JPH0657371A - Low yield ratio fire resistant building steel excellent in weldability - Google Patents

Abstract

PURPOSE:To develop low yield ratio building steel having proof stress at a high temp. as well as excellent weldability by subjecting a low carbon steel contg. small amounts of Nb, Ti, N or the like to hot rolling, thereafter subjecting it to controlled cooling to form the structure of its base metal into a one essentially consisting of bainite and subsequently executing tempering. CONSTITUTION:A slab having a compsn. contg., by weight, 0.05 to 0.20% C, 0.05 to 0.60% Si, 0.50 to 1.80% Mn, 0.020% or less P, 0.005% or less S, 0.004 to 0.020% Nb and 0.0020 to 0.0070% N and in which the carbon equivalent (Ceq) expressed by the formula I is regulated to 0.40% or below or furthermore contg. one or two kinds among 0.05 to 0.40% Cu, Ni, Cr and Mo, 0.005 to 0.06% V and 0.0005 to 0.0050% Ca and in which the carbon equivalent expressed by the formula II is regulated to 0.40% or below is subjected to hot rolling into steel. After that, this steel is subjected to controlled cooling to suppress the precipitation of Nb, and in such a manner that the strengthening of the transformation by solid solution Nb is activated, the structure of its base metal is formed into a one essentially consisting of bainite. After that, tempering is executed to precipitate fine Nb carbonitrides having 80% bainite structural fraction and smaller than 200Angstrom average grain size by the amt. of 1X10<6> pieces/mm<3>.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a refractory steel for construction,
More specifically, the present invention relates to a low yield ratio steel material for construction having a high yield strength even at a high temperature of 600 ° C. and having excellent weldability.

[0002]

2. Description of the Related Art In a steel frame building, when the steel frame is exposed to a high temperature during a fire, its strength decreases and the yield strength of the building decreases.

In the conventional Si-Mn system steel for construction, 35
Long-term proof strength required for structural members in case of fire above 0 ℃
Since it is less than 217 N / mm ² (2/3 of room temperature proof stress), a fireproof coating is applied to prevent the steel frame temperature from exceeding 350 ° C from the standpoint of construction cost and construction period.

However, the recently proposed "new fireproof design method"
Then, if you use a steel material with excellent high temperature resistance (fire resistant steel),
The reduction or omission of the amount of refractory coating has been approved.

At present, as a steel material excellent in high temperature proof stress, there is a Cr-Mo steel sheet which is widely used for boilers and pressure vessels. This steel sheet has a proof stress of 217 N / mm at 600 ° C.
^{Although it has 2} or more, since Ceq (carbon equivalent) is high, weldability and large heat input welded joint toughness are poor, and there is a problem in welding work.

Further, Japanese Patent Laid-Open No. 3-6322 proposes a refractory steel containing a small amount of alloy by adding Mo and applying accelerated cooling. However, the addition of Mo is indispensable for this steel, so it becomes expensive, and it is expected that the weldability and the toughness of the high heat input welded joint will be impaired.

[0007]

As described above, when the conventional Si-Mn steel is used, the high temperature proof stress is low, so that a fireproof coating must be applied, which reduces the construction cost and shortens the construction period. There is. In addition, since Cr-Mo steel and Mo-added steel have poor weldability and high heat input welded joint toughness, there is a problem that preheating and heat input restriction are performed to reduce welding work efficiency.

[0008] Therefore, in order to reduce or omit the fireproof coating construction for building steel, it has a high-temperature proof stress and excellent weldability and large heat input welded joint toughness. A steel material that can be used is needed.

[0009] Further, it is necessary for the building steel material to have a low yield ratio of 80% or less in order to ensure plastic deformability during an earthquake.

The present invention solves the above-mentioned problems of the prior art and provides a low yield ratio refractory steel for construction having high yield strength at 600 ° C., good weldability, and high heat input welded joint toughness. It is intended.

