JP2764007B2 - Low yield ratio refractory steel sheet for building with excellent weldability and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a refractory steel sheet and a method for producing the same, and more particularly, to a building structure having a low yield ratio and a large uniform elongation at room temperature, as well as a sufficient high temperature resistance and good weldability. The present invention relates to a refractory steel sheet and a method for manufacturing the same.

[0002]

2. Description of the Related Art In general, steel buildings have a reduced strength due to a high temperature at the time of fire, and the required strength of the building cannot be maintained. ing. In conventional Si-Mn-based steel sheets for building structures, when the steel material temperature becomes 350 ° C or more at the time of fire, the proof strength decreases to ／ or less of the normal temperature specification value, and the proof strength (long-term allowable stress) required for the structure is reduced. Because of this, the rise in steel material temperature is suppressed by the refractory coating, but there is a problem that the construction cost increases and the construction period is lengthened.

[0003]

However, according to the "New Fire-Resistant Design Law" enacted in 1987, it is possible to reduce or omit the fire-resistant coating if a steel sheet (fire-resistant steel sheet) having a high proof stress at high temperatures is used. became. This fire-resistant steel plate is mainly used in multi-story parking lots, external steel buildings, atriums and the like.

At present, there is a Cr-Mo steel for boilers and pressure vessels as a steel material having excellent proof stress at high temperatures. However, this steel material has a proof stress at 600 ° C. of ／ or more of the normal temperature standard value, but has a high C content, so that the weldability (weld cracking resistance) is high.
In addition, the toughness of the welded joint is poor, and there is a difficulty in welding. For this reason, several refractory steel sheets which have a high proof stress even at 600 ° C., have excellent weldability, and can be designed and implemented in the same manner as conventional steel have been proposed (Japanese Patent Publication No. 4-50362).

[0005] These steel sheets are required to have a reduced yield ratio in order to ensure seismic performance for construction purposes, and steel sheets satisfying the normal requirement of 80% or less have been developed. But,
Demands on seismic performance are becoming more stringent, and in some cases not only low yield ratios but also increases in uniform elongation are required. For such a request, the refractory steel plate proposed so far has not been given sufficient consideration. on the other hand,
For example, the steel disclosed in Japanese Unexamined Patent Publication No. 3-173715 is manufactured by a controlled rolling method with a composite addition of Cr, Mo and Nb, and has an excellent high temperature proof stress. Not intended for thick materials. Also,
The steel disclosed in Japanese Unexamined Patent Publication No. Hei.
The steel to which o is added is accelerated and cooled from the Ar ₃ transformation point or lower, so that the microstructure is a mixed structure of ferrite and bainite, the yield ratio at room temperature is kept low, and the strength at 600 ° C. is secured. However, since the residual stress of the steel material is high under the condition of accelerated cooling, there is a problem that camber and warping shape defects are apt to occur at the time of cutting by gas cutting.

The present invention solves the above-mentioned problems in the prior art, and has a sufficient yield strength at high temperatures while reducing or omitting the refractory coating on the steel frame, and has a low yield ratio at room temperature and a large uniformity. For building structures that have excellent elongation and excellent weldability, as well as thick steel plates used for high-rise and large-span steel structures, which are excellent not only in weldability but also in stripping properties. An object of the present invention is to provide a method for manufacturing a refractory steel sheet.

[0007]

Means for Solving the Problems In order to achieve the above object, the present inventors have conducted intensive studies and, as a result, have limited the chemical components of the raw material and defined the constituent requirements of the microstructure, thereby making it possible to obtain the microstructure at room temperature. The inventors have found that a refractory steel sheet for a building structure having a low yield ratio, a large uniform elongation, and excellent weldability and sectioning characteristics can be manufactured, thereby completing the present invention.

Conventional refractory steel sheets generally utilize precipitation strengthening of Mo, Nb, etc. in order to ensure their high-temperature strength. However, the addition of these elements increases the hardenability of the steel. The microstructure is mainly composed of bainite, and in many cases, a reduction in yield ratio and an increase in uniform elongation cannot be sufficiently achieved. For improvements in these properties, see 590
It is effective to disperse a soft ferrite phase in a microstructure as is known for N / mm2 grade steel sheets and the like, and a specific means for that purpose is, for example, a two-phase heat treatment method.

