JPH079030B2 - Method of manufacturing low yield ratio high strength hot dip galvanized hot rolled steel sheet for construction with excellent fire resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to lightweight structural steels for construction, U-shaped columns, and other low yielding excellent in fire resistance used for various buildings in the field of civil engineering and offshore structures. The present invention relates to a method for manufacturing a high strength galvanized hot rolled steel sheet.

(Prior Art) Fire resistance of a building is important, and various countermeasures have been taken from large buildings to ordinary houses. Especially for general houses, three-story houses are becoming popular due to soaring land prices. In this case, appropriate fire resistance is required. But in general,
Under the current circumstances, fire protection is performed with a fireproof coating as in the technique described in JP-A-63-47451. for that reason,
Construction costs are rising, and the space available for buildings is being narrowed.

Recently, the fire resistance design was reviewed, and the new fire resistance design method for buildings was enacted in 1987, and the conventional allowable steel material temperature (350 ° C or less) at the time of fire was removed. The strength and the load actually applied to the building made it possible to determine the ability of the fireproof coating, and it became possible to use the uncoated steel sheet when the high temperature strength of the raw steel sheet was secured.

Similar to the steel sheet obtained by the present invention, there is a technology proposed by Japanese Patent Application No. 63-143740 for a steel sheet that guarantees high temperature strength for fire resistance, but this technology is mainly for thick plates. However, among the buildings, lightweight steel frames and U
The column-shaped column is often made of hot-rolled steel plate (steel, etc.) or hot-dip galvanized hot-rolled steel plate (steel strip). These manufacturing processes are significantly different from the plank manufacturing processes.

That is, in the hot rolling process, the finishing temperature cannot be unnecessarily lowered or the sheet passing speed cannot be extremely lowered because of the continuous method. In order to produce a larger amount, there is a run-out table quenching process and a winding process.

Next, in the continuous hot-dip galvanizing process, the heating temperature cannot be unnecessarily lowered or the sheet passing speed cannot be lowered, like the hot rolling. Further, there are restrictions on the additive elements from the viewpoint of plating adhesion. In addition, since a heat history is received after the hot rolling process, it becomes more difficult to maintain the supersaturated state of the precipitation strengthening element, which is responsible for the high temperature strength, at room temperature.

For these reasons, it differs significantly from the thick plate process in providing the required room temperature tensile properties and high temperature strength properties.

Furthermore, the technology of the thick plate is based on the addition of Mo, and it cannot be said that the problem has been completely solved in terms of economic efficiency, though not to the extent of high alloy steel.

Therefore, the present inventors focused their attention on the superiority of Cu-based steel, devised a technology related to Cu-based steel, and applied for a patent (Japanese Patent Application No. 1-27297).
issue). The present invention is a further development of the technology of low C-Cu steel.

(Problems to be Solved by the Invention) In conventional steels, it is difficult to secure high-temperature strength by grain growth, coarsening of precipitates, dissolution of carbides, and the like. In addition to Fe-based high-alloy heat-resistant metals, there are Ni-based, Ti-based, etc., but they are economically disadvantageous as they are consumed in large quantities for construction.

The object of the present invention is excellent in high temperature characteristics, capable of reducing or omitting the refractory coating, having high room temperature strength, low yield ratio, and requiring extremely low carbonization as in the technique of Japanese Patent Application No. 1-26225. Manufacturing of low-strength-ratio, high-strength hot-dip galvanized hot-rolled steel sheets or steel strips that have excellent economic efficiency, a steel composition similar to that of non-Mo ordinary steel, and have weather resistance and corrosion resistance required for construction steel. There is a way to do it.

(Means for Solving the Problems) As a result of research on steel plate strength during a fire, the present inventors have found that it is an economical component system and has a yield point strength at 600 ° C. of 60% or more of room temperature strength. Invented a manufacturing method. Furthermore, as a result of studying the steel plate strength during an earthquake, the yield ratio (yield point strength / tensile strength) at room temperature was 80
It was also clarified that a steel sheet with a low yield ratio of less than or equal to 10% had excellent earthquake resistance, and it was also achieved.

