JPH08945B2 - Forged steel product excellent in fire resistance and toughness and method for manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Due to the construction of super-high-rise buildings and the sophistication of building design technology, the fireproof design was reviewed by the Ministry of Construction comprehensive project, and the "New Fireproof Design Law" was enacted in March 1987. . By this regulation, the restriction of fireproof coating to keep the temperature of steel at 350 ° C or less at the time of fire under the old law was lifted, and a fireproof coating method suitable for it was established due to the balance between the high temperature strength of steel and the actual load of the building. I can now decide. That is, when the designed high temperature strength at 600 ° C. can be secured, the fireproof coating can be reduced accordingly.

As a technique for responding to such a trend, the applicants disclosed in Japanese Patent Application Laid-Open No. 2-77523 and Japanese Patent Application Laid-Open No. 3-6322 disclose "a low yield ratio steel material for construction having excellent fire resistance. There is a manufacturing method and a steel material for construction using the steel material. The main point of this technology is to balance the increase in steel cost by adding alloying elements necessary for securing high temperature strength with the reduction of fireproof coating construction cost by securing high temperature strength.
Steels with a yield point at ℃ of 70% or more of that at room temperature are the most economical.
b) and a suitable amount of Mo are added together to reheat a steel slab having a compositional composition at a high temperature and then finish rolling at a relatively high temperature, or further water cooling from Ar ₃ -100 ° C to an arbitrary temperature of 550 ° C or less. It is obtained by doing.

[0004]

In recent years, forged steel products have been demanded mainly for large-section joints and the like, which are mainly column members for construction and beam members and exceed the production limit of rolled materials. The present inventors have tried to apply the steel material produced by the above-mentioned prior art to various forged steel products, particularly members having a large cross section, and materials for deformed forged steel products having different cross-sectional shapes at both ends, It was found that there are problems such as room temperature strength, high temperature strength, ductility, toughness, etc. within one member, and some parts do not satisfy the standard.

Further, while the rolled material finishes rolling over its entire length at a substantially constant temperature in a relatively short time, in the case of a forged steel product, the forging process gradually takes a long time from one end of the material to the other end. As a result of the application, it was found that the processing temperature, that is, the forging finishing temperature, was different in each part, and in the case of the irregular shape, it was caused by the difference in the forging ratio and the cooling rate after the forging process.

The present inventors, while pursuing the above problems, found a component condition in which the change in the crystal structure is small even when the forging temperature is changed and the cooling rate is different, and from the component system, It has been found that the problem can be solved by manufacturing the following steel materials under proper forging conditions.

That is, the present invention has been completed based on the above findings, and stably provides a forged steel product having excellent fire resistance and toughness, which has excellent high-temperature characteristics even for a large cross-section member. The purpose is to

[0008]

In order to achieve the above object, the present invention has the following structures. That is, by weight, C: 0.05 to 0.20%, Si: 0.05
~ 0.50%, Mn: 0.4-2.0%, Mo: 0.3
~ 0.7%, V: 0.05 to 0.20%, N: 0.00
60 to 0.0180%, Al: 0.005 to 0.03%
Alternatively, Cr ≦ 0.7%, Ni ≦ 1.0%, Cu
≦ 1.0%, Ti ≦ 0.02%, Ca ≦ 0.001
0.007% of 1 type or 2 types or more and the balance is F
It is a forged steel product having excellent fire resistance and toughness, characterized by comprising the composition of e and unavoidable impurities, and hot forging after reheating a steel slab having the composition of the above item to a temperature range of 1100 to 1300 ° C. Is finished in a temperature range of 700 ° C. or higher, a method for producing a forged steel product excellent in fire resistance and toughness, the steel slab having the composition of the above item is reheated to a temperature range of 1100 to 1300 ° C., and then heated. The method for producing a forged steel product excellent in fire resistance and toughness, characterized in that the hot forging is finished at a temperature range of 700 ° C. or higher, cooled, and then tempered at a temperature of an Ac ₁ transformation point or lower. A normalizing treatment in which a steel slab having the composition is reheated to a temperature range of 1100 to 1300 ° C., then hot forging is finished in a temperature range of 700 ° C. or higher, and after cooling, is heated to a temperature of 900 to 1100 ° C. Fireproof characterized by And a method for producing a forged steel product having excellent toughness, and reheating a steel slab having the composition of the above-mentioned item to a temperature range of 1100 to 1300 ° C., and then finishing hot forging at a temperature of 700 ° C. or higher and cooling, A method for producing a forged steel product excellent in fire resistance and toughness, which comprises performing a normalizing treatment of heating to a temperature between 900 to 1100 ° C., and subsequently performing a tempering treatment at a temperature of an Ac ₁ transformation point or lower. is there.