Therefore, the present inventors investigated the effect of the microstructure on the yield ratio and uniform elongation. As a result, in order to satisfy the requirements for these properties, it is not enough to simply disperse the ferrite phase, and the difference in hardness between the second phase other than ferrite and the ferrite phase is set to a certain value or more. It has been found that when the difference in hardness exceeds a certain value, remarkable deterioration of toughness occurs, so that it is necessary to set an upper limit for the difference in hardness.

According to the present invention based on the above-mentioned findings, the mass ratio of C: 0.04 to 0.15%, Si: 0.05
0.50%, Mn: 0.50 to 1.60%, P: 0.
020% or less, S: 0.005% or less, Mo: 0.10
0.40%, V: 0.005 to 0.070%, Nb:
0.005 to 0.050%, Ti: 0.005 to 0.0
30% Al: contains 0.01 to 0.10%, and satisfies P _CM (weld crack susceptibility composition) is less than 0.21% represented by the following formula, the chemical composition and the balance Fe and unavoidable impurities The fraction of the second phase other than ferrite is 20% or more, and the difference between the average of the measured values of the Vickers hardness of the second phase at 5 points or more and the Vickers hardness of the ferrite phase is 120 to
The gist of the present invention is a low yield ratio refractory steel sheet for building, which is excellent in weldability and characterized in the range of 260. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B

In the present invention, C: 0.04 by mass ratio.
0.15%, Si: 0.05 to 0.50%, Mn:
0.50 to 1.60%, P: 0.020% or less, S:
0.005% or less, Mo: 0.10 to 0.40%, V:
0.005 to 0.070%, Nb: 0.005 to 0.0
50%, Ti: 0.005 to 0.030%, Al: 0.
0.1 to 0.10%, and further Cu: 0.05 to
0.40%, Ni: 0.05 to 0.40%, Cr: 0.
10-0.50%, Ca: 0.0005-0.0050
Contain one or two of%, P _CM represented by the following formula
(Weld crack susceptibility composition) of 0.21% or less,
It has a chemical composition consisting of the balance of Fe and unavoidable impurities, the fraction of the second phase other than ferrite is 20% or more, and the average of the measured values of Vickers hardness of the second phase at 5 points or more and the ferrite phase Vickers hardness difference is 120-260
And a low yield ratio refractory steel sheet for architectural use having excellent weldability, characterized by being within the range described above. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B

On the other hand, it is effective to control rolled steel sheets regardless of the sheet thickness in order to impart excellent stripping characteristics to fire-resistant steel sheets for building structures. the Ceq forced to increase the (carbon equivalent) and P _CM (weld crack susceptibility composition), weldability is degraded. On the other hand, there is an accelerated cooling method as a means for increasing the strength without increasing Ceq. However, if accelerated cooling is performed, the residual stress of the steel material is high, so that the gas cutting performance is poor. In addition, to improve the cutting characteristics by the accelerated cooling method, accelerated cooling +
The tempering method is said to be effective. However, in this method, the microstructure becomes bainite, and Mo and Nb added for improving the high-temperature strength during tempering precipitate to increase the yield ratio at room temperature. Therefore, as a means of increasing the strength without increasing Ceq and securing a low yield ratio at room temperature, after accelerated cooling, reheating is performed in the temperature range of the A _{C1 to} A _C3 transformation point, and air cooling is performed. Furthermore, it has been found that it is effective to make the microstructure a mixed structure of ferrite and bainite by performing a tempering treatment. As a result, it becomes possible to produce a low yield ratio fire-resistant steel sheet for building structures which is excellent not only in weldability but also in stripping properties, even in thick materials.

