 1100 ° C., finishing temperature was 900 ° C., plate thickness: 4 mm, hot rolling was completed, and winding temperature was variously changed under the conditions shown in Table 1. After pickling, the reduction rate of 80% was 0.8 mm,
In continuous annealing, heating rate: 300 ° C./s, annealing temperature: 800 ° C., retention: 0 seconds by electric heating and water cooling,
Annealing was performed at a cooling rate of 100 ° C / s, followed by overaging treatment: 350 ° C x 10 minutes and temper rolling: 1%. After that, as a material evaluation, a No. 5 test piece described in JIS Z 2201 was processed, and a tensile test was performed according to the test method described in JIS Z 2241. The aging effect is 10
%, The load was removed, heat treatment was carried out at 100 ° C. for 60 minutes, and further the tensile test was performed. The results are summarized in Table 1. No. according to the scope of the invention. In 2, 3, 4, 5
It has an elongation of more than 45%, an r value of more than 1.7, an n value of more than 0.23 and an AI of less than 20 N / mm ² , and has excellent workability and aging property. No. In No. 1, the pickling temperature was out of the range of the present invention, and thus the pickling property was poor. on the other hand,
No. In Nos. 6 and 7, since the coiling temperature was out of the range of the present invention, the degree of coagulation of carbides in the hot rolled sheet was poor,
Remelting of carbide occurs not only during heating but also during cooling,
Carbides are finely precipitated in the grains in the subsequent overaging treatment. Therefore, the r value and the n value are low, and the AI is high.

[0020]

[Table 1]

Example 2 C: 0.021 wt%, Si: 0.01 wt%, Mn:
0.10 wt%, P: 0.008 wt%, S: 0.00
58 wt%, Al: 0.035 wt%, N: 0.002
Steel was taken out of the converter from 1 wt%, the balance Fe and unavoidable impurities, and made into a slab by continuous casting. After hot rolling at 1200 ° C., the finishing temperature was 910 ° C. and the plate thickness was 3 mm, hot rolling was completed, and the material was wound at 700 ° C. After pickling, the reduction rate was 73% to 0.8 mm, and continuous annealing was carried out under the conditions shown in Table 2 by electric heating and steam cooling. In addition, overaging treatment was implemented as needed. After temper rolling at 1%, the material was evaluated in the same manner as in Example 1. The results are summarized in Table 3. No. according to the scope of the invention. 14, 15, 1
7, 18, 20 and 22, elongation exceeding 45%,
It has an n value of more than 0.23 and an AI of less than 20 N / mm ² , and has excellent workability and aging property. The heating rate or cooling rate was low and the result was No. In Nos. 11, 12 and 19, the carbide was redissolved during heating and cooling, and the carbide was finely precipitated in the grains after cooling and overaging treatment, so that the AI was 2
It is less than 0 N / mm ² , but the n value is low. No. In No. 13, since the annealing temperature was low and was off, grain growth was insufficient and ductility was low. The annealing temperature was too high and the number was off. In No. 6, since the Ac ₃ point was exceeded, the structure became nonuniform and the n value was low. On the other hand, the holding time deviates from the range of the present invention to the long side N
o. In No. 21 as well, since carbides are redissolved during annealing, fine precipitation in the grains is promoted during overaging treatment, leading to a decrease in n value.

[0022]

[Table 2]

[0023]

[Table 3]

Example 3 Steels having various compositions shown in Table 4 were taken out from the converter and continuously cast into slabs. After heating these slabs at 1150 ° C,
r ₃ points (= 916-509C (wt%) + 27Si (w
(t%)-64 Mn (wt%) (° C)) or higher, the finish hot rolling was completed, and the hot rolling was performed at 700 ° C to obtain a 4 mm hot rolled sheet. After pickling, a cold-rolled sheet of 0.8 mm with a reduction rate of 80% was prepared, and heating rate was 300 ° C / by electric heating and steam cooling.
s, annealing temperature: 810 ° C., holding time: 0 seconds, cooling rate:
Over-aging treatment at 100 ° C./s: continuous annealing was performed at 350 ° C. for 10 minutes. After the temper rolling of 1%, the material was evaluated by the same method as in Examples 1 and 2. The results are summarized in Table 5.
In A, B, C, D, E and F according to the scope of the invention,
Elongation over 45% and n value over 0.23 and 20N
It exhibits an AI of less than 1 / mm ² and has excellent workability and aging. In the case of G and L steels in which the C content is lower or higher, due to residual solid solution carbon or fine precipitation of carbides,
Not only elongation and n value are low, but also AI is high. Also, S
H, I, and J steels in which i, Mn, and P are out of the range of the present invention are hardened and have low ductility and n value. Especially for J steel with a high P, the deterioration of secondary workability is a concern. S
If the amount of K steel is too high, there is concern about hardening and cracking during hot rolling.

