JPH09157788A - Hot rolled steel sheet for deep drawing excellent in secondary working brittleness resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot rolled steel sheet substitutable for a cold rolled steel sheet for deep drawing. SOLUTION: The slab of a steel having a compsn. contg., by weight, <=0.003% C, <=0.02% Si, 0.02 to 0.50% Mn, <=0.02% P, <=0.01% S, <=0.10% sol.Al, <=0.005% N, Ti: (48/12)*C+(48/14)*N+(48/32)*S<=Ti<=1.0 (where the elemental symbol in the inequality denotes the content of each element), furthermore contg., at need, 0.005 to 0.05% Nb, and the balance substantial Fe is heated at 1050 to 1250 deg.C. After that, rough rolling is finished in the temp. range of the Ar3 transformation point to 1050 deg.C, and it is held at >=800 deg.C for >=30sec. Thereafter, lubrication rolling is executed so as to regulate the finishing temp. to 600 to 800 deg.C, after the completion of the rolling, rapid cooling is started, and it is cooled at a cooling rate of >=60 deg.C/s at least to 400 deg.C. Subsequently, recrystallization annealing is executed at the recrystallization temp. to less than the Ac3 transformation point.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet having excellent deep drawability which is used for steel sheets for automobiles, steel sheets for home appliances and the like.

[0002]

2. Description of the Related Art Conventionally, cold-rolled steel sheets having good formability have been used as deep-drawing steel sheets, but in recent years, replacement with hot-rolled steel sheets has been promoted for the purpose of cost reduction and productivity improvement. However, the hot-rolled steel sheet used as the base material of the cold-rolled steel sheet for deep drawing has a random texture, so the deep drawability is significantly inferior to that of the cold-rolled steel sheet,
Such a hot rolled steel sheet could not be used as a substitute for the cold rolled steel sheet.

Therefore, as disclosed in Japanese Patent Application Laid-Open No. 59-226149, lubrication rolling in a temperature range of 500 ° C. to Ar ₃ transformation point forms a texture that is preferable for deep drawability. However, sufficient deep drawability has not been obtained. Therefore, in order to further improve the deep drawability, the slab of aluminum-killed steel is roughly rolled at an end temperature of 1100 to 800 ° C, held in the same temperature range for 1 to 60 minutes, and then lubricated in the temperature range of 800 to 450 ° C. A technique for rolling is proposed in Japanese Patent Laid-Open No. 25521/1990.

[0004]

However, even with this manufacturing method, the r value of the hot-rolled steel sheet obtained is 1.2 to 1.4.
It is as low as about 5, which is inferior to the cold-rolled steel sheet for deep drawing, and is insufficient as a substitute for the recent high r-value cold-rolled steel sheet. The present invention has been made in view of the above problems, and provides a hot-rolled steel sheet having excellent secondary work embrittlement resistance and having deep drawability comparable to that of a cold-rolled steel sheet for deep drawing.

[0005]

The hot-rolled steel sheet of the present invention has a weight percentage of C: 0.003% or less and Si: 0.02.
% Or less, Mn: 0.02 to 0.50%, P: 0.02
% Or less, S: 0.01% or less, sol.Al: 0.10
% Or less, N: 0.005% or less, Ti: (48/12) *
C + (48/14) * N + (48/32) * S ≦ Ti ≦ 1.0%
(However, the symbol of the element in the formula indicates the content of each element.) And, if necessary, Nb: 0.005 to 0.0
After heating a slab of steel containing 5% and the balance consisting of iron and unavoidable impurities to 1050 to 1250 ° C., rough rolling is completed in a temperature range of Ar ₃ transformation point or more and 1050 ° C. or less, and 80
After holding at 0 ℃ or more for 30 seconds or more, finish temperature is 600 ~
Lubricating rolling is carried out to 800 ° C., quenching is then preferably started within 1 second, and cooling is performed at least up to 400 ° C. at a cooling rate of 60 ° C./s or more, and then at a recrystallization temperature or more and less than Ac ₃ transformation point. It is manufactured by performing recrystallization annealing at a temperature.

