JPH08333623A - Production of refractory steel material for building construction, excellent in earthquake resistance and reduced in yield ratio - Google Patents

Abstract

PURPOSE: To produce a refractory steel material excellent in weldability, having sufficient high temp. proof stress, and reduced in yield ratio by specifying the composition of a low carbon low alloy steel and hot-rolling this steel. CONSTITUTION: A steel, having a composition which consists of, by mass ratio, 0.04-0.14% C, 0.10-0.40% Si, 0.50-1.40% Mn, <=0.020% P, <=0.005% S, 0.10-0.40% Cr, 0.10-<0.40% Mo, 0.010-0.100% Al, further either or both of 0.010-0.050% V and 0.005-0.030% Nb, and the balance Fe with inevitable impurities and in which the values of Ceq and Pcm, represented respectively by equations I, II, are regulated to <=0.36% and <=0.24%, respectively, and also C/Mn is regulated to >0.05, is used. A bloom of a steel with this composition is heated to 1050-1250 deg.C and rolling is finished at 850-950 deg.C, by which the steel material having a structure composed of mixed structure of ferrite and pearlite or bainite can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material for building structures such as high-rise buildings. More specifically, it has sufficient high temperature proof stress, excellent weldability, and a yield ratio of 80% or less, excellent earthquake resistance. And a low yield ratio refractory steel for building structures.

[0002]

2. Description of the Related Art The room temperature characteristics of fire-resistant steel materials for building structures have been the standard of "rolled steel materials for welded structures, SM" (JIS G 3106).
Had been applied. Therefore, the yield strength at room temperature is manufactured by specifying only the lower limit of the standard, and in order to secure the proof stress at 600 ° C, which is an important property as a fire-resistant steel material, it is usually about 120 N / mm ² or more from the lower limit of the standard. The current value is high. Therefore, if the yield strength at room temperature is to be kept low from the standpoint of earthquake resistance, the high-temperature yield strength will also decrease and the standard value of yield strength at 600 ° C will decrease.
There was a problem that (2/3 or more of the standard value at room temperature) could not be secured.

However, such a steel material having a yield strength at room temperature significantly higher than the lower limit of the standard is not suitable as a structural member capable of ensuring the safety of a building in terms of earthquake resistance. In other words, when a large-scale earthquake occurs, the components of the building do not deform as intended in the design,
The danger that the building will eventually collapse is extremely high.
Therefore, conventional fire-resistant steel materials are naturally limited in their use in buildings.

Under these circumstances, the upper and lower limits of the yield strength at room temperature were newly set in June 1994, the width was set to 120 N / mm ² or less, and the upper limit of the yield ratio was set to 80%. "Rolled steel for building structures, SN" standard (JIS G 313
6) was enacted, and this standard has also been applied to refractory steel materials.

[0005]

However, in order to improve the high temperature proof stress, it is necessary to increase the amount of alloying elements added, and as a result, the weldability deteriorates. Regarding the steel materials for building structures targeted by the present invention, conventionally, there was no official standard that specifically defines weldability, but Ceq represented by the following formula in JIS G 3136:
And P _CM are regulated to 0.36% or less and 0.26% or less, respectively, and it is an issue to secure sufficient high temperature proof strength as a fire-resistant steel plate while satisfying these regulations. Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (%) P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (%)

Another problem when increasing the amount of alloying elements added in order to improve the high temperature yield strength is that the room temperature yield strength rises above the high temperature yield strength. Another major issue is how to avoid exceeding the upper limit of room temperature yield strength specified in 3136.

The present invention has been made in order to solve the above problems, has a high temperature proof that can reduce or omit the fireproof coating on the steel frame, and has a yield strength at room temperature of 120 N / mm from the lower limit of the standard. ^An object of the present invention is to provide a method for producing a low yield ratio refractory steel material for building structures having a width of ² and a yield ratio of 80% or less, excellent in weldability, and excellent in earthquake resistance.

[0008]

The present inventor has studied the relationship between the chemical composition of steel and the yield strength at room temperature and the proof stress at 600 ° C. As a result, Mo and Cr and one of V and Nb or The combined addition of two types can increase the yield strength at 600 ° C without significantly increasing the yield strength at room temperature, and can improve the C / Mn in the microstructure of ferrite and pearlite or a mixed structure of ferrite, pearlite and bainite. The yield ratio of 80% or less is ensured by controlling the ratio and rolling end temperature, and by limiting the carbon equivalent (Ceq) and weld crack susceptibility composition (P _CM ), it is suitable for building structures with good weldability. It has been found that refractory steel can be obtained.