The gist of the present invention is as follows.

(1) By weight, C: 0.02 to 0.1%, Si ≦ 0.3%, Mn: 0.3 to
Steel containing 1.5%, P ≦ 0.05%, Al ≦ 0.05%, Cu: 0.6 to 2.0% and the balance Fe and unavoidable impurities was made into a slab, and immediately or after being heated to 950 to 1150 ° C., 750 to
When hot-rolled coil is manufactured by hot rolling at 900 ° C and the obtained hot-rolled coil is passed through a continuous hot dip galvanizing process, it is heated to 700 to 850 ° C in a reducing atmosphere, and the average cooling rate is set. 3 ° C
Low yield ratio for building with excellent fire resistance, which has a yield point strength at 600 ° C of 0.6 times or more than the yield point strength at room temperature, which is characterized by cooling at a temperature of at least s / s and immersing it in a hot dip galvanizing bath. Manufacturing method of hot-dip galvanized steel sheet.

(2) By weight, C: 0.02 to 0.1%, Si ≦ 0.3%, Mn: 0.3 to
Immediately after forming a slab of steel containing 1.5%, P ≦ 0.05%, Al ≦ 0.05%, Cu: 0.6 to 2.0%, Ni containing 0.2 to 1.0 of Ni / Cu, and the balance Fe and unavoidable impurities. Or
After heating to 950 to 1200 ℃, hot rolling at 750 to 900 ℃ to produce hot rolled coil, and when the obtained hot rolled coil is passed through the continuous hot dip galvanizing process, 700 in a reducing atmosphere. ~ 850 ° C
It is heated to 60 ℃, cooled at an average cooling rate of 3 ℃ / s or more, and immersed in a hot dip galvanizing bath. The yield strength at 600 ℃ is 0.6 times or more the yield strength at room temperature. A method of manufacturing an excellent high yield hot-dip galvanized steel sheet with a low yield ratio for construction.

That is, the present invention, by using a steel of the component system in which a large amount of Cu is added to the low C-Mn basic component system, to prevent hot cracking due to Cu during slab heating, and to impart further predetermined characteristics. By performing hot rolling and continuous hot dip galvanizing under specific conditions, an attempt is made to produce a low yield ratio high strength hot dip galvanized hot rolled steel sheet for buildings having excellent fire resistance. Further, with respect to hot cracking, an excellent effect can be obtained by reinforcing Ni by adding Ni according to the present invention.

The reasons for limiting the numerical values of the present invention will be described below.

C is 0.02 to 0.1%. In the present invention, C is an element responsible for strength and toughness. If it is less than 0.02%, it is difficult to impart the necessary strength and toughness. In addition, strong vacuum degassing in steelmaking is required, which impairs economic efficiency. On the other hand, 0.1%
If it exceeds the range, a hardened structure such as martensite is likely to be formed and toughness is deteriorated.

Si is 0.3% or less. If it exceeds this, the plating adhesion of this component system deteriorates.

Mn is 0.3 to 1.5%. If it is less than 0.3%, it is difficult to form a structure having sufficient strength and toughness. On the other hand, if it exceeds 1.5%, a hardened structure is likely to be formed, which deteriorates toughness.

P should be 0.05% or less. If it exceeds this, toughness deteriorates. On the other hand, in the present component system, it has been found that the addition of 0.03% or more of P improves the corrosion resistance, so the lower limit is preferably made 0.03%.

Al should be 0.05% or less. Al is necessary as a deoxidizing agent, but if it exceeds 0.05%, the plating adhesion will deteriorate with this component system. The lower limit depends on the deoxidation method, but is usually 0.01% for Al deoxidation.

