JPH04362156A - Steel excellent in fire resistance and toughness in welded joint part - Google Patents

Abstract

PURPOSE:To provide a steel with a low yield ratio having fire resistance sufficiently satisfied as the one for various building structures, free from the generation of the deterioration in HAZ toughness even at the time of high heat input welding and furthermore advantageous in terms of cost. CONSTITUTION:This steel for structures has a chemical compsn. contg. 0.05 to 0.20% C, 0.05 to 0.60% Si, 0.5 to 1.6% Mn, 0.003 to 0.050% Ti, <=0.5% Mo, 0.0010 to 0.0060% N and 0.0004 to 0.0060% O, or furthermore contg. one or >=two kinds among 0.05 to 1.0% Cr, 0.05 to 0.50% Ni, 0.05 to 1.0% Cu, 0.004 to 0.04% Nb, 0.004 to 0.10% V and 0.0003 to 0.002% B and the balance Fe with inevitable impurities, and its structure is formed in such a manner that oxide inclusions having a Ti (O, N) composite crystalline phase with <=3mum grain size in the ratio of 0.001 to 0.100wt.% are dispersed into a matrix.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a low yield ratio steel material with excellent fire resistance and welded joint toughness, suitable for use in various architectural structures such as buildings, bridges, and offshore structures.

0002

[Prior art and its problems] Currently, construction materials for various buildings in the fields of architecture, civil engineering, marine technology, etc. are usually rolled steel materials for general structures (JIS G 3101) and rolled steel materials for welded structures (JIS G 3101). 3106), weather-resistant hot-rolled steel for welded structures (JIS G 3114), highly weather-resistant rolled steel (JIS G 3125), carbon steel pipes for general structures (JIS G 3444), and square steel pipes for general structures (JIS G 3106),
Steel materials standardized as IS G 3466) are used. These steel materials are produced by refining desulfurized and dephosphorized blast furnace pig iron in a converter, converting it into steel slabs by continuous casting or blooming rolling, and then hot rolling them to give them the desired properties. is common.

By the way, among the various structures mentioned above, when the above-mentioned steel materials are used for buildings such as buildings and residences that are closely connected to daily life, it is mandatory to apply sufficient fireproof coating to ensure safety against fire. Building-related laws and regulations stipulate that ``the temperature of steel materials must not exceed 350℃ in the event of a fire.'' In other words, when each of the steel materials mentioned above is heated to about 350°C, the yield strength increases from 60 to 7 at room temperature.
If the steel material is used as a support for a building, it may cause the building to collapse. Therefore, it is necessary to take measures to use it in a way that would otherwise be impossible.

For example, general structural rolled steel materials (JIS G
3101), the surface of the shaped steel is coated with "sprayed material" based on slag wool, rock wool, glass wool, asbestos, etc.
It is necessary to carefully apply a fire-resistant coating, such as by spreading ``felt'' or covering with fire-resistant mortar, or by further covering and protecting the insulation layer with a thin metal plate such as aluminum or stainless steel. was there. As a result, not only is it unavoidable that the cost of fireproof coating construction is relatively high compared to the cost of steel materials, resulting in a significant increase in construction costs, but there is also the problem that the usable space of the building becomes narrower, reducing economic efficiency.

[0005] Therefore, round or square steel pipes are used as construction materials to allow cooling water to circulate therein, thereby suppressing the temperature rise in the event of a fire and preventing a drop in loading capacity. Proposals were also made to reduce costs and expand available space (Jikko Kosho 52-1602).
No. 1, etc.). However, with the above method of improving fire resistance performance by using hollow steel materials in buildings and forcing cooling,
As the structure of the building becomes more complex, not only the design and construction costs but also the equipment costs increase, and maintenance costs also become high.

[0006]Of course, "
It is also possible to use heat-resistant steel materials as structural materials for buildings, but although these heat-resistant steel materials have good high-temperature properties, they require advanced technology in terms of production and construction, and are extremely expensive. Therefore, it was difficult to use it as a building material from an economical point of view.

