JP2000054061A - Low yield ratio type refractory steel material and steel tube and their manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material reduced in YR at ordinary temperature in an manufactured state as it is or after cold working at 5% equivalent strain and increased in yield stress at high temperature and to provide a steel tube reduced in YR after making a tube and increased in yield stress at high temperature. SOLUTION: This low yield ratio type refractory steel material is steel having a composition which consists of, by weight, <=0.03% C, <=1% Si, 0.1-2% Mn, <=0.02% S, 0.01-0.1% Al, 0.04-1% Nb, and the balance essential Fe and in which Nb content satisfies Nb>=0.1+7.74C-1.94Ti+6.63N. Further, the microstructure of this steel is a mixed structure of polygonal ferrite and a bainite-containing phase formed at low temperature, and the area ratio of polygonal ferrite on the average in a sheet thickness direction is 10-95%.

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 refractory steel and a steel pipe having a low yield ratio at room temperature and excellent in high-temperature strength characteristics, and a method for producing the same, which are used in the field of construction. .

[0002]

2. Description of the Related Art If sufficient strength can be secured at high temperatures for building steel materials due to the revision of the Building Standards Law of 1987, rock wool usually applied to the surface of structural parts to suppress the temperature rise of steel materials It is no longer necessary to apply a refractory coating such as In response to such a situation, inventions relating to steel materials that ensure high-temperature strength by adjusting the components have been proposed. For example, Japanese Patent Application Laid-Open No. 2-282419
In the invention disclosed in Japanese Patent Laid-Open Publication No. H11-107, Nb, Mo, and the like, which are carbide forming elements, are added in order to ensure high-temperature strength, and precipitation strengthening of these elements by precipitation of fine carbides at high temperatures is used.

However, recently, it has been strongly desired to lower the yield ratio at room temperature from the viewpoint of earthquake resistance of buildings. As an invention capable of obtaining fire resistance without increasing the yield ratio, JP-A-2-205
As disclosed in Japanese Patent No. 625, a steel material in which Cu that precipitates only at a high temperature is added to IF steel has been proposed. In addition,
As a low-yield-ratio hot-rolled steel strip for architectural use having excellent fire resistance and a method for producing the same, an invention in which precipitates of NbC and TiC are finely precipitated to such an extent that high-temperature strength can be ensured has been proposed in JP-A-5-222484. I have. Also, Japanese Patent Application Laid-Open No. 9-41
No. 035 discloses that one or two of Nb and Ti are (T
It is said that a steel sheet satisfying both low YR and high toughness can be produced by satisfying i + Nb / 2) / C ≧ 4 and forming a microstructure of a polygonal ferrite phase single phase not containing bainitic ferrite. The invention has been disclosed.

[0004]

In response to the demand for lowering the yield ratio at room temperature from the viewpoint of earthquake resistance of a building, the invention described in Japanese Patent Application Laid-Open No. 2-282419 discloses the addition of Nb, Mo, and the like. Since the material precipitates at the winding stage after hot rolling and the yield strength at room temperature increases, and as a result, the yield ratio increases, it has been difficult to obtain a steel sheet having a low yield ratio.
In particular, when steel is subjected to cold working before it is actually used as a building structure, the strain introduced by the cold working causes the yield strength of the steel to be higher than at the time of manufacture, and the actual usage environment Low yield ratio cannot be achieved. Therefore, when cold working (for example, working to a closed section such as a circle or a square) is performed before actual use, it is necessary to achieve a lower yield strength, that is, a lower yield ratio at the stage of completing the production.

Further, 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 impossible to provide an inexpensive steel material for building structural members. Further, even in the steel sheet according to the invention described in JP-A-5-222484, the yield strength at the time of pipe making increases greatly,
There is a problem that a sufficient low yield ratio cannot be obtained after pipe formation. Also, the invention described in JP-A-9-41035 is
Since it has a polygonal ferrite phase single phase microstructure, high toughness and low YR are achieved, but there is a problem that strength at high temperatures is not considered.

In view of the state of the art, a specific object of the present invention is to provide a steel sheet having a yield ratio (YR) at room temperature of 75% or less and an equivalent strain of 5% after cold working of 5%. %, A steel material having a low YR and high high-temperature strength, a low YR having a YR at room temperature of 90% or less after pipe formation, and 60%.
An object of the present invention is to provide a steel pipe having a high high-temperature strength of a yield stress at 0 ° C. of 197 MPa or more, and an inexpensive production method thereof.

