JPH02197520A - Manufacture of hot dip galvanized steel sheet for construction use having low yield ratio and excellent in fire resistance - Google Patents

Abstract

PURPOSE:To manufacture the steel sheet having superior high-temperature characteristics, easy to work at the time of forming, and also excellent in corrosion resistance of base material by subjecting a steel having a specific composition to rolling under prescribed conditions and to winding and then carrying out hot dip galvanizing. CONSTITUTION:A steel having a composition consisting of, by weight ratio, <=100ppm C, 0.1-0.5% Mn, 0.01-0.1% P, <=0.05% Al, 0.6-2.0% Cu, further 0.008-0.2% Ti and/or 0.008-0.10% Nb, and the balance iron with inevitable impurities is formed into a slab. This slab is hot-rolled without delay or after heating up to <=1150 deg.C and hot rolling is finished at >=800 deg.C, and the hot-rolled plate is cooled at >=3 deg.C/sec average cooling rate, wound up at <=600 deg.C, and successively subjected to pickling and electrolytic cleaning, which is then heated in a reducing atmosphere up to 500-600 deg.C and immersed in a hot dip galvanizing bath to undergo plating so as to be formed into the above plated steel sheet in which yield point strength at 600 deg.C is regulated to a value >=0.6 times that at ordinary temp.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is a low-yield-ratio melting steel with excellent fire resistance used in various buildings in the fields of lightweight structural steel, U-columns, and other civil engineering and marine structures. Pertains to a method for manufacturing galvanized steel sheets.

(Conventional technology) Hot-rolled steel plates for construction include rolled steel plates for shell structures (JIS G
3101), rolled steel plates for welded structures (JIS G 31
06) Weather-resistant hot-rolled steel plates for welded structures (JIS G
3114), highly weather resistant rolled steel plate (JIS G 3125)
), (hereinafter referred to as well-known steel sheets), etc. are widely used.

Fire resistance of buildings is important, and various measures have been taken to ensure fire resistance for buildings ranging from large buildings to general residential buildings. However, at present, when fishing on a boat, fire prevention measures are taken by using a fireproof coating as described in Japanese Patent Laid-Open No. 63-47451. As a result, construction costs are rising and the usable space of buildings is becoming smaller.

Recently, fire-resistant design has been reviewed, and the new fire-resistant design law for buildings was enacted in 1986, and the previous regulations regarding the allowable temperature of steel materials in the event of a fire (350"C) were removed, and the high temperature of steel plates was removed. It is now possible to determine the ability of fire-resistant coatings based on the strength and the load actually applied to the building, and it has become possible to use steel plates without coating if the high-temperature strength of the steel plate material is ensured.

However, there is currently no hot-rolled steel sheet with guaranteed high-temperature strength for fire resistance, except for the invention disclosed in Japanese Patent Application No. 143740/1983.

The invention of Japanese Patent Application No. 63-143740 has the same object as the present invention, but this invention mainly relates to thick plates. However, lightweight steel frames and U-shaped columns in buildings are often made of hot-rolled rope or steel plates. Hot-rolled steel strips or steel plates are produced by hot strip mills, but in this process, the finishing temperature cannot be lowered unnecessarily or the threading speed can be extremely reduced due to continuous hot rolling, and in addition, large quantities must be produced. Therefore, there is a runout table quenching process and a winding process.

For these reasons, imparting cold tensile properties and high temperature strength properties is significantly different from the plate manufacturing process.

In addition, we applied for a patent on January 12, 1989 for an invention that applied this thick plate technology to a hot strip mill (
However, this invention is also based on the addition of Mo, and although it is not a high alloy steel, it cannot be said that the problem in terms of economy has been completely solved.

Furthermore, there is currently no technology for hot-dip galvanized steel sheets for construction that guarantees high-temperature strength for fire resistance.

(Problems to be Solved by the Invention) With conventional steels, it is difficult to ensure high-temperature strength due to grain growth, coarsening of precipitates, dissolution of carbides, etc. In addition, high-alloy heat-resistant metals, including iron-based metals, exist, but they are economically disadvantageous as they are consumed in large quantities for construction purposes.

