JPH11302770A - Low yield ratio type fire resisting hot rolled steel sheet and steel pipe and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a low yield ratio type fire resisting hot rolled steel sheet and steel pipe having cold low yield ratio at ordinary temp. and excellent in high temp. strength and to provide methods for producing them. SOLUTION: This low yield ratio type fire resisting steel sheet is the one having a compsn. contg., by weight, <=0.02% C, <=1.0% Si, 0.05 to 2.0% Mn, <=0.02% S, 0.01 to 0.1% Al, 0.08 to 0.3% Nb, <=0.2% Ti, 0.0001 to 0.0020% B, and the balance Fe with inevitable impurities, in which the content of Nb also satisfies Nb>=0.05+7.75C-1.98Ti+6.64N+0.000035/(B+0.0004) and yield ratio at ordinary temp. is regulated to <=70%.

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 use having a low yield ratio at room temperature and excellent high-temperature strength characteristics, a steel pipe used in the construction field, and a method for producing the same. It is.

[0002]

[Prior Art] Based on the revision of the Building Standard Law of 1987,
If sufficient strength can be ensured at high temperatures for building steel materials, it is not necessary to apply a coating for suppressing the temperature rise on the surface of the structural part. In response to such a situation, inventions have been proposed that ensure high-temperature strength by adjusting the components. For example, in the invention disclosed in Japanese Patent Application Laid-Open No. 2-282419, Nb, Mo, etc., which are carbide forming elements, are added in order to secure high-temperature strength, and precipitation of fine carbides of these elements at high temperatures is performed. Take advantage of enhancements.

However, recently, from the viewpoint of earthquake resistance, it has been strongly desired to lower the yield ratio at room temperature. As an invention capable of obtaining fire resistance without increasing the yield ratio, Japanese Patent Application Laid-Open No. 2-205625
As disclosed in Japanese Unexamined Patent Application Publication No. H11-264, there has been proposed an invention in which Cu that precipitates only at a high temperature is added to IF steel. In addition, Japanese Unexamined Patent Publication No. 5-222484 discloses a low-yield ratio hot-rolled steel strip for building having excellent fire resistance and a method for producing the same.
There has also been proposed an invention in which the precipitates are finely precipitated to such an extent that high-temperature strength can be ensured.

[0004]

In response to the demand for lowering the yield ratio at room temperature from the viewpoint of earthquake resistance, Japanese Patent Application Laid-Open No.
In the invention described in Japanese Patent No. 2419, additional elements such as Nb and Mo are precipitated in a winding stage after hot rolling, and the yield strength at ordinary temperature and, consequently, the yield ratio are increased. It was difficult to obtain. In particular, steel pipes used as building structural members that have been formed into a closed section such as a circle or square have a higher yield strength than steel sheets after hot rolling because strain is applied to the steel material from the time of pipe formation. Will be raised more. For this reason, it is desired that the steel sheet used as the material of the steel pipe for building structural members be further reduced in yield ratio.

In the invention described in Japanese Patent Application Laid-Open No. 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 sheet as a material thereof.

Further, even the steel sheet according to the invention described in the above-mentioned Japanese Patent Application Laid-Open No. 5-222484 has a problem that the yield strength at the time of pipe forming is large, and a sufficient low yield ratio cannot be obtained after pipe forming.

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 high-temperature strength characteristics, and a method for producing the same. And

[0008]

Means for Solving the Problems As a result of repeated experiments and studies, the present inventors have 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 normal temperature and excellent strength properties at high temperature can be obtained.

That is, the gist of the present invention is as follows. (1) By weight%, C ≦ 0.02%, Si ≦ 1.0%, Mn: 0.05-2.0%, S ≦ 0.02%, Al: 0.01-0.1%, Nb: 0.08 to 0.3%, Ti ≦ 0.2%, B: 0.0001 to 0.0020%, the balance being Fe and unavoidable impurities, and the Nb content being Nb ≧ 0.05 + 7. .75C-1.98Ti + 6.64
A low-yield-ratio hot-rolled hot-rolled steel sheet satisfying N10 0.000035 / (B + 0.0004) and having a yield ratio at room temperature of 70% or less.

(2) In terms of% by weight, C ≦ 0.02%, Si ≦ 1.0%, Mn: 0.05-2.0%, S ≦ 0.02%, Al: 0.01-0. 1%, Nb: 0.08 to 0.3%, Ti ≦ 0.2%, B: 0.0001 to 0.0020%, the balance being Fe and unavoidable impurities, and the Nb amount being Nb ≧ 0.05 + 7.75C-1.98Ti + 6.64
Hot rolling of a steel slab satisfying N + 0.000035 / (B + 0.0004) is completed at a temperature not lower than the Ar ₃ transformation point, and is 700 at an average cooling rate of 0.1 ° C./sec or more and 30 ° C./sec or less. A method for producing a low-yield-ratio type hot-rolled steel sheet for refractory, wherein the yield ratio at room temperature is 70% or less, wherein the yield ratio is 70% or less.