[0007]

Means for Solving the Problems As a result of repeated studies and experiments, the present inventors have found that C contained in steel is reduced,
It has been found that a steel material having a low yield ratio at room temperature and excellent strength characteristics at high temperatures can be obtained by allowing Nb to be present in a solid solution state and appropriately controlling the microstructure of the steel material.

That is, the gist of the present invention is as follows. (1) By weight%, C: 0.03% or less, Si: 1% or less, Mn: 0.1 to 2%, S: 0.02% or less, Al: 0.01 to 0.1%, Nb : 0.04 to 1%, the balance being Fe-based steel and Nb
The amount of addition satisfies the following expression (1), the microstructure of which is a mixed structure of polygonal ferrite and a low-temperature generation phase containing bainite, and the area ratio of polygonal ferrite in the thickness direction average is 10% or more and 95% or more. % Or less, a low yield ratio type refractory steel material. Nb ≧ 0.1 + 7.74C-1.94Ti + 6.63N (1)

(2) By weight%, one or two of Ti: 0.25% or less, B: 0.0001 to 0.01%, and the Nb content satisfies the following equation (2): The microstructure is a mixed structure of polygonal ferrite and a low-temperature generation phase containing bainite, and the area ratio of polygonal ferrite in the thickness direction average is 10% or more;
The low-yield-ratio type refractory steel according to the above (1), which is not more than 5%. Nb ≧ 0.1 + 7.74C-1.94Ti + 6.63 (N-1.30B) (2) (3) By weight%, Cu: 2% or less, Ni: 1.5% or less, Mo: 1 % Or less; V: 0.25% or less; Cr: 1% or less. The low yielding as described in (1) or (2) above, comprising a total of 2.5% by weight or less. Specific type refractory steel.

(4) Ca: 0.0005-0.005%, Rem: 0.00% by weight
(1) characterized in that it contains 1 to 0.02% of one or two kinds.
The low-yield-ratio type refractory steel material according to any one of (1) to (3). (5) When the YR at room temperature is 75% or less and the equivalent strain is 5
% YR at room temperature even after cold working of 90% or less, the low yield ratio type refractory steel material according to any one of the above (1) to (4).

(6) Any one of the above (1) to (5)
In the production of the steel material described in the item, the cast slab is heated as it is as cast or once cooled to the Ar ₃ transformation point or less, and when working into a predetermined shape by hot working, the working end temperature is Ar _{3 to} -50 ° C or higher, and then at a cooling rate of 0.1 to 50 ° C / sec.
A method for producing a low-yield-ratio type refractory steel material, characterized in that the steel material is cooled to not more than 00 ° C. (7) The method for producing a low-yield-ratio refractory steel material according to (6), wherein the hot-worked steel material is cold-worked after cooling. (8) The method for producing a low yield ratio type refractory steel material according to (7), wherein the heat treatment is performed at a temperature of 450 ° C. or more and 950 ° C. or less.

(9) Any one of the above (1) to (4)
A low yield ratio type refractory steel pipe, wherein the steel material according to the paragraph is a steel pipe. (10) The low yield ratio type refractory steel pipe according to (9), wherein the YR at room temperature is 90% or less and the yield strength at 600 ° C. is 197 MPa or more. (11) In producing the steel pipe according to the above (9) or (10), the steel slab after casting may be used as cast or once Ar.
After being cooled to ₃ transformation points or less, it is heated again, and when working into a steel pipe having a predetermined cross-sectional shape by hot working, the working end temperature is Ar ₃ -50 ° C. or higher, and then 0.1 ° C./se.
c. A method for producing a low yield ratio type refractory steel pipe, wherein the pipe is cooled to 700 ° C. or less at a cooling rate of 50 ° C./sec or less.

