JP2655956B2 - Manufacturing method of low yield ratio refractory steel sheet for building structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a 50 kg class steel sheet used for a building structure, and particularly to a JIS standard structural steel (for example, SM50) in strength, toughness and weldability at room temperature (room temperature). The present invention relates to a method for producing a low yield ratio refractory steel plate for a building structure in which a yield ratio is reduced at ordinary temperature and a yield strength at high temperature is further improved while maintaining equivalent characteristic values.

[0002]

2. Description of the Related Art Conventionally, steel materials for building structures have been JI.
Steel materials specified in SG-3106 “Rolled steel materials for welded structures” and the like are used. However, when this kind of steel is exposed to a high temperature of 350 ° C. or more, the proof stress is significantly reduced.
It is required by law to use fireproof coatings that do not exceed ° C. When such a fireproof coating is applied, not only does construction cost increase significantly, but also the occupied area of pillars and the like increases, which is a factor that greatly impairs the effective use of living space. In view of the circumstances described above, various studies have been made on steel materials having high proof stress even at high temperatures and methods of manufacturing the same in order to reduce or reduce the refractory coating treatment.

[0003] For example, Japanese Unexamined Patent Publication No. 2-77523 discloses a low yield ratio refractory for architectural structures which has a high proof stress both at high temperatures and ordinary temperatures, and has the same ordinary temperature characteristics as JIS standard structural steel (SM50). Steel has been proposed. However, these steels, especially those of the 50 kg class, all have high M
Since it is characterized by containing o, the quenchability is good, and therefore, even if cooling after hot rolling is performed by air cooling, it becomes a bainite single phase structure. As a result, a drawback that the yield ratio at room temperature increases and the plastic deformability decreases often occurs.

[0004]

DISCLOSURE OF THE INVENTION The present invention is superior in weldability to conventional low-alloy steels for high temperatures, and has higher high-temperature strength (particularly, yield strength or proof stress) than conventional welded structural steels. It is an object of the present invention to propose a method for producing a refractory steel sheet having a low yield ratio for a building structure, which has a high yield ratio at room temperature and a low strength at room temperature, and has the same properties as room temperature strength and toughness.

[0005]

SUMMARY OF THE INVENTION The present invention provides, in weight percent,
C: 0.02 to 0.15%, Si: 0.40% or less, M
n: 0.80 to 2.00%, P: 0.020% or less,
S: 0.015% or less, and Mo: 0.15%
0.80%, V: 0.005 to 0.30%, Nb: 0.
005 to 0.10%, Ti: 0.005 to 0.250%
2-phase region temperature between a steel slab containing one or more, is heated to above 1000 ° C., then terminated at a temperature of at least 900 ° C. The hot rolling, after cooling, A _C1 to A _C3 point of And then subjecting the steel slab to reheating and then cooling by a two-phase region reheating quenching method, wherein the steel slab further comprises C by weight%.
u: 0.50% or less, Ni: 1.00% or less, and B:
One or more of 0.0003 to 0.0030% may be contained.

[0006]

The present inventors have conducted various studies and experiments to solve the above-mentioned problems, and as a result, have obtained the following findings. By utilizing the precipitation strengthening of carbides of Mo, V, Nb and Ti, the high temperature proof stress can be increased. By performing the reheating treatment in the (α + γ) two-phase region, the yield ratio at room temperature can be significantly reduced even in the precipitation-strengthened steel. The high-temperature strength of the steel material reheated in the (α + γ) two-phase region can be significantly improved by changing the structure before the reheat treatment to a fine matrix structure containing bainite or martensite. The present invention has succeeded in producing a low-yield-ratio fire-resistant steel sheet for building structures by achieving a balance between a low yield ratio at normal temperature and high-temperature strength by comprehensively utilizing the above findings.

Hereinafter, the reasons for limiting each component of the low yield ratio refractory steel sheet for a building structure according to the present invention and the operation thereof will be described in detail. C: 0.02 to 0.15% C is added to secure a predetermined strength.
%, The effect of addition is small. On the other hand, if it exceeds 0.15%, weldability and toughness are deteriorated. Therefore, the content of C is limited to the range of 0.02 to 0.15%. Si: 0.40% or less Si is added to improve the room temperature strength. But,
If it exceeds 0.40%, the toughness of the heat-affected zone of the weld decreases, so it was limited to 0.40% or less. Mn: 0.80 to 2.00% In the two-phase region reheating quenching method of the present invention, in order to ensure high high-temperature strength, the structure before the reheating treatment is defined as a fine matrix structure containing bainite or martensite. It is necessary to put. For that purpose, Mn needs to be added at 0.80% or more. However, when the content exceeds 2.00%, the weldability is significantly reduced. Therefore, the content of Mn is 0.1.
Limited to 80-2.00%.