According to the present invention, the nitride is precipitated during heating before rough rolling, and after rough rolling, TiC, which is a carbide, can be completely precipitated by holding before lubrication rolling, which is positioned as finish rolling. The rolling suppresses the formation of textures that are unfavorable to deep drawability, and the rapid cooling after lubrication rolling causes annealing while retaining the strain accumulated during lubrication rolling. It is possible to develop a texture of {111} orientation that is favorable for the property, obtain good deep drawability, suppress coarsening of crystal grains, and obtain good secondary work embrittlement resistance.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons for limiting the components of the steel used in the present invention will be explained. All units are% by weight
(Mass%). C: 0.003% or less From the viewpoint of deep drawability, the lower the content of C, the better. If the amount of C is large, the absolute amount of TiC that precipitates becomes large, and it becomes difficult to obtain a high r value. Further, the amount of Ti added to fix C also increases, leading to cost increase and the like, so the allowable upper limit value is set to 0.003%.

Si: 0.02% or less If the Si content is more than 0.02%, the strength becomes high,
Deep drawability and ductility decrease. In addition, scale is likely to be generated, and the pickling property is deteriorated. For this reason, the upper limit is set to 0.
02%. It is preferably 0.01% or less.

Mn: 0.02 to 0.50% Mn requires at least 0.02% to strengthen the steel and prevent hot embrittlement, but if it exceeds 0.50%, deep drawability and Ductility decreases, and workability deteriorates. Therefore, the lower limit of the amount of Mn is set to 0.02% and the upper limit is set to 0.50%.

P: 0.02% or less P is an element that deteriorates ductility and secondary work embrittlement resistance, so it is desirable that the content be small, and the content is limited to 0.02% or less.

S: 0.01% or less S is deteriorated in ductility and deep drawability, so it is preferable that the S content is small. S existing in the steel is fixed as TiS.
For this reason, it is preferable that the amount of Ti to be added be small, and the content is limited to 0.01% or less.

Sol.Al: 0.10% or less Al is added as a deoxidizing element, but even if added over 0.10%, the effect is saturated and inclusions increase,
Since the workability will deteriorate, the upper limit is set to 0.1.
0%. If it is too small, the deoxidizing effect will be insufficient,
It is preferable to contain 0.003% or more.

N: 0.005% or less N is fixed as TiN because it deteriorates the deep drawability. Therefore, from the viewpoint of the cost of Ti added, it is preferable that the amount of N is small. However, since excessive reduction has a problem in steelmaking technology, its upper limit is set to 0.005%.

Ti: (48/12) * C + (48/14) * N +
(48/32) * S ≦ Ti ≦ 1.0% However, the element symbol in the formula indicates the content of each element. To fix C, S and N in the steel, it is necessary to add Ti to these elements in an amount equal to or more than the equivalent value indicated by the value on the left side of the above formula, but exceeding 0.1%. If it is added as a solid solution, the solid solution Ti will increase more than necessary and a high r value will not be obtained, so the upper limit value is made 1.0%.

The steel sheet composition of the present invention contains the above components and the balance Fe and unavoidable impurities. In addition to the above components, Nb can be contained in an amount of 0.005 to 0.05%, if necessary. Nb is added to improve workability, but if it is less than 0.005%, its effect is too small, while if it is added more than 0.05%, the recrystallization temperature rises and high temperature annealing is required, resulting in cost reduction. Since it becomes high, the upper limit value is made 0.05%.

In producing the hot-rolled steel sheet of the present invention, first, the steel slab of the above-mentioned components produced by continuous casting or ingot casting is heated in the range of 1050 to 1250 ° C. Sulfide acts as an exfoliation nucleus of carbide, so it is important to precipitate it in the heating step. However, if it is less than 1050 ° C or exceeds 1250 ° C, it is difficult to precipitate sulfide.
This is because heating for a long time is required for complete precipitation, which is not preferable.

