JP2662198B2 - Manufacturing method of cast steel with excellent fire resistance, strength and toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cast steel product having excellent fire resistance, strength and toughness used as a structural member of a building, in particular, a concentrated member of the building, and a method for producing the same.

[0002]

2. Description of the Related Art In order to cope with the diversification and complexity of both forms and functions of buildings, a rational and comprehensive approach for achieving a certain level of fire protection in accordance with the conditions of individual buildings. The development of fire protection design method was led by the government,
It was put into practical use in March 2003 as the "Comprehensive Fire Protection Design Method for Buildings". With this practical application, in steel buildings, the target level of fire resistance required for individual buildings will be technically clarified, and the fire resistance will be adjusted according to the thermal and mechanical properties of the material (steel material) to achieve the target level. Structural specifications (type and thickness of heat insulating material covering steel) can now be determined rationally.

[0003] That is, the strength decrease at high temperature (at the time of fire) is smaller than that of a general steel material for building structures.
In the case of using a new "steel material" that can set a guaranteed value for the mechanical properties, it became possible to reduce construction costs by using inexpensive low-heat insulation materials and reducing the coating thickness.

As a technique to cope with such a trend, Japanese Patent Application Laid-Open No. 2-77523 and Japanese Patent Application Laid-Open No. 3-6322 disclose a "method of manufacturing a low-yield-ratio steel material for building having excellent fire resistance and its steel material". Or a steel material for architectural use of steel, or JP-A-5-195141, entitled "Forged steel with excellent fire resistance and toughness and a method for producing the same". The gist of the former is that the yield point at 600 ° C is 70% or more of that at room temperature, because of the balance between the increase in steel cost due to the addition of alloying elements necessary for securing high-temperature strength and the reduction in fire-resistant coating construction cost due to securing high-temperature strength. The most economical use of steel is that low-C-low-Mn steel is mixed with a small amount of Nb and Mo to re-heat a steel slab at a high temperature and then finish rolling at a relatively high temperature. Alternatively, Ar ₃ -1
It is obtained by water cooling from 00 ° C. to an arbitrary temperature of 550 ° C. or less. The latter has excellent high-temperature properties even for large-section members due to component composition, forging processing conditions, and heat treatment, targeting large-scale building columns, beam aggregate members, etc. that cannot be manufactured with rolled materials. It provides a forged steel product having excellent strength and toughness.

[0005]

2. Description of the Related Art In recent years, there has been an increasing movement to incorporate claims of various expressions into buildings and buildings by various designs. To satisfy this artistic desire, especially the special shape, with the above prior art,
There is a problem that it is difficult to achieve a balance between economic efficiency as an industrial product such as yield and productivity.

The present inventors have tried to meet this demand by casting excellent in formability. However, the material of cast steel refractory steel is the strength of rolled or forged refractory steel.
Compared with the relationship of toughness, it was found that there was a problem that there was a large variation in toughness, and some portions did not satisfy the standard. In addition, while both rolled and forged materials undergo rolling or forging, the steel structure is uniformly refined, whereas the cast steel material is not subjected to plastic working, and the degree of coarseness / density of the solidified structure directly affects the material. It was also found to be due to.

The present inventors have found a steel component that makes the solidification structure finer in the search for the above problems, and have solved the problem by producing a steel material composed of the component system under appropriate heat treatment conditions. We found that it was possible. That is, the present invention has been completed on the basis of the above findings, and achieves both moldability capable of responding to various designs and economic efficiency as an industrial product, and a cast steel product excellent in fire resistance and strength / toughness. The purpose is to provide stable.

[0008]

Means for Solving the Problems To achieve the above object, the present invention has the following constitution. C: 0.05 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.4 to 2.0%, Mo: 0.2 to 1.0%, V : 0.02 to 0.2%, N: 0.0060 to 0.0180%, Ca: 0.005 to 0.01%, Al: 0.03% or less as a basic component, or Cr ≦ 1 0.0%, Ni ≦ 1.0%, Cu ≦ 1.0%, Nb ≦ 0.1%, Ti ≦ 0.03%, with the balance being Fe and unavoidable impurities After heating a cast steel product consisting of the composition of
A method for producing a cast steel product having excellent fire resistance, strength and toughness, characterized by performing at least one normalizing process at a cooling rate of 2 to 30 ° C./min during cooling. After heating the article to a temperature between 800 and 1100 ° C, the cooling rate is 2-30 ° C /
A cast steel product having excellent fire resistance, strength and toughness, characterized by performing normalizing treatment at least once, followed by tempering at least once in a temperature range below the Ac ₁ transformation point. After heating the cast steel product having the composition described in the above item to a temperature between 800 and 1100 ° C., when cooling, the cooling rate is 2 to 30 ° C. /
Performs normalizing treatment to regulate the amount, followed by baked back normalizing at a temperature range Ac ₁ transformation point - refractory and strength and toughness, characterized in that the tempering process is performed twice or more excellent cast steel It is a manufacturing method of.