The present invention has been made based on the above findings.
0.05 to 0.50%, Mn: 0.50 to 1.60%,
P: 0.020% or less, S: 0.005% or less, Mo:
0.10 to 0.40%, V: 0.005 to 0.070
%, Nb: 0.005 to 0.050%, Ti: 0.00
5~0.030%, Al: contains 0.01~0.10%, P _CM (weld crack susceptibility composition) is 0 represented by the following formula.
A steel slab satisfying 21% or less and having a chemical composition comprising the balance of Fe and unavoidable impurities is heated to a temperature of 1050 ° C. or more, and after rolling is completed in a temperature range of 850 to 950 ° C., the _{Ar 3} transformation point after accelerated cooling to 400 to 550 ° C. at a cooling rate of 3 to 20 ° C. / sec from a temperature above, a _C1 to a
_The gist of the present invention is a method for producing a low yield ratio refractory steel sheet for building with excellent weldability, characterized by reheating in the temperature range of the _C3 transformation point and air cooling. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B

The present invention also provides a method for producing a low yield ratio refractory steel sheet for building having excellent weldability as described above, wherein the steel sheet is reheated in a temperature range of A _{C1 to} A _C3 transformation point, air-cooled, and further cooled to A _C1 transformation state. A method for producing a low yield ratio refractory steel sheet for building having excellent weldability, characterized by performing a tempering treatment at a temperature below the point.

Further, the present invention provides a method of the present invention wherein C: 0.04
0.15%, Si: 0.05 to 0.50%, Mn:
0.50 to 1.60%, P: 0.020% or less, S:
0.005% or less, Mo: 0.10 to 0.40%, V:
0.005 to 0.070%, Nb: 0.005 to 0.0
50%, Ti: 0.005 to 0.030%, Al: 0.
0.1 to 0.10%, and further Cu: 0.05 to
0.40%, Ni: 0.05 to 0.40%, Cr: 0.
10-0.50%, Ca: 0.0005-0.0050
Contain one or two of%, P _CM represented by the following formula
A steel slab satisfying (weld crack susceptibility composition) of 0.21% or less and having a chemical composition consisting of the balance of Fe and unavoidable impurities was heated to a temperature of 1050 ° C. or more,
After the completion of rolling in a temperature range of 0 ° C., at a cooling rate of 3 to 20 ° C. / sec from A _r3 transformation point temperature above 400 to 550 ° C.
A method for producing a low yield ratio refractory steel sheet for building with excellent weldability, characterized in that the steel sheet is re-heated in the temperature range of A _{C1 to} A _C3 transformation point and then air-cooled after accelerated cooling to a temperature of A _{C1 to} A _C3 . P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B

The present invention also provides a method for producing a low yield ratio refractory steel sheet for buildings having excellent weldability as described above, wherein the steel sheet is reheated in a temperature range of A _{C1 to} A _C3 transformation point, air-cooled, and further cooled to A _C1 transformation state. A method for producing a low yield ratio refractory steel sheet for building having excellent weldability, characterized by performing a tempering treatment at a temperature below the point.

[0017]

The structure and operation of the present invention will be described. The reasons for limiting the chemical components in the present invention will be described. C is an element that contributes to an increase in strength, but if it is less than 0.04%, it is difficult to secure the strength. On the other hand, if it exceeds 0.15%, the weldability and toughness are deteriorated. Therefore, the C content is in the range of 0.04 to 0.15%. S
i is an essential element for deoxidation, but if it is less than 0.05%, its effect is small, while if it exceeds 0.50%, weldability is deteriorated. Therefore, the Si content is in the range of 0.05 to 0.50%.

Mn is an element necessary for securing strength and toughness. If less than 0.50%, these effects are small, while if added over 1.60%, weldability and toughness deteriorate. Let it. Therefore, the Mn content is in the range of 0.50 to 1.60%. P is 0.020
%, The weldability and toughness deteriorate. Therefore, the P content is set to 0.020% or less. S is 0.0
If it exceeds 05%, coarse A-based inclusions increase and the toughness is deteriorated. Therefore, the S content is set to 0.005% or less.

Mo is an element indispensable for securing high-temperature strength, and greatly increases the proof stress at 600 ° C. However, if the content is less than 0.10%, such an effect cannot be obtained. On the other hand, if the content exceeds 0.40%, the weldability is deteriorated. Therefore, the Mo content is 0.10 to
The range is 0.40%. V is an element effective for increasing the high-temperature strength due to the precipitation effect. However, if it is less than 0.005%, this effect is small, and if it exceeds 0.070%, the weldability deteriorates. Therefore, the V content is 0.005
-0.070%.