(12) In producing the steel pipe according to the above (9) or (10), when the cast slab is heated as it is as cast or once cooled to the Ar ₃ transformation point or lower and then hot worked. the working end temperature Ar ₃ -5
0 ° C or higher, then 0.1 ° C / sec or higher, 50 ° C / se
c. A method for producing a low yield ratio type refractory steel pipe, comprising welding a hot-rolled steel sheet produced by cooling to 700 ° C. or less at a cooling rate of not more than to form a steel pipe having a predetermined sectional shape. (13) The method for producing a low-yield-ratio type refractory steel pipe according to (12), wherein the hot-rolled steel sheet is welded into a steel pipe, and then formed into a steel pipe having a predetermined cross-sectional shape by cold forming. . (14) The method for producing a low-yield-ratio type refractory steel pipe according to (12), wherein the hot-rolled steel sheet is welded into a steel pipe having a circular cross section, and then formed into a rectangular cross section by roll forming.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The steel material and the steel pipe of the present invention increase the high-temperature strength by the interaction with dislocation at high temperature by the solid solution Nb, and at the same time, the solid solution C and the solid solution N
And a low yield strength at room temperature, that is, a low YR, is achieved by reducing the initial movable dislocations introduced in the manufacturing process and avoiding sticking.

First, the reasons for limiting the chemical composition of steel materials will be described. C can combine with other additional elements to form precipitates, or combine with iron to precipitate as cementite, thereby increasing the strength of steel at room temperature and high temperature.
When C exceeds 0.03% by weight (hereinafter, the weight% is abbreviated as%), as described later, the amount of added elements for securing solid solution Nb becomes unnecessarily large, and the room temperature strength of the steel material decreases. Not only does it increase more than necessary, but it becomes difficult to maintain the YR at room temperature (YR = yield strength YS in tensile test / maximum strength TS × 100) at 75% or less, and furthermore, there is an economic disadvantage. , C is 0.03% or less, preferably 0.02% or less.

In particular, when cold working is performed after production is completed and before actual use, C is added to reduce YR after cold working.
Is desirably 0.015% or less. Although the lower limit of C is not particularly limited, C is 0.0005%
If the content is set below, a large load is applied to a process such as degassing in steelmaking, and the production cost is increased. Therefore, it is desirable that C is more than 0.0005%.

Si is used as a deoxidizing material and is a solid solution strengthening element, and can increase the strength of steel at relatively low cost. However, a large amount of Si increases the yield strength at room temperature and increases the YR. Higher. If the amount of Si exceeds 1%, the YR at room temperature is kept at 75% or less, and the equivalent strain is 5%.
Since it becomes difficult to maintain the YR after cold working at 90% or less, the addition amount is set to 1% or less. Further, when particularly high surface quality, avoidance of plating cracks during hot-dip plating, and the like are strongly required, the amount of Si added is desirably 0.02% or less.

Mn, like Si, is a relatively inexpensive solid solution strengthening element. Since setting the amount of Mn addition to less than 0.1% leads to an increase in cost in the process, 0.1% is set as the lower limit of the Mn addition amount. On the other hand, if the Mn content exceeds 2%, the hardenability of the steel becomes unnecessarily high, a predetermined microstructure cannot be obtained, and the upper limit of the Mn content is increased by 2% in order to increase YR at room temperature. And

S is an element that is inevitably contained and causes deterioration of workability and toughness. Therefore, it is desirable to reduce S as much as possible. However, the addition amount is 0.02
% Or less, the above-mentioned tendency of material deterioration is saturated, so the upper limit of the addition amount is set to 0.02%. In the case where particularly strict cold workability is required, it is desirable that the amount of S added be 0.01% or less.

Al is used together with Si as a deoxidizing agent, but in order to exhibit this effect, 0.01%
% Or more. On the other hand, if the addition amount of Al exceeds 0.1%, the amount of Al-based oxides in the steel increases, and particularly the toughness is deteriorated. Therefore, the upper limit of the addition amount is set to 0.1%.

Nb is the most important additive element in the present invention. Nb is usually added as a precipitation strengthening element in many cases, and in this case, it is general that the component is designed so that almost all the Nb element takes the form of a precipitate by combining with C or N. . However, such precipitates that increase the strength of steel also increase the yield strength at room temperature, making it difficult to obtain a low YR.

On the other hand, according to the study of the present inventors, Nb existing in a solid solution state effectively interacts with dislocations at the time of deformation at a high temperature, and the movement of dislocations and the movement of ferrite grain boundaries are inhibited. Effective high suppression achieves high high-temperature strength. Therefore, it is necessary to adjust the amount of Nb added so that the added Nb is not consumed by bonding with C or N. To increase the strength at high temperatures, it is desirable to increase the amount of Nb added. However, if the amount exceeds 1%, the effect of increasing the high-temperature strength is saturated and the production cost is increased. Was set to 1% or less. When the amount of Nb added is 0.
When it is less than 04%, a high high-temperature strength cannot be obtained even by adjusting other additive elements, so the lower limit of the additive amount is set to 0.04%.

