JP3462968B2 - Low yield ratio type hot rolled steel sheet for refractory, steel pipe, and method for producing them - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low yield ratio type hot-rolled steel sheet for refractory and a steel pipe, which has a low yield ratio at room temperature and is excellent in high temperature strength properties, and a manufacturing method thereof, which are used in the construction field. Is.

[0002]

2. Description of the Related Art With the revision of the Building Standards Act of 1987,
As long as sufficient strength can be secured at high temperatures for building steel materials, it is no longer necessary to provide a coating for suppressing the temperature rise on the surface of structural members. In response to such a situation, an invention has been proposed in which the high temperature strength is secured by adjusting the components. For example, in the invention disclosed in Japanese Unexamined Patent Application Publication No. Hei 2-282419, in order to secure high temperature strength,
Carbide forming elements such as Nb and Mo are added, and precipitation strengthening by precipitation of fine carbides of these elements at high temperature is utilized.

However, recently, from the viewpoint of earthquake resistance, it has been strongly desired to reduce the yield ratio at room temperature. As an invention capable of obtaining fire resistance without increasing the yield ratio, JP-A-2-205625 is known.
As can be seen in the publication, an invention has been proposed in which Cu that precipitates only at high temperatures is added to IF steel. In addition, as a low yield specific heat-rolled steel strip for construction excellent in fire resistance and a method for manufacturing the same, Japanese Patent Laid-Open No. 522484/1993 discloses NbC and TiC.
An invention has also been proposed in which the above-mentioned precipitate is finely precipitated to the extent that high temperature strength can be secured.

[0004]

With respect to the demand for a low yield ratio at room temperature from the viewpoint of earthquake resistance, the above-mentioned Japanese Unexamined Patent Publication No. 2-2 has been proposed.
In the invention described in Japanese Patent No. 82419, additional elements such as Nb and Mo are precipitated in the winding stage after hot rolling, and the yield strength at room temperature and eventually the yield ratio are increased to obtain a steel sheet with a low yield ratio. Was difficult. In particular, in steel pipes used for building structural members, which have been formed into closed sections such as circles and squares, strain is applied to the steel material during pipe making, so the yield strength is higher than that of steel sheets after hot rolling, and the yield ratio is It will be higher. Therefore, it is desired that the steel plate, which is a material for the steel pipe for building structural members, has a further lower yield ratio.

Further, in the invention described in JP-A-2-205625, it is necessary to add expensive Ni at the same time, and it is not possible to provide an inexpensive steel pipe for building structural members and a steel plate as a raw material thereof. Further, the above-mentioned Japanese Patent Laid-Open No. 5-2
Even the steel sheet according to the invention described in Japanese Patent No. 22484 has a drawback that the yield strength is greatly increased during pipe making, and a sufficiently low yield ratio cannot be obtained after pipe making.

In view of such circumstances, an object of the present invention is to provide a low yield ratio type hot rolled steel sheet for refractory and a steel pipe having a low yield ratio at room temperature and excellent in high temperature strength properties, and a method for producing them. It is what

[0007]

[Means for Solving the Problems] As a result of various experiments and researches, the present inventors reduced C contained in steel materials,
It has been found that by adding a large amount of Nb, a steel material having a low yield ratio at room temperature and excellent strength properties at high temperature can be obtained. That is, the gist of the present invention is as follows.

(1) In weight%, C ≦ 0.04%, Si ≦
1.0%, Mn: 0.05 to 2.0%, S ≦ 0.02
%, Al: 0.01 to 0.1%, Ti ≦ 0.2%, the Nb content is Nb: 0.1 to 2.0%, and Nb ≧ 0.08 + 7.75C-1. .98Ti + 6.
A low-yield-ratio hot-rolled steel sheet for refractory, which satisfies 64N, the balance consists of Fe and unavoidable impurities, and has a yield ratio of 70% or less at room temperature.

(2) C≤0.04%, Si≤
1.0%, Mn: 0.05 to 2.0%, S ≦ 0.02
%, Al: 0.01 to 0.1%, Ti ≦ 0.2%, the Nb content is Nb: 0.1 to 2.0%, and Nb ≧ 0.08 + 7.75C-1. .98Ti + 6.
A steel piece satisfying 64 N and the balance of Fe and unavoidable impurities was hot-rolled at a temperature of Ar ₃ transformation point or higher and cooled at a cooling rate of 0.1 ° C./sec or more and 50 ° C./sec or less. A method for producing a low yield ratio hot-rolled hot rolled steel sheet having a yield ratio of 70% or less at room temperature, which is characterized by cooling to 800 ° C or less.