The object of the present invention is to provide a non-Mo material that has excellent high-temperature properties, can reduce or omit the need for fire-resistant coating, can be easily processed when forming U-columns, etc., has excellent base material corrosion resistance, and is highly economical.
An object of the present invention is to provide a method for manufacturing a hot-dip galvanized steel sheet or copper strip having a low yield ratio and having a steel composition close to that of ordinary steel.

(Means for Solving the Problems) As a result of research on the strength of steel plates in the event of a fire, the present inventors have found that a steel plate with an economical composition system whose yield point strength at 600°C is 60% or more of the normal temperature strength. This led to the invention of a manufacturing method. Furthermore, as a result of studying the strength of steel plates during earthquakes, we found that the yield ratio (yield point strength/tensile strength) at room temperature
It has also been revealed that low yield ratio steel plates with a yield ratio of 80% or less have excellent earthquake resistance, and they have also achieved this goal.

The gist of the present invention is as follows.

(1) Weight ratio: C≦100ppm, Mn: 0.
1-0.5%, P: 0.01-0.1% 1M≦
In addition to 0.05% and Cu: 0.6 to 2.0%, Ti:
0.008~0.2% or/and Nb: 0.008~
After making a slab of steel containing 0.10% Fe and unavoidable impurities, immediately or at 1150°C
After heating, hot rolling is carried out and the rolling is completed at a temperature of 800°C or higher, after which cooling is performed at an average cooling rate of 3°C/s or higher, winding is performed at a temperature of 600°C or lower, followed by pickling and electrolytic cleaning. After heating to 500-600°C in a reducing atmosphere,
Low yield ratio melting for construction with excellent fire resistance, characterized in that the yield point strength at 600°C is 0.6 or more of the yield point strength at room temperature, characterized by applying hot dip galvanization by immersing it in a hot dip galvanizing bath. Method of manufacturing galvanized steel sheet.

(2) Weight ratio, C≦l OOppm, Mn: 0
．． 1-0.5%, P: 0.01-0.1%, /
In addition to Al≦0.05%, Cu: 0.6 to 2.0%,
Ti: 0.008-0.2% or/and Nb: 0.0
08 to 0.10%, further B: 1 to 30 ppm or/and Ni: 0.2 to 0.7%, balance Fe
After making the steel containing unavoidable impurities into a slab, it is hot-rolled immediately or after heating to 1150℃ or less.
After finishing the rolling at a temperature of 0 o'c or more, cooling at an average cooling rate of 3°C/s or more, winding at 600°C or less, followed by pickling and electrolytic cleaning, reducing reducibility 60 characterized by heating to 500 to 600°C in an atmosphere and then immersing it in a hot-dip galvanizing bath to apply hot-dip galvanizing.
The yield point strength at 0℃ is 0 of the yield point strength at room temperature.
．． A method for producing a low yield ratio hot-dip galvanized steel sheet for construction with excellent fire resistance of 6 or more.

That is, the gist of the present invention is to add Ti or/to ultra-low carbon steel.
In addition, the steel containing Nb and a large amount of Cu is hot-rolled under specific hot-rolling conditions that prevent hot cracking due to Cu and sufficiently impart predetermined properties. In some cases, strength may be adjusted with P, and Ni may be added to strengthen the steel against hot cracking.

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

C shall be 1100 pp or less. In the present invention, C is
So-called IF steel (Interst
Ti and/or Nb are added. If there is a large amount of C, a large amount of Ti and/or Nb is required for IF, which not only impairs economic efficiency but also deteriorates workability and toughness due to these carbides.

In this sense, it is preferable that C be 50 ppm or less.

Ti: 0.008-0.2% or/and Nb: 0
．． 0.008 to 0.10% is necessary to form these carbides and fix C. If it is less than the lower limit, the IF will not be sufficient, and the formability and corrosion resistance will not be achieved, and if it exceeds the upper limit, the atomic equivalent of C will be exceeded, which not only impairs economic efficiency but also solid solution Ti,
Formability deteriorates due to Nb. Preferably, 0.05≧12/48 (Ti(χ)) +12/
93 Ti and/or Nb is added within the range shown by (Nb(χ)) ≧ (C(χ)).