The steel slab heated to the above temperature range is A
After the completion of rough rolling with r ₃ 1050 ° C. lower than the transformation point or less, 8
Hold at 00 ° C or higher for 30 seconds or longer. Hold after rough rolling is T
800C to 1050C for the purpose of complete precipitation of iC
The temperature range of is most suitable. When the rough rolling is finished in a temperature range exceeding 1050 ° C., the holding temperature also exceeds 1050 ° C. and the precipitation of TiC is hindered, so the upper limit of the rough rolling finish temperature is set to 1050 ° C. Also, if the end temperature is below the Ar ₃ transformation point, it is not preferable for deep drawing {10
Since a texture with developed 0} orientation will be formed, the lower limit temperature is the Ar ₃ transformation point. Further, if the holding time after rough rolling is less than 30 seconds, precipitation of TiC will be insufficient and solid solution C will remain, so that a high r value cannot be obtained. Therefore, the holding time is 30 seconds or more.

After the rough rolling, the temperature is maintained at a predetermined temperature, and then as finishing rolling, rolling is performed while lubricating (lubrication rolling), and the rolling end temperature is set in the range of 600 to 800 ° C. When lubrication is not applied, shear stress is applied to the surface layer of the steel sheet due to the friction between the steel sheet and the roll, and a texture that is not preferable for deep drawability is formed. Further, if the end temperature exceeds 800 ° C., the strain accumulated by rolling is released before annealing, and good deep drawability cannot be obtained. 600 ° C
If less than, there is no problem in material properties, but the rolling load increases,
It is not preferable in terms of productivity and cost.

After the lubrication rolling, quenching is started before the strain due to rolling is released, and is rapidly cooled to at least 400 ° C. at a cooling rate of 60 ° C./s or more. Such rapid cooling is performed by supplying cooling water to the steel plate surface. It is better to start quenching as soon as possible after lubrication rolling.
It is preferable to start the rolling within 1.5 seconds, preferably within 1 second after the rolling. By starting within 1 second, the release of strain accumulated during the lubrication rolling can be sufficiently suppressed, the deep drawability can be further improved, and the coarsening of the crystal grain size can be further suppressed. Secondary working brittleness can be further improved. However, even if the cooling is started within 1 second, if the cooling rate is as low as less than 60 ° C./s, the accumulated strain cannot be frozen and the deep drawability may be deteriorated during the recrystallization annealing. Since it is not possible to develop a preferable texture of {111} orientation, good deep drawability and secondary work embrittlement resistance cannot be obtained. Since the release of the strain hardly occurs below 400 ° C., the rapid cooling as described above may be performed at 400 ° C. or higher. However, 400 ° C
Quenching in the temperature range below does not adversely affect the material properties.

After cooling to at least 400 ° C., recrystallization annealing is performed at a temperature not lower than the recrystallization temperature and lower than the Ac ₃ transformation point.
Recrystallization is not completed below the recrystallization temperature. The preferred {111} orientation for deep drawability increases with the progress of recrystallization. Therefore, if recrystallization is not completed, {111} texture remains undeveloped, and good deep drawability cannot be obtained. On the other hand, if the Ac is ₃ or more, austenite transformation occurs and the texture is randomized, and in this case also, good deep drawability cannot be obtained. The annealing is usually performed by continuous annealing after cooling, but it may be wound once after cooling and then subjected to continuous annealing or box-type annealing. By the above process, a hot-rolled steel sheet for forming can be obtained that has excellent secondary work embrittlement resistance and deep drawability that are comparable to conventional cold-rolled steel sheets for deep drawing.

[0021]

【Example】

(1) First Example A steel of the present invention having the following composition was melted, and a continuous cast slab was used for 120
After heating to 0 ° C., rough rolling was completed at 900 ° C., and the same completion temperature was maintained for various times shown in Table 1 below.
Lubricating rolling was performed at 0 ° C., after which cooling was performed to 350 ° C. under various cooling conditions shown in the same table, followed by heating and holding at 830 ° C. for recrystallization annealing. The r value and the secondary work embrittlement resistance of the obtained hot rolled steel sheet were examined. The results are shown in the same table. The secondary work brittleness resistance was determined by subjecting a cup with a drawing ratio of 2.5 to a vertical cracking test at various temperatures every 10 ° C.
The case of 80 ° C or lower was evaluated as good (◯ in the table), and the case of higher than 80 ° C was evaluated as poor (x in the table). Steel composition C: 0.0020%, Si: 0.005%, Mn: 0.15%, P: 0.00
7%, S: 0.005%, N: 0.0030%, sol.Al: 0.03
%, Ti: 0.027%, balance substantially Fe