[0009] The gist is mass%, C: 0.04 to 0.14%,
Si: 0.10 to 0.40%, Mn: 0.50 to 1.40%, P: 0.020% or less, S: 0.005% or less, Cr: 0.10 to 0.40%, Mo: 0.10% to less than 0.40%, Al: 0.010 to 0.100% And further
It contains one or two selected from V: 0.010 to 0.050% and Nb: 0.005 to 0.030%, and Ceq represented by the following formula is 0.
Steel pieces containing 36% or less, P _CM of 0.24% or less, C / Mn of more than 0.05, and the balance of Fe and inevitable impurities are 1050 ~
Low yield for building structure with excellent earthquake resistance, which is heated in the temperature range of 1250 ℃, finished rolling in the temperature range of 850-950 ℃, and has a microstructure of ferrite and pearlite or a mixed structure of ferrite, pearlite and bainite. This is a method for manufacturing a specific refractory steel material. Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (%) P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (%)

Further, as a chemical component, Cu: 0.05 to 0.40
%, Ni: 0.05 to 0.40%, Ti: 0.005 to 0.020%, Ca: 0.00
This is a method for producing a low yield ratio refractory steel material for building structures, which has one or more selected from the range of 05 to 0.0050% and is excellent in earthquake resistance.

The yield strength is lower than the standard lower limit +12
A method for producing a low yield ratio refractory steel material for a building structure, which has an excellent earthquake resistance and has a yield ratio of 80% or less in a range of 0 N / mm ² .

[0012]

The reason for limiting the chemical components in the present invention will be described. C is an element that contributes to the increase in strength, but if the content is less than 0.04%, it is difficult to secure strength,
On the other hand, if it exceeds 0.14%, the weldability and toughness deteriorate.
Therefore, the C content is set to the range of 0.04 to 0.14%.

Si is an essential element for deoxidation,
If the content is less than 0.10%, its effect is small, while 0.40%
%, The weldability and toughness deteriorate. Therefore, the Si content is set to the range of 0.10 to 0.40%.

Mn is an element that contributes to the increase in strength,
If the content is less than 0.50%, its effect is small, while 1.40
%, The Ceq becomes high, but the yield strength at 600 ° C cannot be expected. Therefore, the Mn content is 0.50-
The range is 1.40%.

When P exceeds 0.020%, P deteriorates weldability and toughness due to microsegregation. Therefore, the P content should be 0.020% or less.

If S is contained in an amount of more than 0.005%, coarse A type inclusions are likely to be formed and the toughness is deteriorated. Therefore, the S content should be 0.005 or less.

Cr is an element effective in improving the yield strength at 600 ° C. without significantly increasing the yield strength at room temperature, but if the content is less than 0.10%, this effect is small, while on the other hand, it exceeds 0.40%. But for the rise in Ceq, 6
The effect of increasing proof stress at 00 ° C is small. Therefore Cr
The content is in the range of 0.10 to 0.40%.

Mo is an indispensable element for ensuring high temperature strength, and significantly increases the yield strength at 600 ° C.
However, when the content is less than 0.10%, this effect is small. On the other hand, when the content is 0.40% or more, the increase in Ceq is 600 ° C. for the increase in
The effect of increasing yield strength is small. Therefore, the Mo content is set to a range of 0.10% or more and less than 0.40%.

Al is an element that contributes to grain size regulation of ferrite grains through the formation of nitrides in addition to the deoxidizing effect, but if the content is less than 0.010%, these effects are small, while
If it exceeds 0.100%, the amount of oxide inclusions increases and the toughness deteriorates. Therefore, the Al content is in the range of 0.010 to 0.100%.

V is an element effective for increasing the strength at room temperature and high temperature due to precipitation strengthening, but its content is 0.010
If it is less than 0.05%, these effects are small, while if it exceeds 0.050%, the weldability deteriorates. Therefore, the V content is 0.0
The range is 10 to 0.050%.

Nb is an element effective for increasing the strength at normal temperature and high temperature due to the grain refining action and precipitation strengthening, but if the content is less than 0.005%, these effects are small, while on the other hand, it exceeds 0.030%. The effect of increasing the yield strength at 600 ° C is small compared to the increase in yield strength at room temperature. Therefore, the Nb content should be in the range of 0.005 to 0.030%.