Ti, which is selectively added, combines with C and N which may combine with Nb, and has the function of increasing the high-temperature strength by reducing the waste of solid-solution Nb effective for increasing the strength at high temperatures. However, if the addition amount exceeds 0.25%, the workability is degraded, the steel material strength is unnecessarily increased, and the Y
R increases, the upper limit of the amount added is 0.25.
%.

The selectively added B binds to N which may bind to Nb, and not only reduces the waste of solid solution Nb effective for increasing the strength at high temperatures, but also allows the addition of Nb in combination with Nb. Promotes the effect of solid solution Nb on high temperature strength. Although the reason for this is not clear at present, the effect is not recognized when the amount of B added is less than 0.0001%, and the effect is saturated when the amount of B exceeds 0.01%. It is preferably set to 0001 to 0.01%.

In addition to the above components, C which is selectively added
u, Ni, Mo, V, and Cr are effective strengthening elements for steel materials, and also have a function of increasing high-temperature strength. However, with the addition amount of these elements, the addition amount of Cu exceeds 2%,
When the Ni content exceeds 1.5%, the Mo content exceeds 1%, the V content exceeds 0.25%, or the Cr content exceeds 1%, the yield strength at room temperature increases. It becomes difficult to keep the YR at room temperature at 75% or less and the YR after 5% cold working at 90% or less at a considerable strain. The upper limit was set. Also, even in the above range, when the total amount of one or more of these additives exceeds 2.5%, the amount of increase in yield strength at room temperature increases, and the YR at room temperature is 75% or less. , And it becomes difficult to maintain the YR after cold working of 5% with a considerable strain at 90% or less, so this is set as the upper limit of the total amount of one or more of the above-mentioned elements added. These elements may be added aggressively, or the mixing from scrap or the like may be used effectively.

Ca and Rem, which are selectively added, are elements that improve the sour resistance, toughness, weldability, and the like by controlling the form of sulfide. However,
When Ca is less than 0.0005% and Rem is less than 0.001%, the effect is not exhibited.
If the content exceeds 05% and Rem exceeds 0.02%, not only these effects are saturated, but also the toughness is deteriorated due to oxides.

Next, the manufacturing conditions of the present invention will be described below. When the components are adjusted in the above-mentioned limited range, since the function of increasing the strength at high temperature is in the solid solution Nb as described above, it is necessary to secure a certain amount of the solid solution Nb. Regardless of the presence or absence of cold working (corresponding to 5% cold working with equivalent strain), the ratio of the yield strength at room temperature to the yield strength at 600 ° C. increases with an increase in the calculated solid solution Nb amount, As shown in FIG. 2, when the following formula (2) is satisfied, the yield strength is less deteriorated, and the low YR
High yield strength at 0 ° C. is compatible. Nb ≧ 0.1 + 7.74C-1.94Ti + 6.63 (N-1.30B) (2) However, when the amount of Ti added and the amount of B added are 0, Ti = 0 and B in the above equations, respectively. = 0. As a result, when the amount of Nb added satisfies the above expression, the YR after cold working of 5% with a substantial strain becomes 90% or less as shown in FIG.

FIGS. 1 and 2 show that, after casting the steels shown in Table 1, hot-rolled steel sheets were produced under the production conditions within the scope of the present invention, and hot-rolled as they were, and cold-rolled at 5% (pipe making into square steel pipes). YR at normal temperature and yield stress at 600 ° C. were measured, and their ratio (yield stress at 600 ° C./YS at normal temperature) was measured. Is plotted on the vertical axis in FIG. 2 and YR after 5% cold rolling is plotted on the vertical axis in FIG. 1, and the calculated solid solution Nb amount = left side−right side of the above equation = Nb− ｛0.1 + 7.
74C-1.94Ti + 6.63 (N-1.30B)}
Was plotted on the horizontal axis.