Cu is 0.6 to 2.0%. Cu is a very important element in the present invention. That is, the high temperature strength, which is the main object of the present invention, is ensured and also the room temperature strength is taken. Although the strengthening mechanism has not been clarified yet, it is considered that the room temperature strength is due to solid solution strengthening or slight cluster strengthening of Cu, and the high temperature strength is due to cluster strengthening or precipitation of Cu. If it is added in an amount of less than 0.6%, the degree of supersaturation of Cu at room temperature will be insufficient and the required high temperature strength will not be imparted. On the other hand, if the content exceeds 2.0%, these effects tend to be saturated and hot cracking becomes difficult to avoid.

Ni is Ni / Cu and is 0.2 to 1.0. Ni is added to completely eliminate hot cracking. If it is less than the lower limit, it is not effective, and if it exceeds the upper limit, Ni is an expensive metal and impairs economical efficiency, which is one of the major objects of the present invention.

The effects of the present invention are not brought about by specifying only the above component systems. That is, hot rolling and continuous hot dip galvanizing conditions are also extremely important requirements. In particular, in the steel containing a large amount of Cu as in the present invention, the current situation is that hot cracking called so-called Cu embrittlement occurs and sufficient hot rolling cannot be performed. In the present invention, the hot rolling conditions are specified as follows.

Hot rolling is performed immediately after slab casting (CC-direct rolling) or after heating. The heating temperature is 950 to 1150 ° C when Ni is not added, and 950 to 1200 ° C when Ni is added.
If the heating temperature exceeds the upper limit, hot cracking cannot be avoided.
Further, when Ni is added, the upper limit value is relaxed. CC-In the case of direct rolling, heat retention or some heating of the end may be performed. The lower limit of the heating temperature shall be 950 ° C, which can be used in the current continuous hot rolling equipment. Under this condition, C
Solutionization of u is sufficient.

The hot rolling finish temperature is 750 to 900 ° C.

If the temperature is lower than 750 ° C, Cu is strain-induced precipitation due to rolling, and the amount of supersaturated Cu at room temperature required for high temperature strength cannot be secured. To obtain a sufficient amount of supersaturated Cu, 800 ℃
The above is preferable. The upper limit is determined by the relationship with low temperature heating, but the current continuous hot rolling equipment does not put a load on 900 ℃
And

When passing the hot-rolled steel strip obtained by the above method in a continuous hot dip galvanizing step, the heating temperature in the reducing atmosphere is 70
0 to 850 ℃ If it is less than the lower limit value, Cu precipitates and required high temperature characteristics and room temperature characteristics cannot be obtained. Although the upper limit depends on the relationship with the plate thickness, it is set to 850 ° C, which does not apply a load when passing through current equipment.

The average cooling rate is 3 ° C / s or more. If it is slowly cooled at a value lower than this cooling rate, Cu will precipitate during cooling and the required high temperature characteristics and room temperature characteristics cannot be obtained. Although the upper limit depends on the plate thickness, the effect will continue even if the current equipment can be set to 20 ° C / s or higher.

The steel which is the subject of the present invention may be usually melted in a converter and subjected to secondary refining by vacuum degassing or the like. And usually it is continuously cast into slabs. Further, a hot-rolled coil is formed in the continuous hot-rolling step, and the hot-rolled coil is subjected to hot-dip galvanizing in a continuous hot-dip galvanizing step. After that, the plating layer may be alloyed by post-heating.

The addition of 0.01 to 20% of Al in the galvanizing bath does not impair the effects of the present invention.

Next, examples of the present invention will be described.

After the steel having the components shown in Table 1 was tapped in a converter, slabs were continuously cast, and immediately or after heating, hot rolling was performed, and the obtained hot rolled coil was subjected to a continuous hot dip galvanizing process. The plate was passed and hot-dip galvanized.