In the present invention, in addition to the above elements, Cu,
It may contain one or more of Ni, Ti and Ca.

Cu is an element effective for increasing the strength by precipitation strengthening, but if the content is less than 0.05%, this effect is small, while if it exceeds 0.40%, cracking and weldability during hot working occur. Cause deterioration. Therefore, the Cu content is 0.
The range is from 05 to 0.40%.

Ni is an element effective for improving toughness,
When the content is less than 0.05%, this effect is small, while 0.40
Even if it exceeds%, the effect is saturated, and since it is an expensive element, it is economically useless. Therefore, the Ni content is 0.
The range is from 05 to 0.40%.

Ti is an element effective in improving the toughness of the base material and the weld heat affected zone by forming a nitride, but its content is
If it is less than 0.005%, this effect is small, while if it exceeds 0.020%, the above effect cannot be expected. Therefore, the Ti content should be in the range of 0.005 to 0.020%.

Ca is an element that controls the morphology of sulfides and is effective in improving the toughness and the characteristics in the plate thickness direction, but its content is 0.00
If it is less than 05%, these effects are small. On the other hand, if it exceeds 0.0050%, large inclusions of CaS and CaO are generated in large amounts, which rather deteriorates the toughness. Therefore, the Ca content is 0.00
The range is from 05 to 0.0050%.

Further, in order to secure a low yield ratio of 80% or less required from the viewpoint of seismic resistance for building structures, the ratio of C / Mn in the mixed structure of ferrite and pearlite or ferrite, pearlite and bainite is set to It is set to exceed 0.05, and in order to provide better weldability, the carbon equivalent (Ce
q) is 0.36% or less, weld crack susceptibility composition (P _CM ) is 0.24%
Limited to:

Next, the reasons for limiting the manufacturing conditions of the present invention will be described. The lower limit of the heating temperature is 1050 ° C, because Cr, Mo, V and Nb added to improve the proof stress at 600 ° C must be sufficiently dissolved. On the other hand, if the heating temperature is too high, the austenite grains become coarse and the toughness of the base material deteriorates. Therefore, the upper limit of the heating temperature is 1250 ° C.

Further, when the rolling end temperature is less than 850 ° C., it is not possible to secure a low yield ratio of 80% or less, which is required for steel materials for building structures, from the viewpoint of earthquake resistance due to the refinement of ferrite grains. The acoustic anisotropy is increased due to the tissue, and the refraction angle and the flaw detection sensitivity are changed in the ultrasonic oblique flaw detection, which makes it difficult to detect the welding defect portion. On the other hand, when the rolling end temperature exceeds 950 ° C, the austenite grains become coarse and the toughness of the base material deteriorates. Therefore, the rolling finish temperature is limited to the temperature range of 850 to 950 ° C.

[0030]

EXAMPLES Examples of the present invention will be described below.
The test steel is a steel plate having a chemical composition shown in Table 1 and finished into a steel plate having a thickness of 25 mm under the heating and rolling conditions shown in Tables 2 and 4. Test pieces were taken from these steel sheets and subjected to a tensile test at room temperature, a Charpy impact test, a high temperature tensile test at 600 ° C and a maximum hardness test. Furthermore, the microstructure was observed with a microscope. The results are shown in Table 3.
And shown in Table 5. The maximum hardness test is JIS Z 3101.
It was carried out according to.

Table 1 shows examples A to G of the present invention and comparative examples H to P.
Table 2 shows the heating and rolling conditions of the present invention and comparative examples. However, the rolling conditions are within the limits of the present invention. Table 4 shows the tensile properties, impact properties, high temperature properties and weldability at room temperature. The microstructure is a mixed structure mainly composed of ferrite and pearlite.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

As is clear from Table 3, A to A of the examples of the present invention.
Both G, has a high temperature yield strength yield strength with excellent 2/3 (157N / mm ²⁾ or more at room temperature standard value of 400 N / mm ² class steels at 600 ° C., characteristic tensile at room temperature 400 N / mm ² class Standard value of steel
(Yield point or 0.2% proof stress: 235 to 355 N / mm ² , tensile strength: 400
〜510N / mm ² , yield ratio: 80% or less) are fully satisfied.
Also, the fracture surface transition temperature of the base metal in the Charpy impact test is good at -26 ℃ or less. Furthermore, the maximum hardness is 265 or less in Vickers hardness, and it has good weldability. All of these values satisfy the required characteristics as a fire-resistant steel material for building structures.