[0007] Incidentally, in recent years, building design technology has improved further and its reliability has also increased due to the remarkable progress in building heights, so a review of the fireproof design of buildings has been carried out. In 1986, a new fireproof design law for buildings was enacted. As a result, the aforementioned “35
The ability of fireproof coating can now be determined based on the high-temperature strength of the steel material and the load actually applied to the building, without being restricted by the 0℃ temperature limit, and in some cases, steel materials can be used without coating. It has also become possible.

[0008] In response to this, new structural steel materials that are economically advantageous and have excellent fire resistance have been developed, and for example, the following method of manufacturing structural steel materials has been proposed. (a) A method in which Nb and Mo are added in combination to low C-low Mn steel, or steel to which Cr, V, etc. are added is heated to a high temperature and then the rolling is finished at a relatively high temperature (JP-A-2-1999)
No. 77523). (b) A method of heating V or V-Mo added steel to a high temperature and then finishing rolling at a relatively high temperature (Japanese Patent Application Laid-Open No. 2-1633
No. 41).

However, in all of the above proposals, alloying elements such as Mo, Cr, Nb, and V are added to increase the strength of the steel material at high temperatures, but on the other hand, the addition of these alloying elements Base metal part (welding heat affected zone: HAZ)
This also caused the inconvenience of "reducing the toughness of the steel." When such steel materials are used as structural materials for buildings, it is inevitable that high heat input welding, such as "welding of the diaphragm part of a box column", will be performed, and therefore the above-mentioned In steel with alloying elements added, HA
It was pointed out that this is not preferable because the deterioration of the toughness of Z becomes noticeable.

[0010] Therefore, an object of the present invention is to have fire resistance performance that is sufficiently satisfactory for use in various building structures, to prevent deterioration of HAZ toughness even during high heat input welding, and to reduce costs. It was also placed to provide advantageous low yield ratio steel materials.

[0011]

[Means for Solving the Problems] In order to achieve the above object, the inventor of the present invention has repeatedly conducted many experiments and researched, and as a result, has discovered the following: Mo addition or Mo and Cr, Ni, Cu,
When combined addition of Nb, V, B, or Ca is performed, and at the same time, Ti oxide-based inclusions are finely dispersed in the matrix,
It is possible to realize a steel material that exhibits sufficient fire resistance for use in building structures, does not cause noticeable deterioration in HAZ toughness even during high heat input welding, and has excellent toughness in the base metal and weld metal parts. ”.

[0012] The present invention was completed based on the above-mentioned findings, etc., and is based on the above-mentioned findings. ), Si: 0.05 to 0.60%,
Mn: 0.5 to 1.6%, Ti
: 0.003 to 0.050%, Mo: 0.5% or less, N: 0.0010 to 0.0060%,
O: Contains 0.0004 to 0.0060%,
Or further Cr: 0.05-1.0%, Ni: 0.
05~0.50%, Cu:0.05~1.0
%, Nb: 0.004 to 0.04%, V: 0
．． 004 - 0.10%, B: 0.0003 - 0.
0.002%, the balance is Fe and unavoidable impurities, and the base material contains 0.002% or more of
．． By creating a structure in which oxide-based inclusions having a Ti(O, N) composite crystal phase with a particle size of 3 μm or less are dispersed at a proportion of 0.001 to 0.100% by weight, the toughness of the base material and the weld metal are improved. Its major feature is that it not only has excellent joint toughness, but also fire resistance and welded joint toughness that are sufficiently satisfactory as a structural material for buildings.

As described above, the present invention adds Mo or Mo and Cr, Ni, and C to steel whose C, Si, and Mn components have been adjusted.
By adding alloying elements such as u, Nb, and V, we improve the fire resistance of steel materials, and at the same time, by finely dispersing stable Ti-based composite oxynitride particles, we can maintain HA even during high heat input welding.
The main point is to prevent deterioration of Z toughness, but if a large number of fine particles as mentioned above are dispersed in the steel, as shown in Figure 1, this also prevents the strength from decreasing at high temperatures. The suppressing effect is ensured. Therefore, if Ti-based composite oxynitride particles are finely dispersed in steel, H
Alloy components that reduce AZ toughness (Mo, Cr, Ni, C
u, Nb, V, etc.).
Finely dispersing the composite oxynitride particles contributes to improving HAZ toughness in two ways.