Table 2 shows the hot rolling conditions, the hot dip galvanizing conditions, and the characteristic values of the obtained steel sheet. The tensile test at room temperature is JIS Z
2201 No. 5 test piece was used and the test was performed according to JIS Z 2241. In the high temperature tensile test, attach a high temperature extensometer to the test piece,
The temperature was raised to 600 ° C. at a rate of 10 ° C./s, the temperature was maintained for 15 minutes, and then a tensile test was performed to measure 0.2% proof stress or yield strength.

Further, when the manufactured hot rolled coil was passed through a continuous hot dip galvanizing process, the surface condition due to so-called Cu heggling was observed on the front surface over the entire length of the coil, and the following scoring was performed. . ◎: Good (same as general material), ○: Minor (shipped product), △: Slightly recognized (shipping impossible depending on the destination), ×: Large generation (defective product).

The toughness of the material is JIS Z 2202 of the sub-size closest to the original thickness.
It carried out according to JIS Z 2242 using a Charpy V notch test piece.

The plating adhesion of the material was evaluated by an impact test. The method is to drop a hemispherical punch (diameter: 12.7 mmφ) onto a steel plate, attach it to the formed circular recess with a tape, make sure it adheres well, then peel it off, and visually check the amount of plating on the tape. It was judged by. The evaluation is as follows. ⊚: Several pieces of dot peeling (good), ○: Some amount of dot peeling (good product shipped), Δ: Some peeling is slightly recognized (needs maintenance),
X: Large occurrence (defective product).

As shown in Table 2, the steel according to the present invention shows excellent values at both room temperature and high temperature characteristic values, whereas the comparative steel shows that both room temperature and high temperature characteristic values are superior to those of the present invention steel. Obviously it is inferior.

(Effects of the Invention) Building fire countermeasures are a social issue, and high-performance housing is also required for general housing, and fire countermeasures are extremely important items.

The present invention, in such a situation, the iron-based material having excellent high-temperature characteristics, with a component system close to ordinary steel, is manufactured by a hot strip mill that can be supplied in large quantities, and further melted with excellent weather resistance and corrosion resistance. It enables galvanizing, and is considered to greatly contribute to the solution of the social issues mentioned above.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location C23F 17/00 8414-4K

Claims (2)

[Claims] 1. By weight ratio, C: 0.02-0.1%, Si ≦ 0.3%, M
n: 0.3-1.5%, P ≦ 0.05%, Al ≦ 0.05%, Cu: 0.6-2.0
%, And the balance Fe and unavoidable impurities were made into slabs, and immediately or after heating to 950 to 1150 ° C, hot rolling at 750 to 900 ° C to produce hot rolled coils, which were obtained. When passing the hot-rolled coil in the continuous hot dip galvanizing process, heat it to 700 to 850 ℃ in a reducing atmosphere, cool it at an average cooling rate of 3 ℃ / s or more, and immerse it in the hot dip galvanizing bath. A method for producing a hot dip galvanized steel sheet having a low yield ratio and high strength for construction, which has excellent fire resistance and has a yield strength at 600 ° C of 0.6 times or more the yield strength at room temperature. 2. By weight ratio, C: 0.02 to 0.1%, Si ≦ 0.3%, M
n: 0.3-1.5%, P ≦ 0.05%, Al ≦ 0.05%, Cu: 0.6-2.0
%, Ni 0.2 to 1.0 in Ni / Cu, and the balance Fe and inevitable impurities made into steel, and immediately or after heating to 950 to 1200 ° C, hot rolling at 750 to 900 ° C. When producing hot-rolled coils and passing the obtained hot-rolled coils in a continuous hot dip galvanizing process, the
Heat to 0 ~ 850 ℃, cool at an average cooling rate of 3 ℃ / s,
600 ℃ characterized by being immersed in hot dip galvanizing bath
A method for manufacturing a hot dip galvanized steel sheet with a low yield ratio and high strength for construction, which has an excellent fire resistance and has a yield strength of 0.6 times or more the yield strength at room temperature.