Next, Mn is added in a range of 0.1 to 0.5%. If it is less than the lower limit, FeS embrittlement tends to occur, and if it exceeds the upper limit, the plating adhesion deteriorates. In addition, in this component system, it is not possible to expect a large solid solution strengthening of Mn, so making it unnecessarily high will impair economic efficiency.

P is an element with large solid solution strengthening and provides corrosion resistance through interaction with Cu. Therefore, it is necessary to add 0.01%. On the other hand, addition of more than 0.1% increases embrittlement, so the upper limit is set to 0.1%.

M is necessary as a deoxidizing agent, but if it exceeds 0.05%, plating adhesion decreases.

Next, Cu is an extremely important element in the present invention. That is, it secures high-temperature strength, which is the main objective of the present invention, and also has room-temperature strength and room-temperature yield ratio, and also has excellent corrosion resistance due to interaction with P. Although the strengthening mechanism is not clear, it is thought that the room temperature strength is due to Cu solid solution strengthening or some cluster strengthening, and the high temperature strength is due to CuO cluster strengthening or precipitation strengthening. If less than 0.6% of Cu is added, the degree of supersaturation of Cu will be insufficient and strength will not be imparted.

This is especially noticeable at high temperatures. Furthermore, if more than 2.0% is added, these effects tend to be saturated, while hot cracking becomes unavoidable, so the upper limit of the addition value is set at 2.0%.

In the present invention, B and/or Ni are further added depending on the case. B is a grain boundary strengthening element, and IF as in the present invention
Similarly, in m, there is little solid solution carbon, which is also a grain boundary strengthening element, and B is added to compensate for this.

If it is less than 1 ppm, there is no effect, and if it exceeds 30 ppm, the effect is saturated. Further, Ni is added in order to completely eliminate hot cracking. If the Ni content is less than 0.2%, the effect of reducing hot cracking due to Ni is not recognized, and if the Ni content exceeds 0.7%, Ni is an expensive metal, which is one of the major objectives of the present invention. This impairs certain economic efficiency and reduces plating adhesion.

Of course, the effects of the present invention are not brought about by specifying only the above-mentioned component system. That is, hot rolling conditions are also extremely important requirements. In particular, in steels to which a large amount of Cu is added, such as those of the present invention, hot embrittlement called so-called Cu embrittlement occurs, making it impossible to perform sufficient hot rolling. In the present invention, hot rolling conditions are specified as follows.

Hot rolling is done immediately after slab casting (so-called CC-direct rolling)
When heating or heating, the temperature should be 1150°C or lower. If this condition is not met, hot cracking is unavoidable. CC-
When direct rolling is performed, there is no problem in keeping it warm or heating the edges to some extent.

The lower limit of heating temperature is 1000, which can be achieved with current continuous hot rolling equipment.
It is about ℃. Under these conditions, solutionization of Cu is sufficient. The hot rolling end temperature is 800°C or higher. If rolling is performed at a temperature lower than this temperature, Cu will be strain-induced precipitated by rolling, and it will be useless to ensure high-temperature strength later. That is, one aspect of the hot rolling conditions of the present invention is to keep Cu dissolved in supersaturated iron. Cooling and winding conditions at the runout table are also determined from this point of view. The former is an average cooling rate of 3° C./s or more.

If it is slowly cooled at a cooling rate lower than this, Cu will precipitate during cooling, making it impossible to ensure room temperature strength, room temperature yield ratio, and high temperature strength. Preferably, the temperature is 10°C/S or more.
This is preferable because it can better maintain the solid solution state. The upper limit of the cooling rate depends on the plate thickness, but the upper limit of the cooling rate is 50,000
The effect of the present invention is maintained even if the temperature is increased to .degree. C./s. Also,
The winding temperature shall be 600°C or less. If this temperature is exceeded, Cu will precipitate over aging during slow cooling after winding, making it impossible to obtain the necessary tensile properties. In order to obtain more stable characteristics in consideration of variations in winding, the winding temperature is preferably 520° C. or lower. Further, in order to obtain sufficient supersaturated Cu over the entire length of the hot-rolled coil and to obtain sufficient room temperature strength, yield ratio, high temperature strength, etc., it is more preferable that the coiling temperature is 450° C. or lower. There is no particular lower limit to the coiling temperature, and it may be room temperature; however, if the coiling temperature is too low, the IF steel may not be sufficient, and excessive solid solution carbon may remain, impairing the ductility of the steel. In this sense, the lower limit of the winding temperature is 30
The temperature is preferably 0°C. The fact that the cooling conditions at the run-out table and the winding temperature conditions are wide in this way is also one of the features of the present invention for Mo-based fire-resistant steel.