Nb is an element capable of increasing the high-temperature strength due to the precipitation effect and improving the toughness due to grain refinement.
Addition of less than 0.005% reduces these effects, while addition of more than 0.050% deteriorates the weld joint toughness. Therefore, the Nb content is 0.005 to 0.0
The range is 50%.

Ti is an element that forms nitrides and is effective for suppressing the coarsening of austenite grains during heating and improving the toughness by accelerating the formation of ferrite. Less, whereas 0.030%
If added in excess of, the base metal toughness will be degraded. Therefore, the Ti content is in the range of 0.005 to 0.030%. Al is an element that is necessary for deoxidation and contributes to the refinement of crystal grains. However, if less than 0.01%, these effects are small. It deteriorates toughness. Therefore, Al
The content is in the range of 0.01 to 0.10%.

According to the present invention, Cu, Ni, C
One or more of r and Ca may be contained. Cu is an element effective for increasing the strength due to the precipitation effect, but if the content is less than 0.05%, such an effect is small, while if it exceeds 0.40%, hot workability and weldability are impaired. Therefore, the Cu content is 0.05 to
The range is 0.40%. Ni is an element effective for improving the strength and toughness, but such an effect is small at less than 0.05%, while the effect is saturated even if added over 0.40%, and economically wasteful. It is. Therefore, the Ni content is in the range of 0.05 to 0.40%.

Cr is an element effective for increasing the high-temperature strength, but if it is less than 0.10%, this effect is small.
If added in excess of 0%, weldability and weld joint toughness deteriorate. Therefore, the Cr content is 0.10 to 0.50%
Range. Ca is an element that slightly improves the properties in the thickness direction. However, if less than 0.0005%, this effect is small, while if it exceeds 0.0050%, nonmetallic inclusions in the steel increase. Causes internal defects. Therefore, the Ca content is in the range of 0.0005 to 0.0050%. Further in the present invention, a low temperature cracking omitted purpose preheating performed in order to prevent the welding, limiting weld cracking susceptibility composition of (P _CM) below 0.21%.

Next, the reason for limiting the organizational structure in the present invention will be described. As described above, in order to reduce the yield ratio while maintaining the strength of the steel, it is effective to disperse the soft ferrite phase in the hard phase. However, if the dispersion ratio of the ferrite phase is too high, the predetermined strength cannot be secured. Therefore, the fraction of the second phase other than ferrite is set to 2
It is defined as 0% or more.

Next, the reason for limiting the difference in Vickers hardness between the second phase and the ferrite phase will be described. The present inventors have
In order to simultaneously secure a yield ratio of 0% or less and a uniform elongation of 20% or more, the influence of the microstructure on these properties was investigated in detail. That is, by applying various heat treatment methods, steel sheets having different microstructures were produced, and a tensile test, a Charpy impact test, and a hardness measurement of the second phase and the ferrite phase were performed. The chemical composition of the steel sheet used and the heat treatment conditions are shown in Table 1 below.

[0026]

[Table 1] Note 1: Steel chemical composition (% by mass). C: 0.10, Si: 0.20, Mn: 1.05, P: 0.007, S: 0.001, Mo: 0.37, V: 0.046, Nb: 0.020, Ti: _{0.012, Al: 0.030, P CM} : 0.18 Note 2: cooling rate at the time of a weak cooling: 2 ° C. / s, the cooling stop temperature: 580 ° C., Note 3, strongly cooled at a cooling rate: 10 ° C. / S, cooling stop temperature: 450 ° C, Note 4;

The hardness of the second phase was an average of the measured values at five or more points. Also, in order to measure the hardness of a micro area,
The load at the time of measuring Vickers hardness was 5 g. The uniform elongation was determined as the strain at the point where the load decreased to 95% of the maximum load in the stress-strain curve. FIG. 1 shows the relationship between the difference in Vickers hardness between the second phase and the ferrite phase of the steel sheet obtained by these treatments, the yield ratio, and the uniform elongation. FIG. 2 shows the relationship between the difference in Vickers hardness and vTrs (impact characteristics).