P: 0.020% or less, S: 0.015% or less P and S are contained as unavoidable impurities. However, since both decrease the ductility and toughness of steel materials, their contents are respectively P. : 0.020% or less, S: 0.015%
Limited to the following. Mo: 0.15 to 0.80% Mo is an element that precipitates carbides stable at high temperatures and acts extremely effectively to improve high-temperature strength by precipitation strengthening. In order to ensure sufficient high-temperature strength even after high-temperature two-phase region reheating quenching, addition of at least 0.15% is necessary. On the other hand, if added in excess of 0.80%, the weldability and toughness are significantly reduced, so the range was limited to the range of 0.15 to 0.80%.

V: 0.005 to 0.30% V is also a carbide precipitation element like Mo, and at least 0.005% must be added to secure high-temperature strength by precipitation strengthening. On the other hand, if added over 0.30%, M
Since the weldability and toughness are significantly reduced as in the case of o, the range is limited to the range of 0.005 to 0.30%. Nb: 0.005 to 0.10% Nb is also one of the carbide precipitation elements, and at least 0.005% of Nb is necessary to secure high-temperature strength by precipitation strengthening. On the other hand, if it is added in excess of 0.10%, the weldability and toughness are significantly reduced.
It was limited to the range of 005 to 0.10%. Ti: 0.005 to 0.250% Ti is also one of the carbide precipitation elements, and at least 0.005% of addition is necessary for securing high-temperature strength by precipitation strengthening. On the other hand, if added in excess of 0.250%, the weldability and toughness are significantly reduced, so the range was limited to the range of 0.005 to 0.250%.

[0010] Cu: 0.50% or less, Ni: 1.00%
Hereinafter, B: at least one of 0.0003 to 0.0030% Cu, Ni and B are elements effective for improving hardenability similarly to Mn, and the structure before the reheating treatment is changed to bainite or martensite. It is effective to add one or more of Cu, Ni and B in order to obtain a fine base structure including and to obtain higher high temperature proof stress. However, if Cu is added in excess of 0.50%, the hot workability is degraded, so the upper limit is set to 0.50%. If Ni is added in excess of 1.00%, it becomes expensive and economically disadvantageous, so the upper limit was set to 1.00%. B is required to be added in an amount of 0.0003% or more in order to improve the hardenability. However, if added over 0.0030%, the weldability, ductility and toughness are significantly reduced. 00
The range was limited to 30%.

Next, the reasons for limiting the manufacturing conditions such as the slab heating temperature and the effects thereof will be described in detail. Slab heating temperature: 1000 ° C. or more To sufficiently form a solution of Mo, V, Nb and Ti, the slab heating temperature must be at least 1000 ° C. or more. Hot rolling end temperature: 900 ° C. or more In order to make the microstructure before reheating in the two-phase region into a fine matrix structure containing bainite or martensite, it is desirable to end rolling at a high temperature. When a strong pressure is applied in a temperature range of 900 ° C. or less, the transformation of ferrite is promoted by the introduction of rolling strain, so that it is difficult to secure high-temperature strength. Therefore, the hot rolling end temperature needs to be 900 ° C. or higher.

Cooling after rolling: air cooling The cooling after rolling is air cooling.

Heat treatment: Reheating and quenching in the two-phase region between A _{C1 and} A _C3 points In order to ensure high-temperature strength and a low yield ratio at room temperature tensile, air-cooling after rolling, and A _{C1 to} A
It is necessary to reheat to the temperature in the two-phase region between points _C3 and cool down. That is, samples were taken from a steel plate manufactured by hot rolling a continuous cast slab having the chemical composition of steel type A in Table 1, and reheated to various temperatures, held for 30 minutes, air-cooled, and then cooled to room temperature and 600 ° C. A tensile strength test was performed. The effects of the reheating temperature on the room temperature strength (YS _RT , TS _RT , YR _RT ) and the high temperature strength (YS ₆₀₀ ) are shown in FIG. The heating, hot rolling and cooling conditions at this time were performed according to A1 in Table 2.