[0011]

As a result of intensive studies to solve the above-mentioned problems, the present inventor suppressed the precipitation of Nb by accelerated cooling after hot rolling and strengthened transformation by solid solution Nb. After making the base metal structure mainly of bainite, a large number of fine Nb carbonitrides are precipitated by tempering, and the precipitation strengthening secures high temperature strength with low Ceq. It has been completed.

That is, according to the present invention, C: 0.05 to 0.2.
0%, Si: 0.05 to 0.60%, Mn: 0.50 to 1.80
%, P ≦ 0.020%, S ≦ 0.005%, Nb: 0.00
4 to 0.020%, Ti: 0.005 to 0.030%, N:
It contains 0.0020 to 0.0070%, the balance Fe and unavoidable impurities, and the Ceq value defined by the following formula is 0.40% or less, and the bainite structure fraction is 80% or more. Furthermore, the average particle size is 200 Å or less,
Further, the gist is a low yield ratio refractory steel material for construction with excellent weldability, which is characterized by containing Nb carbonitrides in a number of 1 × 10 ⁶ pieces / mm ³ or more. Ceq = C + Si / 24 + Mn / 6 (%) ...

Further, according to another aspect of the present invention, C: 0.05 to 0.2.
0%, Si: 0.05 to 0.60%, Mn: 0.50 to 1.80
%, P ≦ 0.020%, S ≦ 0.005%, Nb: 0.00
4 to 0.020%, Ti: 0.005 to 0.030%, N:
It contains 0.0020 to 0.0070%, and further Cu: 0.0.
5 to 0.40%, Ni: 0.05 to 0.40%, Cr: 0.05
~ 0.40%, Mo: 0.05-0.40%, V: 0.005
˜0.060%, Ca: 0.0005 to 0.0050%, and one or more of them, with the balance being Fe and unavoidable impurities, and the value of Ceq defined by the following formula: Is 0.40% or less, and the bainite structure fraction is 80.
% Or more, the average particle size is 200 Å or less, and
The gist is a low yield ratio refractory steel material for construction with excellent weldability, which is characterized by containing Nb carbonitrides in a number of 1 × 10 ⁶ pieces / mm ³ or more. Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (%) ...

[0014]

[Action]

The present invention will be described in more detail below. First, the reasons for limiting the chemical components in the present invention are as follows.

C: C is an element that contributes to the increase in strength, but if it is less than 0.05%, it is difficult to secure the strength, and if it is added in excess of 0.20%, toughness and welding And high heat input HAZ (heat affected zone) toughness are deteriorated. Therefore, the amount of C is set in the range of 0.05 to 0.20%.

Si: Si is an essential element for deoxidation, but if it is less than 0.05%, its effect is small, and it is 0.60.
If it exceeds%, the weldability is deteriorated. Therefore, the amount of Si is set in the range of 0.05 to 0.60%.

Mn: Mn is an element necessary to secure the strength and toughness of steel, but if it is less than 0.50%, such an effect is small, and if it exceeds 1.80%, a large amount is added. It deteriorates weldability and toughness. Therefore, Mn
The amount is set to 0.50 to 1.80%.

P: P as an impurity is limited to 0.020% or less because the lower the P content is, the more the toughness and weldability of the welded portion are improved. In particular, it is effective for preventing segregation and cracks in welds caused by inclusions.

S: S which is an impurity is limited to 0.005% or less in order to suppress hydrogen defects due to MnS.

Nb: Nb is an essential element for increasing the high temperature strength by precipitation hardening. But 0.004
If it is less than%, such an effect cannot be obtained, and it is 0.02.
If it is added in excess of 0%, the yield ratio increases and the large heat input welded joint toughness deteriorates. Therefore, the amount of Nb is set in the range of 0.004 to 0.020%.

Ti: Ti is an element effective in reducing the brittleness of the high heat input welded joint toughness because it suppresses coarsening of austenite grains and produces fine ferrite. However, if it is less than 0.005%, such an effect cannot be exhibited, and if it exceeds 0.030%, the weld joint toughness is deteriorated. Therefore, the Ti amount is 0.005
The range is 0.030%.