As is apparent from FIG. 1, only when the difference between the Vickers hardness of the second phase and the ferrite phase of the steel sheet is 120 or more, a low yield ratio of 80% or less and a large uniform elongation of 20% or more are obtained. It can be seen that they can be secured at the same time. On the other hand, as can be seen from FIG. 2, when the difference between the Vickers hardnesses of the second phase and the ferrite phase exceeds 260 or is less than 70, vTrs becomes −20 ° C. or more, and it is found that remarkable deterioration in toughness occurs. The difference between the Vickers hardness of the second phase and the Vickers hardness of the ferrite phase is limited to the range of 120 to 260 as a range that simultaneously satisfies the above-described targets of yield ratio, uniform elongation, and toughness.

The reason why a low yield ratio of 80% or less and a large uniform elongation of 20% or more can be simultaneously secured only when the difference between the Vickers hardness of the second phase and the ferrite phase is 120 or more is as follows. Can be considered. In other words, when a soft phase is present, the tensile strength decreases according to the mixing rule according to the strength and fraction of each phase, while the yield point is such that strain concentrates on the soft phase at the beginning of deformation and the hard phase Since the soft phase yields prior to the tensile strength, the yield decreases more at the yield point than at the tensile strength. This tendency becomes more remarkable as the difference in hardness between the second phase and the ferrite phase increases, and it is considered that the yield ratio defined by “yield point / tensile strength” decreases. Also,
It is considered that the uniform elongation increases because the amount of work hardening up to the maximum load increases as the yield point decreases.

On the other hand, if the difference in Vickers hardness between the second phase and the ferrite phase exceeds 260, the cause of the remarkable deterioration in toughness is considered as follows. That is, the second phase is a low-temperature transformation product such as martensite or bainite, but if the difference in hardness is large, these are considered to be brittle and deteriorate in toughness. The reason why the toughness is deteriorated even when the difference between the Vickers hardness of the second phase and the ferrite phase is less than 70 is considered that the microstructure is mainly composed of upper bainite having poor toughness.

Next, the reasons for limiting the manufacturing conditions of the present invention will be described. Prior to rolling, the temperature at which the slab is heated is set to a lower limit of 10 in order to form a solid solution of Nb necessary for securing high temperature proof stress.
50 ° C. If the rolling end temperature is lower than 850 ° C., the yield ratio of 80% or less required for building structural steel cannot be secured from the viewpoint of seismic resistance due to the refinement of the structure. As a result, the acoustic anisotropy increases, and the angle of refraction and the flaw detection sensitivity change in ultrasonic angle beam flaw detection, making it difficult to detect a weld defect. on the other hand,
If the rolling end temperature exceeds 950 ° C., the austenite grains become coarse and the base material toughness deteriorates. Therefore, the rolling end temperature is in the range of 850 to 950 ° C.

After the completion of the hot rolling under the above-described conditions, the steel sheet is subjected to accelerated cooling. When the cooling start temperature is lower than the Ar3 transformation point, the rise in high-temperature strength is reduced due to the formation of ferrite. Therefore, the cooling start temperature is set to the Ar3 transformation point or higher. Further, when the cooling rate exceeds 20 ° C./sec, the strength exceeds the upper limit of the specification, while when the cooling rate is less than 3 ° C./sec, the increase in strength is small. Therefore, the cooling rate is in the range of 3 to 20 ° C./sec. Further, the cooling stop temperature is 400 to 550 ° C.
Range. This is because if the temperature is less than 400 ° C., the base material toughness is deteriorated and it is difficult to perform hot straightening after accelerated cooling. On the other hand, if the temperature exceeds 550 ° C., the increase in strength is small.