[0014]

[Function] The function of the steel material according to the present invention will be explained in more detail below. Now, in order to increase the strength of steel materials in the event of a fire, it is effective to add Mo and Cr as alloy components, and that the above-mentioned effects are promoted by adding Nb, V, etc. in combination in addition to these. is known. but,
As mentioned above, when such alloy components are included, H
If the alloy component is increased until the AZ toughness deteriorates and the desired fire resistance is secured, normal welding for building materials will no longer be possible. Therefore, it has not been possible to take full advantage of the economic advantage of being able to make the fireproof coating thinner by adding alloying components.

However, in the present invention, Mo, Cr, Nb
Based on the results of studies on methods to increase high-temperature strength while minimizing the amount of added elements such as We succeeded in paving the way to both securing HAZ toughness.

In other words, the presence of finely dispersed particles in steel makes the structure of the steel finer, and at the same time suppresses recrystallization and grain growth at high temperatures, resulting in the "Petch relationship" (yield strength is inversely proportional to the square root of the grain size). )”, the strength increases. However, if the particles to be dispersed are ordinary carbide particles, they will begin to dissolve into the steel at temperatures above 600°C, so the effect will gradually diminish, but Ti-based oxynitrides are extremely stable, so they cannot be used at high temperatures. Even if there is, the effect of suppressing strength reduction is significant. Moreover, among various oxide-based fine particles, “Ti”
Oxynitrides are relatively easy to finely disperse in steel, and can be industrially and economically treated for dispersion and precipitation from the steelmaking process to the continuous casting process. Can be done.

As mentioned above, finely dispersing Ti-based oxynitride particles in the matrix increases the high-temperature strength of the steel material and improves the fire resistance. Therefore, Mo or Cr added to improve the fire resistance , Nb, V, etc., can be added as low as possible. And this Mo, Cr, N
Since B, V, etc. are elements that have a large adverse effect on HAZ toughness, it is extremely preferable to be able to reduce the amount of these added in order to prevent deterioration of HAZ toughness.

Furthermore, the Ti-based oxynitride itself also has the following effects, and therefore, the fine dispersion of the Ti-based oxynitride has a great effect on improving HAZ toughness from a more positive perspective. become.

That is, when welding steel materials, the HAZ is heated by welding heat to just below the melting point of the steel, so in the case of ordinary steel, the austenite grains in the HAZ become extremely coarse. On the other hand, there is a situation in which the subsequent cooling rate in this portion is extremely fast. In this way, H
In AZ, the hardenability increases due to the large austenite grains, and at the same time the cooling rate is fast, so martensitic transformation or bainite transformation dominates in this area, and generally a hard and coarse structure is formed, resulting in a decrease in toughness. .

However, in the steel material according to the present invention, "Ti-based oxynitrides that do not melt and disappear even with welding heat" are present in the matrix.
Since the oxynitride fine particles are dispersed at a predetermined high density, the oxynitride fine particles change the structure of the HAZ through the following two actions. a) Fine oxynitride particles suppress the growth of austenite grains, prevent them from becoming coarse, and prevent bainite transformation and martensitic transformation from easily occurring, thereby making the HAZ structure somewhat finer. Soften. b) During the γ→α transformation, finely dispersed oxynitride particles act as nuclei and promote the formation of ferrite, changing the structure of the HAZ to a “Widmanstätten-like structure mainly consisting of ferrite side plates” or a “ferrite-like structure”.・Change to a state consisting mainly of pearlite. Therefore, in the steel material according to the present invention, even if high heat input welding is performed, the deterioration in HAZ toughness is very small, and good HAZ toughness is maintained.

[0021] It is generally said that the effect shown in item a) is most effective if the particle size of the precipitated inclusions is 0.02 μm or less. However, when the precipitated inclusions are ordinary oxides, it is difficult to make even precipitates with a particle size of 3 μm or less remain in the steel, not to mention such small ones. Therefore, in the present invention, Ti
, a predetermined amount of N during the formation of Ti oxide (TiO) by O.
Ti-O-N is added at the same time, and the resulting crystal phase
It was made into a composite phase. This composite phase is more difficult to form than TiO and also has a slower growth rate, so as a result, TiO
More finely dispersed precipitation occurs than in the case of individual precipitation. Furthermore, it was discovered that the effect shown in item a) above can be effectively utilized even when the average particle size is 3 μm or less in the oxynitride having the composite phase, and this has led to the completion of the present invention. Ta.