(3) In weight%, C ≦ 0.04%, Si ≦
1.0%, Mn: 0.05 to 2.0%, S ≦ 0.02
%, Al: 0.01 to 0.1%, Ti ≦ 0.2%, the Nb content is Nb: 0.1 to 2.0%, and Nb ≧ 0.08 + 7.75C-1. .98Ti + 6.
A low yield ratio type refractory steel pipe satisfying 64 N, the balance consisting of Fe and unavoidable impurities, having a yield ratio at room temperature of 90% or less and a yield strength at 600 ° C. of 197 MPa or more.

(4) In weight%, C ≦ 0.04%, Si ≦
1.0%, Mn: 0.05 to 2.0%, S ≦ 0.02
%, Al: 0.01 to 0.1%, Ti ≦ 0.2%, the Nb content is Nb: 0.1 to 2.0%, and Nb ≧ 0.08 + 7.75C-1. .98Ti + 6.
Low yield ratio characterized by satisfying 64 N, the balance consisting of Fe and unavoidable impurities, the yield ratio of the flat part of the square steel pipe at room temperature being 90% or less, and the yield strength at 600 ° C. being 197 MPa or more. Type fireproof square steel pipe.

(5) In weight%, C ≦ 0.04%, Si ≦
1.0%, Mn: 0.05 to 2.0%, S ≦ 0.02
%, Al: 0.01 to 0.1%, Ti ≦ 0.2%, the Nb content is Nb: 0.1 to 2.0%, and Nb ≧ 0.08 + 7.75C-1. .98Ti + 6.
A steel piece satisfying 64 N and the balance of Fe and unavoidable impurities was hot-rolled at a temperature of Ar ₃ transformation point or higher and cooled at a cooling rate of 0.1 ° C./sec or more and 50 ° C./sec or less. The hot rolled steel sheet obtained by cooling to 800 ° C. or lower
A method for producing a low yield ratio type fire-resistant steel pipe having a yield ratio at room temperature of 90% or less and a yield strength at 600 ° C. of 197 MPa or more, which is characterized by forming into a circular cross section by a conventional method and welding.

(6) The yield ratio at room temperature of the flat portion of the square steel pipe is 90%, which is obtained by forming a circular cross section by the method described in (5) above, welding, and then forming a square cross section. A method for producing a low yield ratio type fire-resistant rectangular steel pipe having a yield strength at 600 ° C. of 197 MPa or more.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. First, the reasons for limiting the steel components will be described. C
Combines with other additive elements to form a precipitate, or precipitates as cementite to increase the yield ratio at room temperature. FIG. 1 shows the relationship between the amount of added C and the yield strength measured at 600 ° C. when the amount of added Nb is 0.2 to 0.5%.
If the addition amount exceeds 0.04%, the yield ratio at room temperature increases rapidly and exceeds 70%. If the C content is 0.04% or less, the yield ratio at room temperature is 70% or less, so the upper limit of the C content is 0.04%. In order to further suppress the yield ratio increase during pipe making, it is preferable that the C content be 0.025% or less.

Since Si is a solid solution strengthening element and can increase the strength of the steel sheet at a relatively low cost, it is added in the present invention in the sense of adjusting the strength. However, if the addition amount increases, the strength increases. Since it is too much, it is set to 1.0% or less.
If the strength can be adjusted with other elements, Si can be omitted. Similar to Si, Mn is a relatively inexpensive solid solution strengthening element and is effective for strength adjustment. In the present invention, Mn is added for strength adjustment, but if it is less than 0.05%, hot workability cannot be secured. Therefore, it is necessary to add 0.05% or more. On the other hand, the unduly added strength of the steel for construction becomes too high and the formability is lowered, so the content is made 2.0% or less.

S is an element that is unavoidably contained and causes deterioration of workability, so it is necessary to reduce S as much as possible.
By setting the content to 0.02% or less, the problem of workability is solved, so the range is set to 0.02% or less. In the case of difficult-to-work applications, it is desirable to set it to 0.01% or less. Al is used as a deoxidizing agent, but in order to exert this effect, it is necessary to contain 0.01% or more in steel. On the other hand, if the Al content exceeds 0.1%, an increase in oxide inclusions is caused, so the upper limit is made 0.1%.

Nb is usually added as a precipitation strengthening element, but when precipitation strengthening is exerted, the desired low yield ratio cannot be obtained. It does not work and is added as a solid solution strengthening element in the present invention. Further, Nb remarkably increases the high temperature strength. This is because the solid solution Nb interacts with the dislocations that are moving when it is deformed at a high temperature, which reduces the mobility of the dislocations. In FIG. 2, 0.005
The results of investigating the relationship between the added Nb amount of steel containing% C, 0.03% Ti, and 0.003% N and the yield strength at 600 ° C are shown. The yield strength at 600 ° C. greatly increases when 0.1% or more of Nb is added. However, if the amount of Nb added is excessively increased, the workability during hot working is impaired.
The added amount is 0.1 to 2.0%. In addition, 600 ℃
From the viewpoint of further increasing the yield strength, it is preferable that the addition amount is 0.2% or more, and in order to keep the steelmaking cost low, the Nb addition amount is preferably 1.0% or less. .