After the heat treatment, A _{C1 to} A _{C3 are} added to make the microstructure of the steel material a mixed structure of ferrite and bainite.
Re-heat at the temperature of the two-phase region at the transformation point and perform air cooling. The reason why the reheating temperature is limited to the two-phase region temperature is that the yield ratio at room temperature is high below the A _C1 transformation point, while the strength is reduced above the A _C3 transformation point. Further, when it is necessary to reduce the residual stress in the steel material generated by the heat treatment in the previous stage and improve the toughness, a tempering treatment is performed. The tempering temperature at this time is lower than the A _C1 transformation point, but is preferably in the range of 500 to 650 ° C.

[0034]

EXAMPLES Examples of the present invention will be described below, but the present invention is not limited thereto. Example 1 A test steel sheet is a JIS SM490 grade steel sheet having a thickness of 55 to 80 mm and having a difference between hardness and chemical composition shown in Table 2 below (standard value; yield point or 0.2% proof stress: 295 N / mm ² or more) Tensile strength: 490 to 610 N / mm ² ).
Specimens were sampled from these steel plates and subjected to tensile tests at room temperature,
Charpy impact test, high temperature tensile test at 600 ° C,
The highest hardness test was performed. The results are shown in Table 3 below.
The maximum hardness test was performed according to JIS Z 3101. The uniform elongation was obtained as a strain at a point where the load was reduced to 95% of the maximum load in the stress-strain curve.

[0035]

[Table 2]

[0036]

[Table 3]

As is clear from Table 3, the steels A to G of the present invention
Each have a proof stress at 600 ° C. of 2/3 of the normal temperature specification value.
(197 N / mm ² ) or more and high temperature proof stress, and the tensile properties at room temperature are 490 N / mm ² class specification value (yield point or 0.2% proof stress; 295 N / mm ² or more, tensile strength; 490-610 N / mm ² ), and 80
% And a uniform elongation of 20% or more.
In addition, the fracture surface transition temperature (vTrs) of the base material in the Charpy impact test is as good as −30 ° C. or less, the maximum hardness is less than 300, and it has excellent weldability.

On the other hand, Comparative Steel H because the C content and P _CM is out to enhance the scope of the present invention, is poor weldability. In Comparative Steel I, Mo is higher than the scope of the present invention,
Poor weldability. The comparative steel J has poor base metal toughness because no Ti is added. In comparison steels K and L, the former is M
o, but the latter was not added with Nb.
Low proof stress at 00 ° C. Furthermore, since the comparative steel M also does not contain V, the yield strength at 600 ° C. is low.

The comparative steels A1, A2 and D1, D
Nos. 2 and D3 were produced by changing the heat treatment method using the same chemical components as the steels A and D of the present invention, respectively. Comparative steels A1 and D1 have poor toughness because the difference in Vickers hardness between the second phase and ferrite phase exceeds 260, and comparative steels A2 and D2 have a yield ratio because the difference in Vickers hardness is less than 120. And uniform elongation is small. In addition, comparative steel D3
Has a good yield ratio and uniform elongation, but does not have a tensile strength of 490 N / mm ² because of a low fraction of the second phase.

Example 2 In carrying out the method of the present invention, a test steel sheet was prepared by finishing a slab having the chemical components shown in Table 4 below to a thickness of 60 mm according to the heating, rolling and heat treatment conditions shown in Table 5 below. . Test pieces were taken from these steel sheets and subjected to a tensile test at normal temperature, a Charpy impact test, a high-temperature tensile test at 600 ° C., and a maximum hardness test. The results are shown in Table 6 below. The maximum hardness test was performed according to JIS Z 3101. Table 4 shows the chemical compositions of the invention steels A to F and comparative steels G to L. Table 5 shows the heating, rolling and heat treatment conditions.
Table 6 shows tensile properties at normal temperature, impact properties, high temperature properties, and weldability.

As is clear from Table 6, the steels A to F of the present invention were used.
Each have a proof stress at 600 ° C. of 2/3 of the normal temperature specification value.
(197 N / mm ² ) or more, high temperature proof stress, and a tensile property at room temperature of 490 N / mm ² class, and a yield ratio of 80% or less required for building structural steel from the viewpoint of earthquake resistance. Are satisfied enough. Also, the vTrs of the base material in the Charpy impact test is as good as −25 ° C. or less,
The maximum hardness is also less than HV289, and has excellent weldability.