In order to obtain such a steel material, it is essential to appropriately control the microstructure of the steel material. The microstructure of the steel of the present invention has a polygonal ferrite (having a clear grain boundary when observed at a position corresponding to the L section in hot rolling;
A low-temperature generation phase containing bainite (here, low-temperature other than polygonal ferrite) in addition to a grain having an axial ratio of 2.5 or less, which is the ratio of the grain size in the thickness direction to the grain thickness direction, is defined as polygonal ferrite. The generated phase has a mixed structure of bainite, massive ferrite, martensite, and the like.) Examples of these organization photographs are shown in FIG.

Most of C is formed by carbide forming elements such as Nb.
In a steel in which is fixed in the form of precipitates, bainite, massive ferrite and martensite have very similar microstructures. In the steel within the scope of the present invention, most of the structure other than polygonal ferrite determined by an optical microscope is bainite and massive ferrite, and as shown in FIG.
If the axial ratio is more than 2.5 or the axial ratio is less than 2.5, the grain boundary may not be linear, or the grain boundary may be corroded strongly due to defects in the grains. When observed at a magnification of an electron microscope level, the inside of the bainite grains may or may not contain carbide, and an island-like structure containing martensite and partially austenite may be observed.

When the composition of the steel satisfies all of the above requirements, the area ratio of polygonal ferrite exceeds 95% on average in the sheet thickness direction, as described later, when observed at a position corresponding to the L-section in hot rolling. In the case of, a sufficiently high high-temperature strength cannot be obtained, and particularly when cold working is performed, 600 ° C
The ratio of YS at room temperature to YS at room temperature is low, and low Y at room temperature.
Since it is difficult to achieve both R and high yield strength at high temperatures, this is set as the upper limit of the polygonal ferrite area ratio in the average in the thickness direction.

When the area ratio of polygonal ferrite decreases, the strength at high temperatures increases, but at the same time, the yield strength at room temperature increases. In particular, when the average area ratio of polygonal ferrite in the thickness direction is less than 10%, the amount of increase in the yield strength at room temperature increases, and the YR at room temperature is maintained at 75% or less, and the equivalent strain is 5%. 90% YR after cold working
Since it is difficult to maintain the area ratio below, this is set as the lower limit of the polygonal ferrite area ratio on the average in the thickness direction.

In order to obtain a microstructure in such a range, an appropriate combination of components and manufacturing conditions is required. In particular, Nb, Mn, and B are effective elements for obtaining a low-temperature generation phase containing bainite. If each of them is within the scope of the present invention, the area of polygonal ferrite can be reduced within a wide range of production conditions as described below. The ratio can be limited to 10% or more and 95% or less, and the remainder can be a low-temperature generation phase containing bainite.

The reason why the high-temperature strength, particularly the high-temperature strength after cold working becomes high when a low-temperature generation phase containing bainite having such an area ratio is present is not clear at present, but is not clear. Clustered Nb and Nb
And C aggregates interact with the dislocations associated with the transformation introduced into the grains of the low-temperature formed phase containing bainite, and into the ferrite grains surrounding the low-temperature formed phase due to the formation of these low-temperature formed phases. It is thought that the dislocations introduced by GaN are hard to recover even at high temperatures.

Further, in the case where the steel material has a microstructure containing more than 95% of polygonal ferrite, the element which suppresses the transformation of austenite from austenite to ferrite such as solid solution Nb as in the steel material of the present invention. If it is contained in a relatively large amount, the microstructure of the weld zone and the heat affected zone (HAZ zone) changes due to the effect of the rapid thermal history after welding during welding, and it hardens or conversely softens. Or you may. However, the steel of the present invention contains an appropriate amount of a low-temperature generation phase from the beginning and, at the same time, reduces the solid solution C and N in the steel. The hardness change of the HAZ portion is small, and as a result, various material deterioration (toughness, fatigue characteristics, workability, etc.) of the welded portion is suppressed.

The microstructure was determined at a position corresponding to the L-section of the hot-rolled steel sheet from the thickness surface toward the center of the thickness.
Three places of 0.1 times, 0.25 times the plate thickness and the center of the plate thickness were observed with an optical microscope, and after photographing, the area ratio of each tissue at each position was measured using a point count method and averaged. . At this time, the measurement of the tissue area ratio may be performed using image processing.

In the case of producing such a steel material, a steel having the above-mentioned composition is cast, and the obtained steel slab is directly or once cooled to an Ar ₃ transformation point including room temperature and then reheated, followed by hot working. I do. Although the reheating temperature at this time is not particularly limited, considering the productivity and the production cost, the reheating temperature is 100%.
A range of 0 ° C to 1300 ° C is desirable.