[0009]

The present invention will be described in detail below.

The high temperature strength of steel materials is about 1/2 of the melting point of iron.
At a temperature of 700 ° C. or lower, the strengthening mechanism at room temperature is almost the same, and it is governed by the refinement of the ferrite crystal grain size, solid solution strengthening by alloying elements, dispersion strengthening by the hardening phase, precipitation strengthening by fine precipitates, and the like.

Generally, the increase in high temperature strength is achieved by adding Mo and Cr to increase the resistance to softening at high temperature due to their precipitation strengthening and the effect of suppressing the reduction and disappearance of dislocations due to the bonding with atomic vacancies. ing. However, Mo,
Addition of Cr significantly increases hardenability, and the base material ferrite +
The bainite structure is easily made into a bainite structure. When a component system that easily forms a bainite structure is applied to a forged steel product,
As a result of the forging process being divided and performed depending on the shape of the part, differences occur in the forging finishing temperature, forging, and cooling rate at each part, so that the bainite structure ratio greatly changes at each part. As a result, it was found that the room temperature strength, the high temperature strength, the ductility, and the toughness were varied, and a part that did not meet the standard was generated.

The feature of the present invention is to promote ferrite transformation from austenite by precipitation of VN for the purpose of reducing the change in the composition ratio of bainite and ferrite in order to reduce the variation of the material in each part of the forged steel product. It is in the place of maximizing the utilization of precipitation hardening. That is, in the forging process, the strain caused by working at a temperature of 700 ° C. or higher penetrates deeply into the work material as compared with the rolling process, and not only the transformation interface but also many dislocations introduced at this time in the cooling process after working. Precipitation of VN occurs on the upper side, and the generation and strengthening of ferrite are performed more effectively and the intended purpose can be achieved. Processing temperature is 7 in the above forging process
When the temperature is lower than 00 ° C., it is necessary to reheat to secure the temperature. Therefore, in the present invention, the forging temperature and the end temperature thereof are 700 due to the treatment such as reheating on the way.
Do not drop below ℃.

Next, the reasons for limiting the basic composition range of the 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 for structural steel cannot be obtained. Since the material toughness, weld cracking property, weld heat affected zone (hereinafter referred to as HAZ) toughness, etc. are significantly reduced, the upper limit was made 0.20%.

Next, Si is necessary to secure the strength of the base material, but when it exceeds 0.5%, high carbon martensite (hereinafter referred to as M *) having a hardened structure is formed in the heat-treated structure,
Remarkably reduces toughness. If it is less than 0.05%, the required pre-deoxidation of molten steel cannot be performed, so the Si content is limited to this range.

Mn is 0.1 to secure the strength and toughness of the base material.
It is necessary to add 4% or more, but the upper limit was set to 2.0% within the allowable range of the toughness and crackability of the welded portion.

Al is usually added as a deoxidizing agent,
If it exceeds 0.03%, the deoxidation product reduces the cleanliness of the steel, so that not only the internal defects increase but also the toughness decreases. Further, when the solid solution Al becomes excessive, the combination with N increases and the amount of VN precipitation, which is a feature of the steel of the present invention, is reduced, but as described above, in the forged material, the precipitation of VN is more effective than the rolled material. It was found that the effect was small up to 0.03%. Therefore, the upper limit was limited to 0.03%. On the other hand, during heat treatment, especially in normalizing treatment, Al
It forms N, helps to refine the structure, and improves room temperature and high temperature strength. Therefore, it is necessary to contain at least 0.005% or more.

N is an extremely important element for VN precipitation. If it is less than 0.007%, the amount of VN precipitation is insufficient, a sufficient amount of ferrite structure is not obtained, and high temperature strength at 600 ° C. Since it cannot be ensured, the content is set to 0.006% or more. If the content exceeds 0.0180%, the toughness of the base material is reduced and the surface cracks of the steel slab are caused.
Limited to less than%.

Mo is an element effective in securing the strength of the base material and the high temperature strength. If it is less than 0.3%, sufficient high temperature strength cannot be ensured due to the combined action with the precipitation strength of VN,
If it exceeds 0.7%, the hardenability is too high, and the base metal toughness,
Since the HAZ toughness deteriorates, it was limited to 0.3 to 0.7%.