On the other hand, Comparative Steel G Since the amount of C and P _CM is out to enhance the scope of the present invention, is poor base metal toughness and weldability. The comparative steel H has poor weldability because Mo is slightly out of the range of the present invention. The comparative steel I has poor base metal toughness because no Ti is added. In addition, comparative steel J, K
In the former, Mo is not added to the former, and Nb is not added to the latter, so that the yield strength at 600 ° C. is low. Furthermore, since the comparative steel L also does not contain V, the yield strength at 600 ° C. is low.

[0043]

[Table 4]

[0044]

[Table 5]

[0045]

[Table 6]

The comparative steels A1 to A11 correspond to the steel A of the present invention.
Was manufactured outside the manufacturing conditions of the present invention. The comparative steel A1 has a high room temperature strength and a high yield ratio of 85% since the reheat treatment is not performed at the two-phase region temperature of the A _{C1 to} A _C3 transformation point while maintaining the accelerated cooling. Comparative steel A2
Since the heating temperature is slightly lower than the range of the present invention, Nb does not form a solid solution, and the proof stress at 600 ° C. is low.

Since the rolling end temperature of the comparative steel A3 is slightly lower than the range of the present invention, the structure becomes finer, and the yield ratio at room temperature exceeds 80%. Furthermore, comparative steel A4 is
Since the rolling end temperature is out of the range of the present invention, austenite becomes coarse and the base material toughness is poor. Comparative steel A5 has a cooling start temperature lower than the _Ar3 transformation point, and
Low proof stress at 00 ° C.

The comparative steels A6 and A7 have a cooling stop temperature out of the range of the present invention. A6 has a low temperature of 375 ° C. and thus has poor base metal toughness, while A7 has a high temperature of 575 ° C. and has a normal temperature. And strength at 600 ° C. is low. Also,
The comparative steels A8 and A9 had a cooling rate outside the range of the present invention, and A8 was as low as 2 ° C./sec.
Since the strength at 0 ° C. is low, and A9 is as high as 22 ° C./sec, the room temperature strength exceeds the upper limit of the standard. Further, the comparative steels A10 and A11 have reheating temperatures outside the range of the present invention, and since A10 is 680 ° C. and lower than the A _C1 transformation point, the yield ratio at room temperature is as high as 82%, while
11 is 890 ° C., which exceeds the A _C3 transformation point, and therefore has low strength at room temperature and 600 ° C.

[0049]

Since the present invention is constructed as described above, it has high strength even at a high temperature of 600 ° C. even in a thick steel material used for a high-rise building or a large span steel frame building. In addition, it is possible to provide a steel material with a low yield ratio at room temperature and excellent weldability, and it is possible to reduce or omit the fireproof coating, and to secure the safety of the structure in terms of earthquake resistance, It has the same workability as conventional steel materials for welded structures, and is extremely useful in industry.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing the relationship between a difference in Vickers hardness between a second phase and a ferrite phase of a steel sheet of the present invention, a yield ratio, and uniform elongation.

FIG. 2 shows the difference between Vickers hardness of the steel sheet of the present invention and vTrs.
FIG.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-7-173532 (JP, A) JP-A-3-173715 (JP, A) JP-A-3-6322 (JP, A) JP-A-2- 263916 (JP, A) JP-A-2-254134 (JP, A) JP-A-2-254133 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00-38 / 60 C21D 8/02

Claims (6)