In addition, the precipitated Ti-based composite oxynitride has a particle size of 0.0 from the viewpoint of ease of preparation and ferrite nucleation ability.
5 to 3 μm is preferable, and in order to more fully ensure the effect of the action shown in item b) above, the average particle size of the Ti-based composite oxynitride dispersed in the steel base should be 1 μm or less. It has also become clear that this is more preferable.

In addition, in the steel material according to the present invention, the content ratio of each chemical component constituting the steel material is regulated in a well-balanced manner, as described above, so that the base metal and weld metal part toughness is also lower than that of conventional steel. It shows an equally excellent value.

Next, the reason why the composition of the steel material according to the present invention is numerically limited as described above will be explained. C C is an effective component to ensure the strength of the base material and to exhibit the effect of adding Mo, but its content is 0.05
If it is less than 0.20%, the effect is not sufficient;
The C content was determined to be 0.05 to 0.20% because if the C content exceeds 0.2%, the HAZ toughness and weldability deteriorate.

Si Si is an effective element as a deoxidizing agent for steel, and also has the effect of ensuring the required strength of the base material, but if its content is less than 0.05%, the desired effect due to the above effect is not achieved. On the other hand, if the Si content exceeds 0.60%, it will cause deterioration of HAZ toughness and weldability, so the Si content should be 0.
．． It was set at 0.05 to 0.60%.

Mn Mn has the effect of ensuring the required strength and toughness of the base material, but if its content is less than 0.5%, the desired effect of the above effect cannot be obtained; If the Mn content exceeds 0, the weldability will deteriorate, so the Mn content should be 0.
．． It was set at 5% to 1.6%.

Ti, N and O Ti, N and O are important components that characterize the HAZ structure of the steel material according to the present invention. That is, when adding Ti to molten steel, Ti oxides will be formed if Ti is added when the amount of dissolved oxygen in the molten steel is relatively large, but the Ti oxides formed at this time are generally coarse. The number is also small, and therefore, the effect of significantly improving HAZ toughness, which is the objective of the present invention, cannot be expected. In order to form a "structure in which fine Ti oxide-based inclusions are dispersed in steel at a high density" that fully exhibits the above-mentioned effects aimed at by the present invention, it is necessary to increase the amount of dissolved nitrogen as well as the amount of dissolved oxygen. It is necessary to deoxidize the molten steel, which has been adjusted to a high level within a predetermined range, immediately before pouring, and to perform continuous casting. Only by taking such measures will the oxide produced be Ti(O,N)
It becomes a composite crystal phase of the system, and is dispersed in extremely fine form and at high density in the steel.

[0027]For that purpose, at least 0.0
It is necessary to ensure a Ti content of 0.3%, and if the Ti content is less than this, the desired structure in which fine Ti-based composite oxynitrides are dispersed cannot be achieved. On the other hand, 0.
If Ti is contained in an amount exceeding 0.050%, there is a concern that ``excessive oxynitride formation'' and ``the coarsening of oxynitrides'' may occur, as well as deterioration of the toughness of the base metal and weld metal. Therefore, the Ti content is
It was set at 0.003 to 0.050%.

[0028] Also, when the contents of N and O are lower than 0.0010% and 0.0004%, respectively, it is not possible to realize a structure in which the desired fine Ti-based composite oxynitride is dispersed. On the other hand, if the content of N or O exceeds 0.0060%, it will also lead to "excessive oxynitride formation" and "coarsening", as well as the toughness of the base metal and weld metal. Since this would lead to deterioration, the N content was set at 0.0010% to 0.0060%, and the O content was set at 0.0004% to 0.0060%.

As mentioned above, a predetermined amount of Ti and O are essential components for generating Ti oxide-based inclusions in the steel, which form the basis of the steel material according to the present invention, while a predetermined amount of N is an indispensable component for the fine dispersion effect ensured by converting this Ti oxide into a Ti(O,N) composite crystal phase. The oxide inclusions having the Ti(O,N) composite crystal phase finely dispersed in the steel matrix form the austenite grain boundaries during the cooling process of the heated steel material, which has become an austenite state. This independently promotes the formation of ferrite from within the grains, and prevents the HAZ structure from consisting only of coarse bainite, thereby contributing to improving the toughness of the HAZ.