On the other hand, in Comparative Examples H and I, the former is
Since Cr and the latter do not contain Mo, 600 ℃
The yield strength is low. Further, since neither Comparative Example J nor V or Nb was added, the yield strength at 600 ° C. was low.

In Comparative Example K, the weldability is poor because the Cr content is out of the range of the present invention. Comparative Example L
Has a Mo content outside the limited range of the present invention, so that the weldability is poor and the yield strength at room temperature is close to the standard upper limit.

Further, in Comparative Example M, the V content is out of the limited range of the present invention, so that the yield strength at room temperature exceeds the standard upper limit value. Further, Comparative Example N is Nb
Since the content of is out of the limited range of the present invention, the yield strength at room temperature exceeds the standard upper limit,
Moreover, the yield ratio is high, and there is little margin for the standard value.

In Comparative Example O, since the C content and Ceq are out of the limited range of the present invention, the base material toughness and weldability are poor, and the tensile strength at room temperature is also high. In Comparative Example P, C / Mn is 0.03, which is outside the limited range of the present invention, so that the yield ratio is high and exceeds 80% of the standard value.

Comparative Examples A1 to A4 shown in Tables 4 and 5
Is a steel slab of Example A of the present invention manufactured under heating and rolling conditions outside the limits of the present invention. Table 4 shows heating and rolling conditions, and Table 5 shows test results.

[0041]

[Table 4]

[0042]

[Table 5]

In Comparative Example A1, the heating temperature was deviated so low that Cr, Mo and V were not sufficiently solid-dissolved and the yield strength at 600 ° C. was low. On the other hand, in Comparative Example A2, since the heating temperature is deviated to a high temperature, austenite becomes coarse grains and the base material toughness is poor. Further, in Comparative Example A3, since the rolling end temperature is deviated relatively low, the ferrite becomes fine grains, and the yield strength and the yield ratio at room temperature are high, and the values are close to the upper limit of the standard value.
Further, in Comparative Example A4, since the rolling end temperature is deviated to a higher level, the austenite becomes coarse grains and the base material toughness is poor.

[0044]

As is apparent from the above description,
According to the present invention, it is possible to manufacture a steel material having a low yield strength and a low yield ratio at room temperature, a high yield strength even at 600 ° C, and excellent weldability. Therefore, the steel material according to the present invention has an excellent effect that the fireproof coating can be greatly reduced or omitted, and the safety of the building structure can be improved from the viewpoint of earthquake resistance and welding construction. It is a thing.

Claims (3)

[Claims] 1. In mass%, C: 0.04 to 0.14%, Si: 0.10 to
0.40%, Mn: 0.50 to 1.40%, P: 0.020% or less, S: 0.005
% Or less, Cr: 0.10 to 0.40%, Mo: 0.10% to less than 0.40%, Al: 0.010 to 0.100%, and V: 0.010 to 0.
050%, Nb: contains one or two selected from 0.005 to 0.030%, Ceq represented by the following formula is 0.36% or less, P
_A steel piece with _CM of 0.24% or less and C / Mn of more than 0.05 and the balance of Fe and inevitable impurities is heated in the temperature range of 1050 to 1250 ° C, and the rolling is finished in the temperature range of 850 to 950 ° C. A microstructure is a mixed structure of ferrite and pearlite or a mixture of ferrite, pearlite and bainite, and a method for producing a low yield ratio refractory steel material for building structures having excellent earthquake resistance. Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (%) P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (%) 2. Further, as a chemical component, Cu: 0.05 to 0.40
%, Ni: 0.05 to 0.40%, Ti: 0.005 to 0.020%, Ca: 0.00
The method for producing a low yield ratio refractory steel material having excellent earthquake resistance for a building structure according to claim 1, which contains one or more selected from 05 to 0.0050%. 3. The yield strength is lower than the lower limit of the standard and lower than the lower limit of the standard +
The method for producing a low yield ratio refractory steel material for a building structure having excellent earthquake resistance according to claim 1 or 2, wherein the yield ratio is 80% or less in the range of 120 N / mm ² .