N: N has the effect of improving the large heat input welded joint toughness when combined with the above Ti. However, if it is less than 0.0020%, such an effect cannot be exhibited, and if it is added in excess of 0.0070%, the weld joint toughness deteriorates. Therefore, the amount of N is set to the range of 0.0020 to 0.0070%.

In the present invention, in addition to the above elements, if necessary, one or more of Cu, Ni, Cr, Mo, V and Ca may be added in an appropriate amount. it can.

Cu: Cu is an element effective for increasing strength by solid solution strengthening, but if it is less than 0.05%, such an effect is small, and if it is added in an amount of more than 0.40%, hot workability and hot workability are improved. Weldability is impaired. Therefore, the amount of Cu is 0.05-
The range is 0.40%.

Ni: Ni is an element effective in improving the toughness, but if it is less than 0.05%, such an effect cannot be obtained.
Further, even if added in excess of 0.40%, such effects are saturated, which is economically wasteful. Therefore, the amount of Ni is set to the range of 0.05 to 0.40%.

Cr: Cr is an element effective for increasing the high temperature strength, but if it is less than 0.05%, such an effect cannot be expected, and if it is added in excess of 0.40%, the weldability deteriorates. To do. Therefore, the Cr amount is set to be in the range of 0.05 to 0.40%.

Mo: Mo is also an element effective in increasing the high temperature strength, but if it is less than 0.05%, such an effect cannot be expected, and if it is added in excess of 0.40%, the weldability deteriorates. To do. Therefore, the Mo amount is set in the range of 0.05 to 0.40%.

V: V increases the high temperature strength due to precipitation hardening, but if it is less than 0.005%, such an effect can hardly be expected, and if it is added in excess of 0.060%, the weldability deteriorates. To do. Therefore, the V amount is 0.005
The range is 0.060%.

Ca: Ca is an element that improves the properties in the plate thickness direction in a trace amount, but if it is less than 0.0005%, there is no such effect, while if it is added in an amount exceeding 0.0050%,
Such an effect is saturated, and since large inclusions are generated, ultrasonic defects are likely to occur. Therefore, the amount of Ca is set in the range of 0.0005 to 0.0050%.

Although each component is contained within the above range,
Prevents cold cracking during welding and HA during high heat input welding
In order to reduce the amount of island martensite formed in Z, the carbon equivalent (Ceq) is limited to 0.40% or less. Where Ce
q is defined by a formula or formula. Ceq = C + Si / 24 + Mn / 6 (%) ... Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (%) ...

Next, in the present invention, in the microscopic structure,
The bainite structure fraction must be 80% or more. This is to promote this precipitation by generating a large number of dislocations that become Nb carbonitride nucleation sites effective for increasing the high temperature strength. Further, in order to obtain the target high temperature strength, a fine and large amount of Nb carbonitride is required. Specifically, the average particle size is 200Å or less, and the number is 1 × 10 ⁶ / mm ³ or more is required.

Needless to say, the steel materials referred to in the present invention include various steel materials such as bar steel and shaped steel in addition to thick steel plates and thin steel plates. Manufactured by a method in which precipitation of Nb is suppressed by controlled cooling after hot rolling and transformation strengthening by solid solution Nb is utilized, the base metal structure is mainly bainite, and then many fine Nb carbonitrides are precipitated by tempering. You can

Examples of the present invention will be shown below.