[0022]

[Table 1]

From Table 1, it is possible to obtain 30 before finish rolling (lubrication rolling).
When the rapid cooling was started after finishing rolling for more than 2 seconds (Example: Sample Nos. 3, 4, 6, and 7), there was no solute carbon, and about 1.9 like cold-rolled steel sheet. The above high r values were obtained. Also, the crystal grain size after recrystallization annealing is fine,
It can be seen that the secondary processing brittleness resistance is also excellent. On the other hand, when finish rolling was performed immediately without holding after rough rolling (Comparative Example: Sample No. 1), solid solution carbon remained in the hot rolled sheet, and the r value was 1.35. It was a low value. In addition, when the holding time after lubrication rolling is long (Comparative example: Sample No. 5) and the cooling rate is as low as 30 ° C./s (Comparative example: Sample No. 5).
In 2), it can be seen that the r value is 1.45 or less and the secondary work embrittlement resistance is also poor.

(2) Second Example A hot rolled steel sheet was manufactured under the manufacturing conditions shown in Table 3 by using a steel slab having the chemical composition shown in Table 2, and the material properties were evaluated. The evaluation of the secondary processing brittleness resistance is the same as in the first embodiment.
The evaluation results are also shown in the same table. In addition, in Table 3, "-" indicates that the corresponding process was not performed and the next process was performed in the state of being left to cool.

[0025]

[Table 2]

[0026]

[Table 3]

From Table 3, the hot rolled steel sheets according to the Examples have a high r value of r value of 1.9 or more, and are excellent in deep drawability as compared with the hot rolled steel sheets of Comparative Example. It is also found that the secondary processing brittleness resistance is also good.

[0028]

EFFECT OF THE INVENTION According to the hot-rolled steel sheet of the present invention, since it has excellent deep drawability and secondary work embrittlement resistance equivalent to or better than those of conventional cold-rolled steel sheets, it can be used as a substitute for cold-rolled steel sheets for deep drawing. Therefore, the steel plate cost can be reduced. Further, also in the manufacturing process, the cold rolling process can be omitted, and the productivity can be improved.

Claims (3)

[Claims] 1. By weight%, C: 0.003% or less,
Si: 0.02% or less, Mn: 0.02 to 0.50
%, P: 0.02% or less, S: 0.01% or less,
sol.Al: 0.10% or less, N: 0.005% or less,
Ti: (48/12) * C + (48/14) * N + (48/32) *
A steel slab containing S ≦ Ti ≦ 1.0% (however, each element symbol in the formula represents the content of each element), and the balance being iron and unavoidable impurities, is heated to 1050 to 1250 ° C. After that, rough rolling is finished in a temperature range of Ar ₃ transformation point or more and 1050 ° C. or less, and after holding at 800 ° C. or more for 30 seconds or more, lubrication rolling is performed so that the finishing temperature becomes 600 to 800 ° C., and then rapid cooling. A secondary resistance, characterized by being produced by starting, cooling to at least 400 ° C. at a cooling rate of 60 ° C./s or more, and then performing recrystallization annealing at a temperature not lower than the recrystallization temperature and lower than the Ac ₃ transformation point. A hot-rolled steel plate for deep drawing with excellent work brittleness. 2. The hot-rolled steel sheet for deep drawing excellent in secondary work embrittlement resistance according to claim 1, wherein quenching is started within 1 second after lubrication rolling. 3. In addition to the components of claim 1, Nb: 0.
The hot-rolled steel sheet for deep drawing having excellent secondary work embrittlement resistance according to claim 1 or 2, which contains 005 to 0.05%.