[0025]

[Table 4]

[0026]

[Table 5]

[0027]

EFFECTS OF THE INVENTION The present invention clarifies a method for producing a cold-rolled steel sheet excellent in aging and workability by conventional continuous annealing with a low carbon steel component. According to the present invention, it is possible to provide a cold-rolled steel sheet having excellent aging and workability, which cannot be obtained by conventional continuous annealing.

[Brief description of drawings]

FIG. 1 is an explanatory view of a heat cycle in an experiment leading to the present invention,

FIG. 2 is a diagram showing a range of a heating rate in the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kosaku Ushioda 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Corp. Kimitsu Steel Co., Ltd.

Claims (4)

[Claims] 1. C: 0.01 to 0.05 wt% Si: 0.01 to 1.0 wt% Mn: 0.05 to 0.5 wt% P: 0.10 wt% or less S: 0.01 wt% or less Al : 0.005 to 0.1 wt% N: 0.005 wt% or less After slabs made of steel consisting of the balance Fe and inevitable impurity elements by continuous casting, reheating or Ar immediately after casting
Finish hot rolling at a temperature of ₃ transformation points or higher and 650-80
After winding in a temperature range of 0 ° C., pickling, and cold rolling by a usual method, continuous annealing is performed at a heating rate of 100 to 3000 ° C. /
s, heating to a temperature range of 750 to 910 ° C. without holding or holding for 30 seconds or less, cooling at a cooling rate of 70 ° C./s or more, and further temper rolling, and aging and workability. Of excellent cold rolled steel sheet. 2. C: 0.01 to 0.05 wt% Si: 0.01 to 1.0 wt% Mn: 0.05 to 0.5 wt% P: 0.10 wt% or less S: 0.01 wt% or less Al : 0.005-0.1 wt% N: 0.005 wt% or less B: 0.0003-0.004 wt% Steel made of balance Fe and unavoidable impurity elements is continuously cast into a slab and then reheated or cast Immediately after Ar
Finish hot rolling at a temperature of ₃ transformation points or higher and 650-80
After winding in a temperature range of 0 ° C., pickling, and cold rolling by a usual method, continuous annealing is performed at a heating rate of 100 to 3000 ° C. /
s, heating to a temperature range of 750 to 910 ° C. without holding or holding for 30 seconds or less, cooling at a cooling rate of 70 ° C./s or more, and further temper rolling, and aging and workability. Of excellent cold rolled steel sheet. 3. C: 0.01-0.05 wt% Si: 0.01-1.0 wt% Mn: 0.05-0.5 wt% P: 0.10 wt% or less S: 0.01 wt% or less Al : 0.005 to 0.1 wt% N: 0.005 wt% or less After slabs made of steel consisting of the balance Fe and inevitable impurity elements by continuous casting, reheating or Ar immediately after casting
Finish hot rolling at a temperature of ₃ transformation points or higher and 650-80
After winding in a temperature range of 0 ° C., pickling, and cold rolling by a usual method, continuous annealing is performed at a heating rate of 100 to 3000 ° C. /
s, after heating to a temperature range of 750 to 910 ° C., without holding or after holding for 30 seconds or less, cooling at a cooling rate of 70 ° C./s or more, followed by overaging treatment, and further temper rolling. A method for producing a cold-rolled steel sheet having excellent aging and workability. 4. C: 0.01 to 0.05 wt% Si: 0.01 to 1.0 wt% Mn: 0.05 to 0.5 wt% P: 0.10 wt% or less S: 0.01 wt% or less Al : 0.005-0.1 wt% N: 0.005 wt% or less B: 0.0003-0.004 wt% Steel made of balance Fe and unavoidable impurity elements is continuously cast into a slab and then reheated or cast Immediately after Ar
Finish hot rolling at a temperature of ₃ transformation points or higher and 650-80
After winding in a temperature range of 0 ° C., pickling, and cold rolling by a usual method, continuous annealing is performed at a heating rate of 100 to 3000 ° C. /
s, after heating to a temperature range of 750 to 910 ° C., without holding or after holding for 30 seconds or less, cooling at a cooling rate of 70 ° C./s or more, followed by overaging treatment, and further temper rolling. A method for producing a cold-rolled steel sheet having excellent aging and workability.