[0009]

The present invention will be described below in detail. In general, to increase the high-temperature strength, Mo, Cr and the like are added to increase the resistance to softening at high temperatures by the effect of precipitation hardening by carbides and the effect of suppressing the dissipation of dislocations due to the bond between the element itself and atomic vacancies. This has been achieved by:
However, the addition of Mo and Cr increases the hardenability and increases the proportion of bainite of ferrite-bainite in the base metal structure. That is, the dependency of the crystal structure ratio between ferrite and bainite on the cooling rate from the austenite region to room temperature is increased. In addition, as a result of detailed investigations by the present inventors,
As described above, it was found that the crystal structure of the cast steel product was coarse and showed 1.5 to 2 times the crystal grains of a rolled or forged material subjected to the same heat treatment. As a result of these, unlike a simple rolled or relatively simple forged product,
Room temperature strength, high temperature strength, ductility and toughness of cast steel products that are complex and have various cooling rates vary depending on the part, and there are parts where the toughness does not meet the standard, especially where the solidification rate during casting is relatively low and the crystal It was found to be a coarse part of the tissue.

A feature of the present invention is to reduce the variation in the composition ratio of ferrite and bainite due to the cooling rate by reducing the crystal structure in order to reduce the material variation in each part of the cast steel product having different cooling rates. It is to make the most of the nitride precipitation. That is, VN mainly promotes ferrite transformation from austenite during cooling during solidification and during normalizing, and AlN, NbN, and TiN during heating during normalizing.
The intended purpose is attained by combining these as necessary, such as the ferrite-pearlite-bainite structure can be stably obtained as the final structure by making the austenite grain refinement by austenite grain refinement by single or composite precipitation. Is what you can do. Therefore, in the steel of the present invention, normalizing is an essential process. In addition to such basic ideas, during the course of research, the present inventors
Has a remarkable effect on the densification of the solidified structure. Furthermore, when combined with normalizing treatment, the ferrite crystal grains are further refined and at the same time, the pearlite-
It has been found that spheroidization and fine dispersion of the bainite region occur uniformly, and as a result, high toughness can be obtained. The present invention is configured based on these findings.

Next, the reasons for limiting the range of the basic components of the cast steel of the present invention will be described. First, C is added as an effective component for improving the strength of steel. If it is less than 0.05%, the strength required as a structural member cannot be obtained, and if it exceeds 0.20%, the base material toughness and welding Crackability, heat affected zone (hereinafter H
AZ) Since the toughness and the like are significantly reduced, the upper limit is set to 0.20%.

Next, Si is important not only for securing the strength of the base material but also for improving the castability.
If it exceeds 0%, high carbon martensite having a hardened structure is formed in the structure after the heat treatment, and the toughness is significantly reduced. If the content is less than 0.05%, the fluidity of the molten steel is remarkably reduced to cause structural defects. Therefore, the Si content is limited to this range.

Mn is used in an amount of 0.1 to ensure the strength and toughness of the base material.
Although addition of 4% or more is necessary, the upper limit is set to 2.0% within an allowable range such as toughness and hydrogen cracking of the welded portion.

Mo is a particularly important element because it delays recovery and secures high-temperature strength. However, if it is less than 0.2%, sufficient high-temperature strength cannot be secured, and if it exceeds 1.0%, hardenability increases. Since the upper bainite structure becomes excessive and base metal toughness and HAZ toughness deteriorate, the content is limited to 0.2 to 1.0%.

V precipitates as VN during solidification and during cooling after normalizing, thereby promoting ferrite transformation from austenite and contributing to the refinement of crystal grains. And contributes to strengthening of the base material by precipitation strengthening. It is particularly effective for securing high-temperature strength. However, if the addition amount is less than 0.02%, the effect is small, and excessive addition is harmful to the base material toughness and HAZ toughness, so the upper limit is 0.2%.

N is used to reduce the crystal structure and strengthen precipitation.
It is a particularly important element in the present invention utilizing various nitrides. Therefore, 0.006% or more is required, but if it exceeds 0.0180%, the base material toughness is reduced and cracks are generated on the surface of the slab, so the content was limited to 0.0180% or less.