V is extremely important as VN in order to form a ferrite structure, to make it fine grain, and to secure high temperature strength.
If it is less than 0.05%, the amount of VN deposited is insufficient, so that
If it exceeds 2%, the amount of precipitation becomes excessive and the toughness of the base material decreases, so it was limited to 0.05 to 0.2%.

The amounts of P and S contained as unavoidable impurities are not particularly limited, but since weld cracking and toughness are deteriorated due to solidification segregation, they should be reduced as much as possible, and preferably the amounts of P and S are respectively. 0.02%, 0.0
It is 2% or less.

The above are the basic components of the steel of the present invention. For the purpose of increasing the strength of the base metal and improving the toughness of the base metal, Cr, N are used.
One, two or more of i, Cu and Ca can be contained. First, Ni is an extremely effective element that enhances the toughness of the base material, but the addition of more than 1.0% increases the proportion of the bainite structure and increases the alloy cost, and is not economical, so the upper limit is made 1.0%. did. Cr is effective for strengthening the base material and strengthening at high temperature by improving the hardenability and precipitation hardening. However, excessive addition exceeding the upper limit is harmful from the viewpoint of toughness and curability, so the upper limit was made 0.7%. Cu is
Although it is an element effective for strengthening the base material and weather resistance, the upper limit was set to 1.0% in consideration of hot work cracking and the like. Ti is Al
Similarly, is an element that has a strong affinity for N and reduces the precipitation amount of VN to lower the strength, so addition is not originally desirable, but it is effective in improving the toughness of the welded portion, and if necessary 0.02% Add as a limit. Ca is an effect as a deoxidizing material and subdivides sulfide (MnS), and the ductility of the base material,
It has the effect of improving toughness and suppressing anisotropy. However, if it is less than 0.001%, there is no effect, and if it exceeds 0.005%, coarse Ca sulfide oxides are formed, and ductility and toughness are reduced, so the Ca content was made 0.001 to 0.005%.

Next, the manufacturing method of the present invention will be described. The steel making furnace for the steel of the present invention may be either an electric furnace or a current melting furnace such as a converter and is not particularly specified. Moreover, the current refining technology such as degassing and ladle refining can be applied. The steel ingot thus molten and ingot, or the steel of the present invention formed into a steel slab by ingot-segmentation or continuous casting is forged, and the reheating temperature at that time is 1100 to 13
Regulate in the temperature range of 00 ℃. V, which improves fire resistance
In order to maximize the effect of adding Mo, it is necessary to sufficiently dissolve these elements into a solid solution during heating before hot forging, so the lower limit of the reheating temperature is set to 1100 ° C., and the upper limit is the performance and economical efficiency of the heating furnace. To 1300 ° C.

The end temperature in hot forging is 700 ° C. or higher. A preferable range is 800 to 1050 ° C. This is because when the final forging temperature is lower than 700 ° C., the precipitation of VN on the introduced dislocations rapidly increases, the strength rapidly increases, and the toughness decreases. Therefore, when the processing time is long and the processing end temperature of the steel slab and 700 ° C. cannot be secured, or when the temperature decreases to 700 ° C. or less during the processing, the steel slab is again charged and reheated. Also,
If it exceeds 1050 ° C., it becomes difficult to introduce dislocations, and the strengthening due to VN precipitation cannot be sufficiently achieved.

The present invention is basically based on a manufacturing method in which a product is obtained only by finishing hot forging, but thereafter, a tempering process can be performed at a temperature below the Ac ₁ transformation point.
As a result, the strength at room temperature can be improved without changing the components, that is, while maintaining the weldability.

Further, in the present invention, after the hot forging is completed, a normalizing process of heating to a temperature of 900 to 1100 ° C. can be performed. As a result, not only the material variation due to the hot forging end temperature can be reduced, but also the high temperature strength can be increased. Furthermore, it is also possible to perform a tempering process after the normalizing process. This not only makes it possible to further reduce material fluctuations, but also at room temperature,
High-temperature strength and toughness can be stabilized at a high level.