Ti is C, which may bond with Nb,
N is fixed and added so as to effectively utilize the added Nb as a solid solution Nb. However, if the amount of addition is excessively increased, workability is deteriorated, so the upper limit is made 0.2%.
When a relatively large amount of C and N is contained, a part of the added Nb may not be in a solid solution state and sufficient high temperature strength may not be obtained. In FIG. 3, the horizontal axis is Nb- (0.08+
7.75C-1.98Ti + 6.64N), vertical axis is 60
The results arranged as the yield strength at 0 ° C are shown. 197M
In order to secure the yield strength at 600 ° C. of Pa or higher, Nb ≧ 0.08 + 7.75C-1.98Ti + 6.64N (1) is satisfied in addition to the above conditions for the amount of added Nb.

Hot rolling is performed after casting the steel having such a component and directly or heating the obtained hot piece slab, or by reheating the cold piece. At that time, there is almost no change in properties between the hot strip slab being directly rolled and the rolling after being reheated. The reheating temperature is not particularly limited, but it is preferably in the range of 1000 ° C to 1300 ° C in consideration of productivity.

Hot rolling may be carried out by the usual hot rolling process or finishing.
Continuous hot rolling for joining and rolling slabs in top rolling
Either is possible. Rolling end temperature during hot rolling
Degree Ar₃The temperature is higher than the transformation temperature. This is Ar ₃Strange
When finish rolling is performed at a temperature below the freezing point, the processed structure remains
Therefore, the yield ratio at room temperature may increase.
It The upper limit of rolling end temperature is not particularly limited, but production
From the viewpoint of properties, it is desirable to set the temperature to 1000 ° C. or lower.

The cooling rate after hot rolling is 50 ° C./sec.
If it is faster, a low temperature transformation phase is generated during cooling and the yield ratio increases, so it is set to 50 ° C./sec or less. Further, if the cooling rate is less than 0.1 ° C./sec, there is a high possibility that an unnecessary phase will precipitate at the grain boundaries during cooling, so the cooling rate is set to 0.1 ° C./sec or more. The unnecessary phase that precipitates at the grain boundaries often appears even when the coiling is performed at a temperature higher than 800 ° C., so the above-mentioned 0.1 to 50 ° C./se
The cooling at c is performed up to 800 ° C or lower.

The winding temperature may be 800 ° C. or lower and is not particularly limited. Almost no change in strength due to winding is observed. The hot-rolled steel sheet produced in this manner has a yield ratio of 70% or less at room temperature, and strain is introduced to increase the yield ratio when it is formed into a steel pipe. The yield ratio of is less than 90%.

The present invention can be applied not only to the continuous hot rolling process as described above, but also to an ordinary thick plate rolling process. The present invention is applicable not only to hot rolled steel sheets, but also to surface treated steel sheets made of this material. In this case, it is desirable that the amount of Si added be 0.5% or less from the viewpoint of surface treatment property.

Further, the pipe making method is not particularly limited in the present invention, and a press molding method, a roll molding method or the like can be applied.

[0025]

EXAMPLES Steels having various chemical compositions shown in Tables 1 and 2 (continued from Table 1) were cast, reheated to a temperature of 1050 to 1260 ° C., and then Tables 3 and 4 (continued from Table 1-1). ), Table 5 (Table 3
No. 2), Table 6 (No. 3 in Table 3), Table 7 (No. 4 in Table 3), and Table 8 (No. 5 in Table 3) to produce hot-rolled steel sheets, Tensile properties at room temperature and 600
The tensile properties at ° C were measured. The results are also shown in Tables 3 to 8. Steel type Nos. 1 to 19 are component systems within the scope of the present invention, and as shown in Tables 3 to 8, the yield ratio at room temperature is 70% or less, and the flatness after forming a square steel pipe is flat. The yield ratio is 90% or less in all the results of measurements taken from the parts. In addition, the yield strength Y at 600 ° C after forming a rectangular steel pipe
It is possible to manufacture a steel plate having high strength at a high temperature in which all S are 197 MPa or more.

On the other hand, Steel Grade Nos. 20 to 23 are component systems outside the scope of the present invention, and Steel Grade No. 20 contains too much C, so the yield ratio at room temperature exceeds 70% in the as-hot-rolled material. ,
It exceeds 90% after forming into a rectangular steel pipe. In addition, Steel Grade Nos. 21 to 23 have a smaller amount of added Nb than the amount obtained from the formula (1), and the yield strength at 600 ° C. of the flat portion after pipe making is low.