[0017] In addition to its effect as a deoxidizing agent, Ca has an important function in the steel of the present invention, that is, to make the solidified structure finer. Although the refinement of the solidification structure remarkably refines the crystal structure after the subsequent normalizing treatment and improves the strength and toughness, it is preferable that the solidification structure be improved.
If it is less than 0005%, there is no effect, and if it exceeds 0.01%, coarse Ca sulfide oxides are generated and the ductility and toughness of the steel are reduced. Therefore, it is added in the range of 0.0005 to 0.01%.

Al is usually used as a deoxidizing agent in steelmaking and forms nitrides during normalizing to refine austenite grains. Excessive Al deteriorates the cleanliness of steel and impairs toughness. . Further, when the amount of solute Al becomes excessive, the compounding with N increases, and the amount of precipitation of VN, which is a feature of the present invention, is reduced to lower the strength (particularly, high-temperature strength). Preferably it is 0.010% or less.

Further, the amounts of P and S contained as unavoidable impurities are not particularly limited. However, the amounts of P and S should be reduced as much as possible because they cause welding cracks and decrease in toughness due to solidification segregation. 0.02 for both
% Or less.

The above is the first basic component of the steel of the present invention.
In the steel of the present invention, Ni, Cr, Cu, and T are used to improve the balance between strength and toughness of the base material with respect to the above basic components.
One or more of i, Nb, can be contained. First, Ni is a very effective element for increasing the toughness of the base material. However, if added over 1.0%, the ratio of the upper bainite structure is increased to lower the toughness, and the alloy cost is increased, which is not economical. Was set to 1.0%.

Cr is effective in improving the strength of the base material and the high-temperature strength by improving the hardenability and the precipitation strengthening. However, an excessive addition lowers toughness, so the upper limit was made 1.0%. Cu is an element effective for strengthening the base material and weather resistance, but the upper limit is set to 1.0% in consideration of hot working cracks.

Although the nitride of Ti effectively acts on densification of the crystal structure, an excessive addition forms a carbide and impairs the toughness. Therefore, Ti is added up to 0.03% as necessary. Nb is effective for strengthening the base material, but an excessive addition exceeding the upper limit is harmful to the base material toughness and HAZ toughness, so the upper limit is 0.1%.

Next, the production method of the present invention will be described. The steelmaking furnace for the steel of the present invention may be any furnace, such as an electric furnace or a converter, and is not particularly limited. In addition, the current refining technology such as degassing and ladle refining can be applied. The steel of the present invention to which Ca is added has a remarkably fine solidified structure. The molding of the steel of the present invention produced in this way is not particularly limited by any method such as a current mold such as a sand mold or a metal mold, and centrifugal casting.

The cast steel product of the present invention thus formed is given a specified strength and toughness by heat treatment. Prior to this heat treatment, diffusion annealing, dehydrogenation, strain relief annealing, etc., which are usually performed, may be performed. As the heat treatment, a normalizing process of heating to a temperature between 800 to 1100 ° C. is performed one or more times. In that case, the temperatures do not necessarily have to be the same. Thereby, the combination of strength and toughness can be controlled. When the normalizing treatment is repeatedly performed on the steel of the present invention in which the solidification structure is made fine by adding Ca, the refinement of the austenite crystal grains and the spheroidization and fine dispersion of the pearlite-bainite region in the crystal structure do not add Ca. It is performed more effectively than steel and can significantly improve toughness.

The cooling rate after normalizing is regulated to 2 to 30 ° C./min. The temperature should not exceed this range at least between 800 and 500 ° C. If it is lower than 2 ° C./min, the amount of bainite in the steel structure will be small and the required strength cannot be obtained. If it is higher than 30 ° C./min, the bainite content will increase, resulting in excessive strength and insufficient toughness.

After the normalizing process, a tempering process can be performed. Repeated tempering can suppress variations in strength and toughness. Also in this case, the temperatures do not necessarily have to be the same. Further, it can also serve as strain relief annealing. Furthermore, it is also possible to perform a tempering process immediately after the normalizing process and repeat this process. As a result, not only can the material fluctuation be further reduced, but also room temperature strength, high temperature strength and toughness can be stabilized at a high level.

[0027]

Examples of the present invention will be described below. Table 1 shows the chemical components of the steel of the present invention and the comparative steel, and Table 2 shows the production conditions and mechanical test characteristics. The refractory cast steel product obtained by manufacturing the component steel of the present invention by the manufacturing method of the present invention is a rolled steel material for welded structure JIS-G3106 SM49 exemplified as a target value in Table 2.
OB sufficiently satisfies the required values of the mechanical properties.