(57) [Claims] 1. A mass ratio of C: 0.04 to 0.15.
%, Si: 0.05 to 0.50%, Mn: 0.50 to 0.5%
1.60%, P: 0.020% or less, S: 0.005%
Hereinafter, Mo: 0.10 to 0.40%, V: 0.005 to
0.070%, Nb: 0.005 to 0.050%, T
i: 0.005 to 0.030%, Al: 0.01 to 0.
Containing 10% satisfy the P _CM (weld crack susceptibility composition) is less than 0.21% represented by the following formula, it has the chemical composition and the balance Fe and unavoidable impurities, other than the ferrite phase 2 The weldability, wherein the difference between the average of the measured values of the Vickers hardness of the second phase at five or more points and the Vickers hardness of the ferrite phase is 120 to 260. Excellent low yield ratio refractory steel sheet for construction. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B 2. C: 0.04 to 0.15 by mass ratio
%, Si: 0.05 to 0.50%, Mn: 0.50 to 0.5%
1.60%, P: 0.020% or less, S: 0.005%
Hereinafter, Mo: 0.10 to 0.40%, V: 0.005 to
0.070%, Nb: 0.005 to 0.050%, T
i: 0.005 to 0.030%, Al: 0.01 to 0.
10%, Cu: 0.05 to 0.40%,
Ni: 0.05 to 0.40%, Cr: 0.10 to 0.5
0% Ca: contain one or two of from 0.0005 to 0.0050%, and satisfies P _CM (weld crack susceptibility composition) is less than 0.21% represented by the following formula, the remainder Fe and It has a chemical composition of unavoidable impurities, the fraction of the second phase other than ferrite is 20% or more, the average of the measured values of Vickers hardness of the second phase at 5 points or more, and the Vickers hardness of the ferrite phase. Is low in the range of 120 to 260. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B 3. A mass ratio of C: 0.04 to 0.15.
%, Si: 0.05 to 0.50%, Mn: 0.50 to 0.5%
1.60%, P: 0.020% or less, S: 0.005%
Hereinafter, Mo: 0.10 to 0.40%, V: 0.005 to
0.070%, Nb: 0.005 to 0.050%, T
i: 0.005 to 0.030%, Al: 0.01 to 0.
Containing 10% satisfied P _CM (weld crack susceptibility composition) is less than 0.21% represented by the following formula, a steel slab having a chemical composition and the balance Fe and unavoidable impurities, 1050
° C. was heated to a temperature equal to or higher than, after completion of the rolling at a temperature range of 850 to 950 ° C., 3 to 20 from the A _r3 transformation point or above the temperature
A low yield ratio refractory for architectural use with excellent weldability, characterized in that after accelerated cooling to 400 to 550 ° C. at a cooling rate of 400 ° C./sec, it is reheated in the temperature range of A _{C1 to} A _C3 transformation point and air cooled. Steel plate manufacturing method. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B 4. The method for producing a low yield ratio refractory steel sheet for building according to claim 3, which has excellent weldability, reheats in a temperature range of A _{C1 to} A _C3 transformation point, air-cools, and further cools A _C1. A method for producing a low yield ratio refractory steel sheet for building having excellent weldability, characterized by performing a tempering treatment at a temperature lower than a transformation point. 5. A mass ratio of C: 0.04 to 0.15.
%, Si: 0.05 to 0.50%, Mn: 0.50 to 0.5%
1.60%, P: 0.020% or less, S: 0.005%
Hereinafter, Mo: 0.10 to 0.40%, V: 0.005 to
0.070%, Nb: 0.005 to 0.050%, T
i: 0.005 to 0.030%, Al: 0.01 to 0.
10%, Cu: 0.05 to 0.40%,
Ni: 0.05 to 0.40%, Cr: 0.10 to 0.5
0%, Ca: One or two of 0.0005 to 0.0050% are contained, and the P _CM (weld crack susceptibility composition) represented by the following formula satisfies 0.21% or less, with the balance being Fe and inevitable. Steel slab having a chemical composition of
Was heated to 50 ° C. above the temperature, after completion of the rolling at a temperature range of 850 to 950 ° C.,. 3 to the A _r3 transformation point or above the temperature
A low yield ratio for architectural use with excellent weldability, characterized in that it is accelerated and cooled to 400 to 550 ° C. at a cooling rate of 20 ° C./sec, then reheated in the temperature range of A _{C1 to} A _C3 transformation point and air cooled. Manufacturing method of refractory steel sheet. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B 6. The method for producing a low yield ratio refractory steel sheet for building according to claim 5, which has excellent weldability, reheats in a temperature range of A _{C1 to} A _C3 transformation point, air-cools, and further cools A _C1. A method for producing a low yield ratio refractory steel sheet for building having excellent weldability, characterized by performing a tempering treatment at a temperature lower than a transformation point.