(3) By weight%, C ≦ 0.02%, Si ≦ 1.0%, Mn: 0.05-2.0%, S ≦ 0.02%, Al: 0.01-0. 1%, Nb: 0.08 to 0.3%, Ti ≦ 0.2%, B: 0.0001 to 0.0020%, the balance being Fe and unavoidable impurities, and the Nb amount is Nb ≧ 0.05 + 7.75C-1.98Ti + 6.64
A low yield ratio type refractory steel tube satisfying N + 0.000035 / (B + 0.0004), having a yield ratio at room temperature of 90% or less and a yield strength at 600 ° C. of 197 MPa or more.

(4) By weight%, C ≦ 0.02%, Si ≦ 1.0%, Mn: 0.05-2.0%, S ≦ 0.02%, Al: 0.01-0. 1%, Nb: 0.08 to 0.3%, Ti ≦ 0.2%, B: 0.0001 to 0.0020%, the balance being Fe and unavoidable impurities, and the Nb amount is Nb ≧ 0.05 + 7.75C-1.98Ti + 6.64
N + 0.000035 / (B + 0.0004) is satisfied, and the yield ratio of the flat part of the rectangular steel pipe at room temperature is 90%.
A low yield ratio type refractory steel pipe having a yield strength at 600 ° C. of 197 MPa or more.

(5) By weight%, C ≦ 0.02%, Si ≦ 1.0%, Mn: 0.05-2.0%, S ≦ 0.02%, Al: 0.01-0. 1%, Nb: 0.08 to 0.3%, Ti ≦ 0.2%, B: 0.0001 to 0.0020%, the balance being Fe and unavoidable impurities, and the Nb amount is Nb ≧ 0.05 + 7.75C-1.98Ti + 6.64
Hot rolling of a steel slab satisfying N + 0.000035 / (B + 0.0004) is completed at a temperature not lower than the Ar ₃ transformation point, and is 700 at an average cooling rate of 0.1 ° C./sec or more and 30 ° C./sec or less. The hot-rolled steel sheet obtained by cooling to below ℃ is formed into a circular cross section by a conventional method and welded. The yield ratio at room temperature is 90% or less, and the yield strength at 600 ° C. is 197 MPa or more. A method for producing a low yield ratio type refractory steel pipe, characterized in that:

(6) After forming into a circular cross section, welding, and then further forming into a rectangular cross section, the yield ratio at room temperature of the flat portion of the rectangular steel pipe at room temperature is 90% or less, and the yield strength at 600 ° C. The method for producing a low yield ratio type refractory rectangular steel pipe according to the above (5), wherein

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. First, the reasons for limiting the steel composition will be described. C
Increases the yield ratio at room temperature by combining with other additional elements to form precipitates or precipitate as cementite. If the amount of C exceeds 0.02%, the yield ratio at room temperature increases and exceeds 70%. If the C content is 0.02% or less, the yield ratio at room temperature is 70% or less, so the upper limit of the C content is set to 0.02%. In order to further suppress the increase in the yield ratio during pipe formation, the C content is preferably set to 0.015% or less.

Since Si is a solid solution strengthening element and can increase the strength of a steel sheet relatively inexpensively, it is added in the present invention in order to adjust the strength, but the strength increases as the amount of addition increases. Because it is too much, the content is made 1.0% or less.
If the strength can be adjusted with another element, Si can be not added.

Mn is a relatively inexpensive solid solution strengthening element similar to Si, and is effective in adjusting the strength. In the present invention, Mn is added for adjusting the strength. , It is necessary to add 0.05% or more. On the other hand, unnecessarily adding excessively high strength for a building steel material and lowering the formability, so the content is set to 2.0% or less.

S is an element inevitably contained and causes deterioration of workability, so it is necessary to reduce it as much as possible.
Since the problem with workability is solved by setting the content to 0.02% or less, the range is set to 0.02% or less. In the case of difficult-to-work applications, the content is desirably 0.01% or less.

Although Al is used as a deoxidizing agent, it is necessary to contain 0.01% or more in steel in order to exhibit this effect. On the other hand, if it exceeds 0.1%, an increase in oxide-based inclusions is caused, so the upper limit is made 0.1%.