[0014]

[Table 1]

[0015]

[Table 2]

That is, at room temperature, a low YR value (≦ 80
%) Is obtained at a reheating temperature of A _C1 or more,
A high YS ₆₀₀ value at 00 ° C. (≧ 22 kgf / mm ² ) is obtained at reheating temperatures below the A _C3 point. Therefore, the reheating temperature that satisfies both conditions is limited to a temperature not lower than the _AC1 point and not higher than the _AC3 point. Cooling rate: Either air cooling or water cooling may be used, but water cooling has the effect of slightly increasing the room temperature and high temperature strength. Tempering process: After reheating in the two-phase region, by performing the tempering process, the YS and YR values at room temperature slightly increase. Further, the impact toughness is improved by the tempering treatment, but the YS value at 600 ° C. hardly changes. The tempering temperature is preferably from 400 to 600 ° C.

[0017]

EXAMPLE Steel slabs of various chemical compositions shown in Table 1 were
Heated to above 00 ° C and rolled to a plate thickness of 40-60
After rolling the steel sheet having a thickness of 900 mm or more at a temperature of 900 ° C. or more and cooling it by air cooling, it is reheated to a temperature of at least A _C1 point (760 ° C. in this example) and at most A _C3 point (930 ° C. in this example). After holding for 30 minutes, the mixture was cooled by air cooling. The tensile properties at room temperature and 600 ° C. were investigated for the steel sheet samples adjusted as described above, and the results are shown together with the comparative examples in Table 2. Here, A1 to A4 in Table 2 correspond to Table 1
Is a value obtained when manufacturing conditions such as heating, rolling, and reheating are changed for steel type A in the chemical composition table. Steel type A3,
A4, C2, and E2 are examples in which the chemical composition is within the scope of the present invention, but A3 has not been subjected to the reheating treatment according to the present invention, and as a result, the YR _RT at room temperature shows a high value of 81%. In addition, A4 shows an example in which the YS ₆₀₀ value was reduced to 20 kgf / mm ² because the reheating temperature was 950 ° C., which was inappropriate. Steel types C2 and E2 have a YS ₆₀₀ value of 17 kg each due to a low heating temperature during rolling and a low rolling end temperature.
This is an example in which f / mm ² and 19 kgf / mm ² are reduced.

Steel type G is an example in which the contents of Mo, Cu, Ni and B are low, the hardenability after rolling is insufficient, and the high-temperature strength is significantly reduced. Similarly, steel type H is Mn, C
This is an example in which the high-temperature strength is significantly reduced because the contents of u, Ni and B are extremely low. Steel type I is Mo,
This is an example in which the high-temperature strength is significantly reduced due to the low V, Nb and Ti contents. On the other hand, steel types A1 and A2 and B, C1, D, E1 and F all have high high-temperature strengths with YS ₆₀₀ values of 22 kgf / mm ² or more. That is, the decrease in proof stress at a high temperature is small, and YS ₆₀₀ / YS _RT can be maintained at 60% or more. The tensile properties at room temperature are the same as those of SM50 specified in the current JIS G 3106.
The yield ratio was able to be suppressed to 75% or less while satisfying the specifications of the ~ 58th grade steel.

[0019]

The low yield ratio refractory steel sheet for building structures manufactured by the method according to the present invention has the same properties at room temperature as a conventional rolled steel sheet for welded structures, and has a small decrease in proof stress at high temperatures. High high temperature strength can be maintained. Therefore, a great effect such as reduction or reduction of the fireproof coating at the time of building construction can be obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of reheating temperature on normal temperature and high temperature strength.

[Explanation of symbols]

YS ₆₀₀ : Yield strength at 600 ° C. (kgf / mm ² ) YS _RT : Yield strength at normal temperature (kgf / mm ² ) TS _RT : Tensile strength at normal temperature (kgf / mm ² ) YR _RT : Yield ratio at normal temperature (%)

Continuation of the front page (56) References JP-A-5-339633 (JP, A) JP-A-3-130319 (JP, A)

Claims (2)

(57) [Claims] C .: 0.02 to 0.15% by weight,
Si: 0.40% or less, Mn: 0.80 to 2.00%,
P: 0.020% or less, S: 0.015% or less,
And Mo: 0.15 to 0.80%, V: 0.005 to
0.30%, Nb: 0.005 to 0.10%, Ti:
The steel slab containing one or more of 0.005 to 0.250%, heated above 1000 ° C., then exit the hot rolling at 900 ° C. or higher, after cooling, A _C1 to A _A method for producing a low yield ratio refractory steel sheet for a building structure, comprising reheating to a temperature in a two-phase region between the _C3 points and then performing a two-phase reheating quenching method for cooling. 2. The steel slab further comprises Cu:
0.50% or less, Ni: 1.00% or less, and B: 0.
The method for producing a low yield ratio refractory steel sheet for building structures according to claim 1, wherein the steel sheet contains at least one of 0003 to 0.0030%.