On the other hand, the comparative material # FC-13 has V,
The N content is low, the room temperature strength and the strength at 600 ° C. cannot satisfy the target values, and the toughness is extremely low as cast. In # FC-14 with excessive addition of V, carbonitrides in the upper bainite structure are coarsened and the high-temperature strength is rather reduced. In addition, in the case of # FC-15 and 19 in excess of C and N and # FC-16 and 17 in excess of Ni and Mo, even with the steel of the present invention, FC-7 (27) having a high cooling rate after normalizing was used. In each case, the upper bainite structure develops and coarsens,
The toughness is significantly reduced and the target value cannot be satisfied. FC-18 with excessive Ti does not provide the required toughness.

FIG. 1 is an optical micrograph showing the metal structure of the inventive example (22), and FIG. 2 is an optical micrograph showing the metallographic structure of the comparative example (45). It is a photograph showing that it is No. 7 in the example, and the form and dispersion state of the pearlite-bainite portion.
As described above, in the example of the present invention, the steel structure was refined.
The high toughness shown in Fig. 1 is obtained. That is, when all the requirements of the present invention are satisfied, a refractory cast steel product having sufficient room temperature and high temperature strength and excellent toughness can be manufactured.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

The cast steel product according to the present invention has excellent high temperature properties,
The coating thickness of the refractory material can be significantly reduced compared to the conventional one, and the cost can be significantly reduced by reducing the construction cost and the construction period. In addition, it is possible to manufacture a fireproof building material having a large cross section, and there are extremely significant industrial benefits such as improvement in reliability of a large building, security assurance, and economic effects. In addition, it has a high degree of freedom in formability and greatly contributes to the architect's compatibility between artistic expression and industriality.

[Brief description of the drawings]

FIGS. 1 (a) and 1 (b) show optical microscopic metallographic structures of inventive examples (22 in Table 2).

2 (a) and 2 (b) show optical microscopic metal structures of a comparative example (45 in Table 2).

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tsutomu Tajima 46-59 Nakahara-no-Hama, Ohara, Tobata-ku, Kitakyushu-city, Fukuoka Japan (72) Inventor Yukio Mochizuki Nohama 46-59 Nippon Cast and Forged Steel Co., Ltd. (72) Inventor Rikio Chichiiwa 1 Kimitsu, Kimitsu City, Chiba Prefecture Inside Kimitsu Works, Nippon Steel Corporation (72) Inventor Ryosuke Shizora 2- Otemachi, Chiyoda-ku, Tokyo 6-3 Inside Nippon Steel Corporation (56) References JP-A-6-65676 (JP, A) JP-A-3-249149 (JP, A)

Claims (4)

(57) [Claims] 1. C: 0.05 to 0.20% by weight Si: 0.05 to 0.50% Mn: 0.4 to 2.0% Mo: 0.2 to 1.0% V: 0 0.02 to 0.2% N: 0.0060 to 0.0180% Ca: 0.0005 to 0.01% Al: 0.03% or less, the balance being Fe and the composition of unavoidable impurities After heating the article to a temperature between 800 and 1100 ° C,
A method for producing a cast steel product having excellent fire resistance, strength and toughness, wherein a normalizing process for controlling a cooling rate to 2 to 30 ° C./min is performed once or more when cooling. 2. C: 0.05 to 0.20% by weight Si: 0.05 to 0.50% Mn: 0.4 to 2.0% Mo: 0.2 to 1.0% V: 0 0.02 to 0.2% N: 0.0060 to 0.0180% Ca: 0.0005 to 0.01% Al: 0.03% or less, the balance being Fe and the composition of unavoidable impurities After heating the article to a temperature between 800 and 1100 ° C,
Fire resistance characterized by performing normalizing at least once at a cooling rate of 2 to 30 ° C./min at the time of cooling, and then performing tempering at least once in a temperature range below the Ac ₁ transformation point. And a method for producing cast steel products with excellent strength and toughness. 3. C: 0.05 to 0.20% by weight Si: 0.05 to 0.50% Mn: 0.4 to 2.0% Mo: 0.2 to 1.0% V: 0 0.02 to 0.2% N: 0.0060 to 0.0180% Ca: 0.0005 to 0.01% Al: 0.03% or less, the balance being Fe and the composition of unavoidable impurities After heating the article to a temperature between 800 and 1100 ° C,
When cooling, a normalizing process in which the cooling rate is restricted to 2 to 30 ° C./min is performed, and then a normalizing-tempering process is performed twice or more in a temperature range below the Ac ₁ transformation point. For producing cast steel products with excellent fire resistance and strength and toughness. 4. The cast steel product according to claim 1, wherein Ni: 1.0% or less, Cr: 1.0% or less, Cu: 1.0% or less, Nb: 0 by weight%. .1% or less Production of a cast steel product excellent in fire resistance and strength and toughness, characterized in that it contains one or more of Ti: 0.03% or less, and the balance is composed of Fe and inevitable impurities. Method.