In the test of this example, the room temperature tensile test was conducted according to JIS Z 2241 using JIS No. 5 test piece.
The tensile test at 600 ° C. is performed according to JIS G 0
567.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

[Table 3]

[0031]

[Table 4]

[0032]

[Table 5]

[0033]

[Table 6]

[0034]

[Table 7]

[0035]

[Table 8]

[0036]

As described above, according to the present invention, it is possible to manufacture a steel plate and a steel pipe having a low yield ratio before and after the pipe making at room temperature and a high strength at a high temperature. It is of great value.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the amount of C added and the yield ratio at room temperature.

FIG. 2 is a diagram showing the relationship between the amount of Nb added and the yield strength at high temperature.

FIG. 3 is a diagram showing the relationship between the amounts of Nb, Ti, C, and N added and the yield strength at high temperature.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ identification code FI C22C 38/12 C22C 38/12 (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/02 C21D 8/10

Claims (6)

(57) [Claims] 1. By weight%, C ≦ 0.04%, Si ≦ 1.0%, Mn: 0.05 to 2.0%, S ≦ 0.02%, Al: 0.01 to 0.1. %, Ti ≦ 0.2%, the Nb content is Nb: 0.1 to 2.0%, and Nb ≧ 0.08 + 7.75C-1.98Ti + 6.64.
A low yield ratio type hot rolled steel sheet for refractory, which satisfies N 2, has a balance of Fe and unavoidable impurities, and has a yield ratio of 70% or less at room temperature. 2. By weight%, C ≦ 0.04%, Si ≦ 1.0%, Mn: 0.05 to 2.0%, S ≦ 0.02%, Al: 0.01 to 0.1. %, Ti ≦ 0.2%, the Nb content is Nb: 0.1 to 2.0%, and Nb ≧ 0.08 + 7.75C-1.98Ti + 6.64.
Hot rolling of a steel piece satisfying N 2 and the balance consisting of Fe and unavoidable impurities was completed at a temperature not lower than the Ar ₃ transformation point,
8 at a cooling rate of 1 ° C / sec or more and 50 ° C / sec or less
A method for producing a low yield ratio type hot-rolled hot rolled steel sheet having a yield ratio of 70% or less at room temperature, which is characterized by cooling to 00 ° C or less. 3. By weight%, C ≦ 0.04%, Si ≦ 1.0%, Mn: 0.05 to 2.0%, S ≦ 0.02%, Al: 0.01 to 0.1. %, Ti ≦ 0.2%, the Nb content is Nb: 0.1 to 2.0%, and Nb ≧ 0.08 + 7.75C-1.98Ti + 6.64.
N is satisfied, the balance is Fe and inevitable impurities, the yield ratio at room temperature is 90% or less, and the yield strength at 600 ° C. is 1.
Low yield ratio type fire-resistant steel pipe having a pressure of 97 MPa or more. 4. In% by weight, C ≦ 0.04%, Si ≦ 1.0%, Mn: 0.05 to 2.0%, S ≦ 0.02%, Al: 0.01 to 0.1. %, Ti ≦ 0.2%, the Nb content is Nb: 0.1 to 2.0%, and Nb ≧ 0.08 + 7.75C-1.98Ti + 6.64.
N is satisfied, the balance is Fe and unavoidable impurities, and the yield ratio at room temperature of the flat portion of the rectangular steel pipe is 90% or less and 600
A low yield ratio type rectangular steel pipe for refractory, which has a yield strength at ℃ of 197 MPa or more. 5. By weight%, C ≦ 0.04%, Si ≦ 1.0%, Mn: 0.05 to 2.0%, S ≦ 0.02%, Al: 0.01 to 0.1. %, Ti ≦ 0.2%, the Nb content is Nb: 0.1 to 2.0%, and Nb ≧ 0.08 + 7.75C-1.98Ti + 6.64.
Hot rolling of a steel piece satisfying N 2 and the balance consisting of Fe and unavoidable impurities was completed at a temperature not lower than the Ar ₃ transformation point,
8 at a cooling rate of 1 ° C / sec or more and 50 ° C / sec or less
A hot-rolled steel sheet obtained by cooling to 00 ° C or less is formed into a circular cross section by a conventional method and welded, and the yield ratio at room temperature is 90% or less and the yield strength at 600 ° C is 197.
A method for producing a low yield ratio type steel pipe for refractory having a MPa or more. 6. The yield ratio at room temperature of a flat portion of a square steel pipe is 90%, which is formed into a circular cross section, welded, and then formed into a square cross section by the method according to claim 5.
A method for producing a low yield ratio type fire-resistant rectangular steel pipe having a yield strength at 600 ° C. of 197 MPa or more.