When the hot working is performed in the form of a plate, plate-like rolling such as a thick plate or continuous hot rolling may be performed, and a plurality of slabs are connected at the finishing hot rolling entrance of the continuous hot rolling. And hot-rolled continuously. Further, the hot working may be a manufacturing process of a mold steel, a bar, a wire, a steel pipe, or the like, or may be a seamless steel pipe manufacturing, a hot extrusion, or the like.

At this time, the end temperature of the hot working is set to an Ar ₃ transformation point −50 ° C. or more determined by the composition of the steel. If the hot working completion temperature is lower than this, not only the ferrite working structure remains in the steel material, and not only the workability in the cold deteriorates, but also the YR at room temperature increases. It is the lower limit of the cold working end temperature. The upper and lower limits of the hot working start temperature and the upper limit of the end temperature are not particularly limited, but from the viewpoint of productivity and the surface quality due to scale, the hot working end temperature is desirably 1000 ° C. or lower.

After the hot working, the steel material is cooled to room temperature. At this time, if the average cooling rate to 700 ° C. or less after the hot working is less than 0.1 ° C./sec, the microstructure during the cooling is reduced. Since the coarsening progressed, the strength of the steel material was unnecessarily reduced, and the toughness of the steel material was deteriorated, this was set as the lower limit of the cooling rate. On the other hand, if the average cooling rate to 700 ° C. or less after hot working exceeds 50 ° C./sec, the area ratio of a low-temperature generation phase such as bainite is increased, and an appropriate microstructure cannot be obtained. The upper limit of the cooling rate was set.

If the cooling end temperature is higher than 700 ° C., the microstructure becomes coarse and the toughness is deteriorated, and it is difficult to achieve both low YR at room temperature after cold working and high yield strength at 600 ° C. Therefore, this was set as the upper limit. For the purpose of increasing the surface quality by reducing the scale of the final steel surface, it is desirable to limit the cooling end temperature to 650 ° C. or lower. Although there is no particular need to set the lower limit of the cooling end temperature, if the cooling end temperature is 100 ° C. or lower, an unnecessarily high cooling capacity is required within a limited manufacturing process length, and this is an economical disadvantage. 100 ° C. to produce
Ultra is desirable. After the cooling is completed, the winding process is performed in the case of continuous hot rolling, and air cooling is performed in other cases.
Desirably, the temperature is not lower than 00 ° C.

When such a steel material is formed into a predetermined shape by cold working after hot working, a predetermined cold working (cold rolling) is applied to the hot worked steel material manufactured by the above method. , Cold bending, cold pressing, cold forging, etc.) and heat treatment is performed as it is or afterwards. At this time, if the heat treatment temperature is lower than 450 ° C., the YR decrease at room temperature is not sufficient, and if it exceeds 950 ° C., the production cost is increased.

When a steel pipe is manufactured by using such a steel material, when a steel pipe having a predetermined cross-sectional shape is formed by hot working, a method according to the above-described hot working conditions or the above-described manufacturing method can be used. After cold-working the steel plate manufactured in the above, the steel pipe may be formed by welding, or once formed into the shape of a steel pipe by welding, and then further processed into a predetermined cross-sectional shape by cold working. After that, the case where a square steel pipe is formed by roll forming is also included. Here, cold working means cold roll forming, cold press forming,
Including cold forging. Further, these steel materials and steel pipes may be used as they are or after being subjected to a surface treatment such as hot-dip galvanizing.

[0044]

EXAMPLES Steel of each component shown in Table 1 was cast, and the steel was cast in Table 2 (Table 2).
The results of examining the mechanical properties at room temperature and 600 ° C. of the steel sheet that has been hot-rolled under the conditions shown in Table 1, Table 2-2) are shown in the table. Cold work of 5% in equivalent strain was selected as representative of the strain introduced into the flat part of the steel pipe when producing steel sheet into square steel pipe. Although the strain actually introduced is considered to be about 2.5% to 6% in terms of equivalent strain, it has been found that an evaluation with an equivalent strain of about 5% is appropriate in order to represent the characteristics of the steel material. Here, 5% cold working was selected. The method of imparting cold working compared bending, pressing, and cold rolling, which are close to actual square steel pipe production.
Since there was no difference in the YR at normal temperature and the yield strength at 600 ° C. in these working forms, cold working was represented by cold rolling of 5% here.