[0030] If the average particle size of the Ti-based composite oxynitride exceeds 3 μm, the austenite grain growth inhibiting effect and ferrite forming effect will not be sufficiently exerted, and the desired H
As mentioned above, it is not possible to stably secure the effect of improving AZ toughness, but unless the quantitative density of this Ti-based composite oxynitride is high enough, the number of ferrite nuclei generated will be small and the HAZ toughness will improve. cannot be secured. That is, to ensure the desired HAZ toughness, T with a particle size of 3 μm or less is required.
It is necessary that the i-based composite oxynitride is dispersed at a quantity density of about 5 x 103 cells/mm3 or more. And Ti
As the number of composite oxynitride particles increases, the obtained ferrite becomes finer and the HAZ toughness improves, but if the number of particles increases too much and exceeds 108 particles/mm3, the toughness and ductility of the base material tend to decrease. show. This "quantity density range of Ti-based composite oxynitride particles that provides good performance" is 0.001 to 0.1 with an amount of Ti oxynitride of 3 μm or less.
Since this corresponds to 0.00%, the amount of Ti oxynitride-based inclusions to be dispersed was determined to be 0.001 to 0.100%.

Mo Mo has the effect of improving the strength of the base material at high temperatures, and in the presence of finely dispersed Ti-based composite oxynitrides, a corresponding effect can be obtained even by adding a small amount. 0.
If Mo content exceeds 5%, it will lead to a decrease in HAZ toughness and weldability, so the Mo content should be 0.5%.
It was determined as follows.

Cr, Ni, Cu, Nb, V, and B All of these components have the effect of improving the strength of the base material, so it is preferable to selectively add one or more of them as necessary. However, the appropriate amount range of each component will be explained below along with other effects.

[0033] Like Mo, Cr has the effect of improving the strength of the base material at high temperatures, but if its content is less than 0.05%, the desired effect due to the above effect cannot be obtained;
If the Cr content exceeds 1.0%, HAZ toughness and weldability will decrease, so the Cr content should be 0.05 to 0.05%.
It was set at 1.0%.

[0034] Ni has the effect of improving the strength and toughness of the base metal without impairing weldability or HAZ toughness, but if its content is less than 0.05%, the desired effect cannot be obtained by the above effect. On the other hand, since Ni is an expensive element,
Containing more than 0.5% causes economic disadvantage. Therefore, the Ni content was set at 0.05 to 0.5%.

[0035] Like Ni, Cu has the effect of improving the strength and toughness of the base metal without impairing weldability and HAZ toughness, and also has the effect of increasing high-temperature strength and improving corrosion resistance and weather resistance. However, the content is 0.0
If the Cu content is less than 5%, the desired effect due to the above action cannot be obtained, while if the Cu content exceeds 1.0%, cracks will occur during hot rolling. Therefore, the Cu content should be 0.05 to 0.
It was set at 1.0%.

[0036] Nb has the effect of increasing the strength of the base material at room temperature and high temperature. This effect becomes particularly strong when Mo or Cr is added in combination, but when the content is 0.
If the Nb content is less than 0.04%, the desired effect cannot be obtained, while if the Nb content exceeds 0.04%, the HAZ toughness will be reduced.
It was set at 0.04 to 0.04%.

Although V has less effect than Nb, Nb
Like b, it has the effect of increasing the strength of the base material at room temperature and high temperature, but if its content is less than 0.004%, the desired effect due to the above effect cannot be obtained;
If the V content exceeds 0.004 to 0.1%, it will cause a decrease in HAZ toughness.
It was set as 0%.

[0038] B has the effect of improving the hardenability of steel and increasing the strength of the base metal, but if its content is less than 0.0003%, the desired effect due to the above effect cannot be obtained;
If the content exceeds 0.002%, boron carbide becomes HA.
The B content was determined to be 0.0003 to 0.002% since it precipitates in the Z and causes deterioration of HAZ toughness.