Usually, Nb is often added as a precipitation strengthening element. However, when precipitation strengthening is used, a target low yield ratio cannot be obtained. It does not work, and is added as a solid solution strengthening element in the present invention. Nb also significantly increases the high-temperature strength. This is because during the deformation at high temperature, the dislocations moving with the solute Nb interact with each other to reduce the movability of the dislocations. However, if the addition amount is increased unnecessarily, the hot In order to impair workability, the addition amount is set to 0.08% or more and 0.3% or less. In addition, from the viewpoint of further increasing the yield strength at 600 ° C., the addition weight is desirably 0.15% or more.

Ti has C, which may combine with Nb,
N is fixed and Nb is added to effectively use the added Nb as solid solution Nb. However, if the added amount is excessively increased, workability is deteriorated. Therefore, the upper limit is set to 0.2%.

B promotes the action of solid solution Nb by adding Nb in combination. Although the reason for this has not been clarified at present, it has a very remarkable effect.
However, the effect is saturated even if the amount is excessively increased, and the yield ratio may be increased at room temperature. Therefore, the upper limit is 0.0020%.
And

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 a sufficient high-temperature strength may not be obtained. In FIG. 1, the horizontal axis is Nb-
(0.05 + 7.75C-1.98Ti + 6.64N +
0.000035 / (B + 0.0004)), and the vertical axis is 6
The results arranged as the yield strength at 00 ° C. are shown. 197
In order to ensure a yield strength at 600 ° C. of not less than MPa, the amount of Nb added is in addition to the above-mentioned conditions, and Nb ≧ 0.05 + 7.75C-1.98Ti + 6.64N + 0.000035 / (B + 0.0004) (1) To satisfy.

The other components are not particularly limited. However, the presence of elements such as Cu, Ni, Cr, V and the like mixed from scrap has no effect on the characteristics of the developed steel.

The steel having such a composition is cast, and the obtained hot slab is directly or heated, or the cold slab is reheated to perform hot rolling. At that time, almost no change in the characteristics between directly rolling the hot piece slab and rolling after reheating is observed. Also, the reheating temperature is not particularly limited,
It is preferable that the temperature be in the range of 1000 ° C. to 1300 ° C. in consideration of productivity.

[0026] Hot rolling can be carried out in either a normal hot rolling process or a continuous hot rolling process in which slabs are joined and rolled in finish rolling.

The hot rolling end temperature is set to the Ar ₃ transformation point temperature or higher. This is because when finish rolling is performed at a temperature lower than the Ar ₃ transformation point, a processed structure may remain and the yield ratio at room temperature may increase. Although the upper limit of the rolling end temperature is not particularly limited, it is 100 from the viewpoint of productivity.
It is desirable that the temperature be 0 ° C. or lower.

The average cooling rate after hot rolling is 30 ° C./sec.
In the case of faster speed, a low-temperature transformation phase is generated during cooling to increase the yield ratio. If the cooling rate is less than 0.1 ° C / sec, an unnecessary phase may precipitate at the grain boundary during cooling.
ec or more.

The winding temperature may be 700 ° C. or less from the viewpoint of pipe formability of a hot rolled steel sheet into a steel pipe or a square steel pipe.
Desirably, the temperature is 650 ° C. or lower.

The hot-rolled steel sheet manufactured in this manner has a yield ratio at room temperature of 70% or less. When a steel pipe is formed, strain is introduced to increase the yield ratio. The yield ratio after pipe formation is 90% or less.

The present invention is applicable not only to the above-described continuous hot rolling process but also to a normal thick plate rolling process. In addition, the present invention can be applied not only to a hot-rolled steel sheet but also to a surface-treated steel sheet using the same as a material. In this case, from the viewpoint of surface treatment properties, the amount of Si added is desirably 0.5% or less. Further, the pipe forming method is not particularly limited in the present invention, and a press forming method, a roll forming method, and the like can be applied.

[0032]

EXAMPLE Steels of various chemical compositions shown in Table 1 were cast, and 1
After reheating to a temperature of 050 ° C. to 1250 ° C., a hot-rolled steel sheet was manufactured under the conditions shown in Table 2, and tensile properties at room temperature and 600 ° C.
The tensile properties at ° C. were measured and the results are shown in Table 2.

Steel types 1 to 10 are component systems within the scope of the present invention. As shown in Table 2, the yield ratio at room temperature is 7
It is 0% or less, and all the yield ratios are 90% or less as a result of measurement taken from a flat portion of the square steel pipe after pipe formation. Further, the yield strength Y at 600 ° C. after forming into a square steel pipe.
A high-strength steel plate at a high temperature in which S is all 197 MPa or more can be manufactured.