Steel Nos. 1 to 19 which are the component range of the present invention
As shown in Table 2, when the production conditions are within the range of the present invention, the YR at room temperature of the as-produced steel sheet (indicated as unprocessed material YR (%) in Table 2) is 75%. YR after cold working of 5% with equivalent strain (indicated as 5% pre-processed material YR (%) in Table 2) is 90% or less and 5% pre-processed material at 600 ° C. It can be seen that the high-temperature strength of the sample has a high temperature strength of 197 MPa or more.

On the other hand, in steel Nos. 20 to 27 in Table 1, any one of the components was out of the range of the present invention. As a result, as shown in Table 2, even if the production conditions were in the range of the present invention, Either the as-manufactured steel sheet at room temperature or the YR after 5% cold working with considerable strain is outside the scope of the present invention.

Further, steel No. 2 having a component within the scope of the present invention was used.
6 (Ar ₃ = 793 ° C.), any of the hot rolling completion temperature, cooling rate, and cooling completion temperature are out of the range of the present invention.
Either the YR at room temperature of the as-manufactured steel sheet finally obtained or the YR after 5% cold working with considerable strain falls outside the scope of the present invention. The Ar3 temperature (° C.) was Ar3 = 901−
325 ×% C + 33 ×% Si−92 × (% Mn +% Ni
/ 2 +% Cr / 2 +% Cu / 2 +% Mo / 2 +% Nb /
Calculated using 2).

Table 3 shows that steels Nos. 6, 9, and 21 selected from the steel materials shown in Table 1 were formed into hot-rolled steel sheets, then formed into circular steel pipes by welding, and formed into square steel pipes by cold roll forming. After that, a sample was taken from the flat part of the steel pipe and showed the investigated mechanical properties. Even if the ratio between the plate thickness and the outer diameter of the square steel pipe varies, the steel No.
Nos. 6 and 9 have a flat portion with a YR of 90% or less after pipe formation and have a temperature of 600 ° C.
Yield strength of the flat part becomes 197 MPa or more. However, steel No. 21 of the comparative example which is out of the range of the present invention is:
The YR at room temperature, the YR after pipe formation, and the yield strength at high temperature after pipe formation fall outside the scope of the present invention. It can also be seen that the steel pipe of the present invention also has a very high absorption energy at 0 ° C. In this experiment, the mechanical test at room temperature was performed according to JIS Z2241 using a JIS No. 5 test piece, and the tensile test at 600 ° C. was performed according to JIS G0567. The impact test was performed according to JISZ2202.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

[0052]

[Table 4]

[0053]

As described above, according to the present invention, the YR at room temperature as-produced and after cold working of 5% with an equivalent strain is obtained.
Low and high yield stress at high temperature, and steel pipe with low YR after pipe forming and high yield stress at high temperature, and it is excellent in civil engineering and construction fields where these characteristics are required. The effect is exhibited.

[Brief description of the drawings]

FIG. 1 Calculated solid solution Nb amount of steel = Nb- {0.1 + 7.74-1.94Ti +
FIG. 6 is a graph showing the relationship between 6.63 (N-1.30B)} (% by weight) and YR at room temperature after 5% cold working with a considerable strain (indicated as 5% pre-working in the figure).

FIG. 2 Calculated solid solution Nb amount of steel material = Nb- {0.1 + 7.74-1.94Ti +
6.63 (N-1.30B)} (% by weight), as-manufactured (shown as unprocessed material in the figure), and after 5% cold working with equivalent strain (shown as 5% pre-processed material in the figure) Yield strength at 600 ° C (Y
It is the figure which showed the relationship between S) and the ratio of the yield strength at normal temperature.

FIG. 3 is an optical micrograph showing the microstructure of a hot-rolled steel sheet obtained by hot rolling steel No. 6 of Table 1 under the production conditions within the scope of the present invention. The same location in the thickness direction 1/4 from the surface layer is 200 times (upper part in FIG. 3) and 50
The photograph was taken at 0x (lower part in FIG. 3).