By the way, in order to manufacture steel materials in which Ti-based composite oxynitrides are finely dispersed, the amount of dissolved oxygen is reduced to 20 to 80 ppm and the amount of dissolved nitrogen is reduced to about 80 ppm or less by preliminary deoxidation with Si, Al, etc. (When deoxidized with Al, approximately 0.007% or less Al usually remains), and the molten steel that has undergone other compositional adjustments is treated with Ti in a tundish or the like immediately before pouring in continuous casting. Casting may be performed after adding and deoxidizing. By this, T
The fire resistance and H
Steel materials with excellent AZ toughness, base metal toughness, and weld metal part toughness can be stably mass-produced.

Next, the effects of the present invention will be specifically explained with reference to Examples.

[Example] First, the amount of residual oxygen in molten steel is 0.0020 to 0.
．． 0080%, residual nitrogen amount in molten steel is 0.0040 to 0
．． After deoxidizing by adding Ti to the molten steel (the amount of unavoidably mixed Si: 0.05% or less) which has been adjusted to 0.080% and also adjusted other components, Si is further added. Continuous casting was performed to obtain slabs having the chemical composition shown in Table 1. Next, after heating this to 1100 to 1150°C,
Finish rolling temperature: Hot rolling was performed at 800 to 900°C to produce steel plates having the thicknesses shown in Table 1. In addition, Comparative Examples 11 to 15 are fireproof steel materials prototyped according to conventional knowledge, and Comparative Example 16 is a material related to general steel.

Next, the "room temperature strength" and "strength at 600°C" of each of the above-mentioned steel plates were measured, and in order to investigate the HAZ toughness, each steel plate was reduced to a thickness of 20 mm and then subjected to electroslag large heat input. Welding was performed, and the Charpy absorbed energy value of the HAZ at 0°C was measured. These results are shown in Table 2.

[0042]

[Table 1]

[0043]

[Table 2]

As is clear from the results shown in Table 2, although the steel material according to the present invention has a lower "Ceq", which is an index of weldability, than the comparative steel material, it has a base material that is not significantly different from the comparative steel material. Looking at the yield strength at 600°C, it can be seen that the yield strength increases considerably despite the small amount of Mo added, which is highly effective in ensuring high-temperature strength. Regarding HAZ toughness, it can be confirmed that the steel materials according to the present invention are significantly improved compared to the comparative steel materials.

[0045]

[Summary of Effects] As explained above, the present invention not only has strength, base material toughness, and weld metal toughness that are fully satisfactory as structural materials for construction, but also has excellent fire resistance and HAZ toughness. This brings about extremely useful effects industrially, such as being able to provide steel materials exhibiting a

[Brief explanation of drawings]

FIG. 1 is a graph comparing the high temperature strength characteristics of Ti-based composite oxynitride finely dispersed steel and conventional steel.

Claims (2)

[Claims] [Claim 1] C: 0.05 to 0.20%, Si: 0.0% by weight
5-0.60%, Mn: 0.5-1.6%
, Ti: 0.003 to 0.050%, Mo: 0
．． 5% or less, N: 0.0010 to 0.0
060%, O: 0.0004 to 0.0060%, and the balance is Fe and inevitable impurities, and the base material contains 0.001 to 0.100%.
Ti(O, N) with a particle size of 3 μm or less in weight percent
A steel material with excellent fire resistance and weld joint toughness, characterized by dispersing oxide inclusions with a composite crystal phase. [Claim 2] C: 0.05 to 0.20%, Si: 0.0% by weight
5-0.60%, Mn: 0.5-1.6%
, Ti: 0.003 to 0.050%, Mo: 0
．． 5% or less, N: 0.0010 to 0.0
060%, O: 0.0004 to 0.0060%, and further contains Cr: 0.05 to 1.0%, Ni: 0.
05~0.50%, Cu:0.05~1.0
%, Nb: 0.004 to 0.04%, V: 0
．． 004 - 0.10%, B: 0.0003 - 0.
0.002%, the balance is Fe and unavoidable impurities, and the base material contains 0.002% or more of
．． A fire-resistant welded joint characterized by dispersing oxide-based inclusions having a Ti(O, N) composite crystal phase with a grain size of 3 μm or less at a proportion of 0.001 to 0.100% by weight. Steel material with excellent toughness.