[0026]

EXAMPLES The effects of the present invention will be shown in the following examples. Table 1 shows the chemical composition of the steel of the present invention and the comparative material, and Table 2 shows the manufacturing conditions and mechanical test characteristics.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

[Table 4]

The forged steel product produced from the component steel of the present invention by the method of the present invention sufficiently satisfies the target room temperature strength, high temperature strength, and V notch Charpy impact value at 0 ° C. of 4.8 kgf · m at each part of the product. are doing. On the other hand, comparative materials # 2 and # 7
Has a low Mo and V content and a target high temperature strength at 600 ° C (14.7 kgf / mm ² or higher for 40 kg class, ² for 50 kg class).
(0 kgf / mm ² or more) cannot be secured, and for the # 4 material having a forging finish temperature lower than 700 ° C, the room temperature strength increases and the impact value at 0 ° C decreases. # 9 and # 12 are V and N
The content is low, the bainite structure is coarse, and the toughness is significantly reduced, so that the target value cannot be satisfied.

In FIG. 1, the yield strength at room temperature and the yield strength at 600 ° C. of the # 4 material treated by the method of the present invention shown in Table 2 are plotted and the relationship between them is illustrated. The value of # 8 material is also shown as a comparative material. As is clear from the figure, it is understood that both the materials of the present invention (as-forged, normalized, and tempered) have superior properties to the comparative materials.

[0033]

The forged steel product according to the present invention has excellent high temperature characteristics,
The coating thickness of the refractory material can be significantly reduced compared to the conventional one, and it is possible to significantly reduce the cost by reducing the construction cost and the construction period. Further, since it becomes possible to manufacture a fire-resistant building material made of forged steel having a large cross section, industrial benefits such as improvement in reliability of large-sized buildings, ensuring safety, and economic effects are extremely remarkable.

[Brief description of drawings]

FIG. 1 shows the relationship between the room-temperature yield strength and the yield strength at 600 ° C. in the case where the various production methods of the present invention are applied to the present invention material # 4 together with comparative materials.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Koichi Yamamoto, No. 1 Tsukiko Hachiman-cho, Sakai City, Osaka Prefecture, Nippon Steel Co., Ltd., Sakai Steel Works (72) Rikio Senjoiwa, No. 1 Kimitsu, Kimitsu City, Chiba, Japan Inside the Kimitsu Steel Works, Ltd. (72) Mamoru Ohashi, 1st Kimitsu, Kimitsu City, Chiba Prefecture, Japan (56) Inside the Kimitsu Works, Nippon Steel Co., Ltd. (56) Reference JP-A 63-161117 (JP, A) 59-107063 (JP, A)

Claims (6)

[Claims] 1. By weight, C: 0.05 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.4 to 2.0%, Mo: 0.3 to 0.7. %, V: 0.05 to 0.20%, N: 0.0060 to 0.0180%, Al: 0.005 to 0.03%, with the balance being composed of Fe and inevitable impurities. Forged steel products with excellent fire resistance and toughness. 2. By weight, C: 0.05 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.4 to 2.0%, Mo: 0.3 to 0.7. %, V: 0.05 to 0.20%, N: 0.0060 to 0.0180%, Al: 0.005 to 0.03%, and Cr ≦ 0.7%, Ni ≦ 1. 0.0%, Cu ≤ 1.0%, Ti ≤ 0.02%, Ca ≤ 0.001 to 0.007%, and one or more of them, with the balance being Fe and inevitable impurities. A forged steel product with excellent fire resistance and toughness. 3. A steel slab comprising the steel composition according to claim 1 or 2 is reheated to a temperature range of 1100 to 1300 ° C., hot forging is then performed, and the processing is completed in a temperature range of 700 ° C. or higher. A method for producing a forged steel product having excellent fire resistance and toughness, which is characterized by the following. 4. A steel slab comprising the steel composition according to claim 1 or 2 is reheated to a temperature range of 1100 to 1300 ° C., and then hot forging is finished in a temperature range of 700 ° C. or higher, and after cooling, A method for producing a forged steel product having excellent fire resistance and toughness, which comprises performing tempering at a temperature not higher than the Ac ₁ transformation point. 5. A steel slab comprising the steel composition according to claim 1 or claim 2 is reheated to a temperature range of 1100 to 1300 ° C., and then hot forging is completed in a temperature range of 700 ° C. or higher, and after cooling, A method for manufacturing a forged steel product having excellent fire resistance and toughness, which comprises performing a normalizing treatment of heating to a temperature between 900 and 1100 ° C. 6. A steel slab comprising the steel composition according to claim 1 or 2 is reheated to a temperature range of 1100 to 1300 ° C., and then hot forging is finished at a temperature of 700 ° C. or higher, and after cooling, 900 A method for producing a forged steel product having excellent fire resistance and toughness, which comprises performing a normalizing treatment of heating to a temperature between ˜1100 ° C. and subsequently performing a tempering treatment at a temperature of an Ac ₁ transformation point or lower.