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Jun Itami 1 Kimitsu, Kimitsu-shi Nippon Steel Corporation Kimitsu Works F-term (reference) 4K032 AA01 AA02 AA04 AA08 AA11 AA14 AA15 AA16 AA19 AA21 AA22 AA23 AA24 AA29 AA31 AA35 AA36 AA40 BA03 CA02 CA03 CC04 CD01 CD02 CD03 CH04 CH05

Claims (14)

[Claims] 1. In weight%, C: 0.03% or less, Si: 1% or less, Mn: 0.1 to 2%, S: 0.02% or less, Al: 0.01 to 0.1% , Nb: 0.04 to 1%, with the balance being Fe-based steel and Nb
The amount of addition satisfies the following expression (1), the microstructure of which is a mixed structure of polygonal ferrite and a low-temperature generation phase containing bainite, and the area ratio of polygonal ferrite in the thickness direction average is 10% or more and 95% or more. % Or less, a low yield ratio type refractory steel material. Nb ≧ 0.1 + 7.74C-1.94Ti + 6.63N (1) 2. The composition according to claim 1, wherein one or two of Ti: 0.25% or less and B: 0.0001 to 0.01% by weight are contained, and the amount of Nb added satisfies the following formula (2). The low-yield-ratio type refractory steel material according to claim 1, characterized in that: Nb ≧ 0.1 + 7.74C-1.94Ti + 6.63 (N-1.30B) (2) 3. One or more of the following by weight: Cu: 2% or less, Ni: 1.5% or less, Mo: 1% or less, V: 0.25% or less, Cr: 1% or less. The low-yield-ratio type refractory steel material according to claim 1, wherein the steel material contains not more than 2.5% by weight in total. 4. The method according to claim 1, wherein one or two of Ca: 0.0005 to 0.005% and Rem: 0.001 to 0.02% are contained by weight%. The low yield ratio type refractory steel material according to any one of the preceding claims. 5. The YR at room temperature after the cold working of 5% with equivalent strain is 75% or less at room temperature and the YR at room temperature is 90%.
The low yield ratio type refractory steel material according to any one of claims 1 to 4, wherein: 6. In producing the steel material according to any one of claims 1 to 5, the steel slab after casting is heated again as it is as cast, or once cooled to an Ar ₃ transformation point or lower, and then heated again. When processing into a predetermined shape by processing, the processing end temperature is Ar ₃ -50 ° C. or higher, and then 0.1 ° C.
A method for producing a low-yield-ratio-type refractory steel material, comprising cooling to 700 ° C or less at a cooling rate of 50 ° C / sec or more and 50 ° C / sec or less. 7. The method for producing a low-yield-ratio type refractory steel according to claim 6, wherein the hot-worked steel is cold-worked after cooling. 8. The method according to claim 7, wherein heat treatment is performed at a temperature of 450 ° C. or more and 950 ° C. or less after the cold working. 9. A low yield ratio type refractory steel pipe, wherein the steel material according to any one of claims 1 to 4 is a steel pipe. 10. When the YR at room temperature is 90% or less,
The low yield ratio type refractory steel pipe according to claim 9, wherein the yield strength at ℃ is 197 MPa or more. 11. In producing the steel pipe according to claim 9 or 10, the cast slab is cast as it is or once A
After cooling to a temperature below the r ₃ transformation point and then heating again to form a steel pipe having a predetermined cross-sectional shape by hot working, the working end temperature is set to Ar ₃ −50 ° C. or higher, and then 0.1 ° C. /
A method for producing a low yield ratio type refractory steel pipe, wherein the pipe is cooled to 700 ° C. or lower at a cooling rate of 50 ° C./sec to 50 ° C./sec. 12. In producing the steel pipe according to claim 9 or 10, the steel slab after casting is used as it is or once A
After cooling to the r ₃ transformation point or less and then heating again to perform hot working, the working end temperature is set to Ar ₃ −50 ° C. or more, and then 700 ° C. 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-type refractory steel pipe, comprising welding a hot-rolled steel sheet manufactured by cooling the steel pipe to a temperature of not more than 0 ° C or less to form a steel pipe having a predetermined cross-sectional shape. 13. After welding a hot-rolled steel sheet into a steel pipe,
The method for producing a low yield ratio type refractory steel pipe according to claim 12, wherein the steel pipe having a predetermined cross-sectional shape is formed by cold forming. 14. The method for producing a low yield ratio type refractory steel pipe according to claim 12, wherein the hot rolled steel sheet is welded into a steel pipe having a circular cross section, and then formed into a rectangular cross section by roll forming.