[0035]

Example 1 Table 1 shows the chemical composition of the test steel. This steel slab was heated to 1150 ° C., hot-rolled to a plate thickness of 50 mm, accelerated cooling, and further tempered. Test pieces were taken from these steel plates and subjected to normal temperature tensile test, Charpy impact test, 6
A high temperature tensile test at 00 ° C., a maximum hardness test and a reproduced heat cycle Charpy test were performed. The manufacturing conditions are shown in Table 2. The test results are shown in Table 3. The maximum hardness test is JIS Z
It was carried out according to 3101.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

As is clear from Table 3, the steels A to H of the present invention.
Has a high temperature proof stress of 217 N / mm ² or more at 600 ° C., a maximum hardness of less than HV350, a low weld hardenability, and an absorbed energy vE20 in a reproducible heat cycle Charpy test. Is 100 J or more, and the high heat input HAZ toughness is also good. In addition, the yield ratio fully satisfies 80% or less required for building steel,
The fracture surface transition temperature in the Charpy test is also good at -40 ° C or lower. All of these have a bainite structure fraction of 80%
Above, the average particle diameter of Nb carbonitride is 200 Å or less,
Moreover, the number was 1 × 10 ⁶ pieces / mm ³ or more.

On the other hand, Comparative Steel I has a Nb content of 0.003%, which is below the range of the present invention, and therefore has a low bainite structure fraction and a coarse Nb carbonitride grain size. Low temperature resistance due to small amount. Comparative Steel J has an Nb content of 0.
The yield ratio is 80% or more, although the high temperature proof stress is high because it exceeds the range of the present invention at 025%. Comparative steel K is T
Since i was not added, Comparative Steel L had an N content of 0.00
The high heat input HAZ toughness is poor in all cases, since it is 83%, which exceeds the range of the present invention. Comparative steel M has a C content of 0.04%, which is below the range of the present invention, and therefore has a low yield strength at 600 ° C., and Comparative steel N has a C content of 0.22%, which exceeds the range of the present invention. Therefore, the base metal toughness, weldability, and high heat input H
AZ toughness is poor. Comparative steel O has a Ceq of more than 0.40% and thus has poor weldability.

It should be noted that the above embodiment is an example relating to the production of thick steel plates, but it goes without saying that it can be applied to the production of other steel products such as bar steel and shaped steel in the same manner.

[0042]

As described above, according to the present invention,
High yield strength at 600 ° C, good weldability and large heat input H
It is possible to provide a building steel material having AZ toughness and a low yield ratio. Therefore, it is possible to significantly reduce or omit the conventionally required fireproof coating, and further, it is possible to enhance the safety of the structure in terms of welding work and earthquake resistance. I have.

Claims (2)

[Claims] 1. By weight% (hereinafter the same), C: 0.05-
0.20%, Si: 0.05 to 0.60%, Mn: 0.50 to
1.80%, P ≦ 0.020%, S ≦ 0.005%, Nb:
0.004 to 0.020%, Ti: 0.005 to 0.030
%, N: 0.0020 to 0.0070%, the balance consisting of Fe and unavoidable impurities, and the Ceq value defined by the following formula is 0.40% or less, and the bainite structure fraction Is 80% or more, and the average particle size is 200
A low yield ratio refractory steel material for construction with excellent weldability, characterized by containing Nb carbonitrides of Å or less and a number of 1 × 10 ⁶ pieces / mm ³ or more. Ceq = C + Si / 24 + Mn / 6 (%) ... 2. C: 0.05-0.20%, Si: 0.05-
0.60%, Mn: 0.50 to 1.80%, P ≦ 0.020
%, S ≦ 0.005%, Nb: 0.004 to 0.020%,
Ti: 0.005 to 0.030%, N: 0.0020 to 0.0
070%, Cu: 0.05 to 0.40%, Ni:
0.05 to 0.40%, Cr: 0.05 to 0.40%, Mo:
0.05 to 0.40%, V: 0.005 to 0.060%, C
a: contains one or more of 0.0005 to 0.0050%, the balance being Fe and inevitable impurities,
Further, the value of Ceq defined by the following formula is 0.40% or less, the bainite structure fraction is 80% or more, the average grain size is 200Å or less, and the number is 1 × 10 ⁶
A low-yield ratio refractory steel material for construction with excellent weldability, characterized by containing Nb carbonitrides per unit / mm ³ or more. Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (%) ...