On the other hand, the cooling rate of steel type No. 6 is 50 ° C./sec.
The material has too high a cooling rate, so the yield ratio at room temperature is too high. Steel type numbers 20 to 23 are component systems outside the scope of the present invention, and steel type number 20 has too much C content, so that the yield ratio at room temperature exceeds 70% in hot-rolled material,
It exceeds 90% after the square steel pipe is made. Moreover, the amount of added Nb is slightly smaller than the value obtained from the equation (1),
The yield strength at 600 ° C. cannot be stably secured. Also,
Steel types 21 to 23 have low strength at high temperatures because the amount of added Nb is smaller than the amount obtained from equation (1).

In the test in this example, the room-temperature tensile test was performed according to JIS Z 2241 using a JIS No. 5 test piece, and the tensile test at 600 ° C. was performed according to JIS G05.
67.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

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

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the added amount of Nb, Ti, C, N, and B and the yield strength at high temperatures.

Claims (6)

[Claims] 1. Weight%, C ≦ 0.02%, Si ≦ 1.0%, Mn: 0.05-2.0%, S ≦ 0.02%, Al: 0.01-0.1 %, Nb: 0.08 to 0.3%, Ti ≦ 0.2%, B: 0.0001 to 0.0020%, the balance being Fe and unavoidable impurities, and the Nb content being Nb ≧ 0. .05 + 7.75C-1.98Ti + 6.64
A low yield ratio type hot rolled steel sheet for refractory satisfying N + 0.000035 / (B + 0.0004) and having a yield ratio at room temperature of 70% or less. 2.% by weight: C ≦ 0.02%, Si ≦ 1.0%, Mn: 0.05-2.0%, S ≦ 0.02%, Al: 0.01-0.1 %, Nb: 0.08 to 0.3%, Ti ≦ 0.2%, B: 0.0001 to 0.0020%, the balance being Fe and unavoidable impurities, and the Nb content being Nb ≧ 0. .05 + 7.75C-1.98Ti + 6.64
Hot rolling of a steel slab satisfying N + 0.000035 / (B + 0.0004) is completed at a temperature not lower than the Ar ₃ transformation point, and is 700 at an average cooling rate of 0.1 ° C./sec or more and 30 ° C./sec or less. A method for producing a low-yield-ratio type hot-rolled steel sheet for refractory, wherein the yield ratio at room temperature is 70% or less, wherein the yield ratio is 70% or less. C. 0.02%, Si ≤ 1.0%, Mn: 0.05-2.0%, S ≤ 0.02%, Al: 0.01-0.1% by weight. %, Nb: 0.08 to 0.3%, Ti ≦ 0.2% B: 0.0001 to 0.0020%, the balance being Fe and unavoidable impurities, and the Nb content being Nb ≧ 0. 05 + 7.75C-1.98Ti + 6.64
A low yield ratio type refractory steel tube satisfying N + 0.000035 / (B + 0.0004), 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% by weight, C ≦ 0.02%, Si ≦ 1.0%, Mn: 0.05-2.0%, S ≦ 0.02%, Al: 0.01-0.1. %, Nb: 0.08 to 0.3%, Ti ≦ 0.2% B: 0.0001 to 0.0020%, the balance being Fe and unavoidable impurities, and the Nb content being Nb ≧ 0. 05 + 7.75C-1.98Ti + 6.64
N + 0.000035 / (B + 0.0004) is satisfied, and the yield ratio of the flat part of the rectangular steel pipe at room temperature is 90%.
A low yield ratio type refractory steel pipe having a yield strength at 600 ° C. of 197 MPa or more. 5.% by weight, C ≦ 0.02%, Si ≦ 1.0%, Mn: 0.05-2.0%, S ≦ 0.02%, Al: 0.01-0.1 %, Nb: 0.08 to 0.3%, Ti ≦ 0.2%, B: 0.0001 to 0.0020%, the balance being Fe and unavoidable impurities, and the Nb content being Nb ≧ 0. .05 + 7.75C-1.98Ti + 6.64
Hot rolling of a steel slab satisfying N + 0.000035 / (B + 0.0004) is completed at a temperature not lower than the Ar ₃ transformation point, and is 700 at an average cooling rate of 0.1 ° C./sec or more and 30 ° C./sec or less. The hot-rolled steel sheet obtained by cooling to below ℃ is formed into a circular cross section by a conventional method and welded. The yield ratio at room temperature is 90% or less, and the yield strength at 600 ° C. is 197 MPa or more. A method for producing a low yield ratio type refractory steel pipe, characterized in that: 6. The yield ratio at room temperature of a flat portion of a rectangular steel pipe is 90% or less, and the yield strength at 600 ° C. is characterized by being formed into a circular cross section, welded, and further formed into a square cross section. The method for producing a low yield ratio type refractory rectangular steel pipe according to claim 5, which is at least 197 MPa.