When the above-mentioned steel plate or strip is electrolytically cleaned after pickling and hot-dip galvanized, 500 to 600
Heat to ℃. If this upper limit is exceeded, Cu will precipitate due to over-aging, making it impossible to obtain the necessary tensile properties. Below the lower limit, plating adhesion deteriorates.

The steel of the present invention is usually made into steel in a converter, and because of its extremely low carbon content, it is usually decarburized by a vacuum degassing method. It is then usually continuously cast into slabs. It is made into a hot-rolled coil using a hot strip mill, then pickled, electrolytically cleaned, and hot-dip galvanized. Thereafter, depending on the case, the plated layer may be alloyed by post-heating.

0.01 to 20% M may be added to the Zn plating bath. As a result, Fe-Zn is deposited at the interface between the plating and the base metal.
- It is possible to form an AZ ternary alloy layer and improve plating adhesion.

In addition, in the Zn plating bath, Pb + Cd, Sn, Sb
Even if 1% or less of low melting point alloys such as Mg or Mg are added, the effects of the present invention will not be impaired in any way.

〔Example〕

Next, examples of the present invention will be described.

After steel having the components shown in Table 1 was tapped in a converter, it was made into a slab by continuous casting, and then hot rolled immediately or after heating. Table 2 shows the hot rolling conditions and hot dip galvanizing conditions.

The tensile test at room temperature was conducted in accordance with JIS Z2241 using a JIS Z22015 test piece.

For high-temperature tensile testing, a high-temperature extensometer is attached to the test piece, and 60
The temperature was raised to 0° C. at a rate of 150° C./hour, and at this temperature the sample was pulled and the yield point was measured.

In addition, after pickling the produced hot-rolled coil, samples for unwinding tests were taken on a skin pass line.

When the surface condition of the plate caused by so-called Cu curling was unwound using a skin pass line, the entire length of the coil was observed and evaluated as follows. ◎: Good.

(-Same as shell material), O: Slight (product passed for shipping), Δ: Slightly observed (impossible to ship due to the condition), ×: Large occurrence (defective product).

In addition, the workability of the material was evaluated by a hole expansion test, which shows stretch flangeability, which is most required for such hot-dip galvanized hot-rolled steel sheets. The method is to make a 20mmφ hole with a punch.
The hole is expanded with a conical punch having an apex angle of 3" until a crack occurs, and evaluated by dividing the hole diameter at that time by the original hole diameter (referred to as hole expansion ratio).

The plating adhesion of the material was evaluated by a taping test. In this method, a tape was attached to a steel plate, the tape side was turned inward, the tape was folded in close contact with the steel plate, the tape was peeled off from the steel plate, and the amount of plating attached to the tape was visually determined. The evaluation is as follows. ×: Whole surface peeling (defective product), Δ
2 Partial peeling (shipping is not possible due to the tip), O: Slightly many spot peelings (product passed shipping), ◎ Several pieces of two spot peelings (good) 2nd
The table shows the hot rolling conditions, plating conditions, and characteristic values of the obtained steel. The steel according to the present invention has no problem with the degree of Cu heave at a practical level, and the yield point strength is 25 kgf/mj, which is the standard value, for the room temperature tensile strength of 40 kgf/- class or 50 kgf/- class, respectively. .

It is excellent in that it has a sufficient groove of 33 kgf/- or more and a yield ratio (yield point strength/tensile strength) of 80% or less. Further, the hole expansion ratio is also a good value of about 2.5 or more. Furthermore, the yield point strength at high temperature at 600° C. is sufficiently high, and the ratio to the yield point strength at room temperature fully satisfies the value of 0.6 or more, and is generally a high value of 0.7 or more.

In contrast, steels not according to the invention lack at least one of these characteristic values.

(Effects of the invention) Building fire prevention is a social issue, and high-performance housing is also required for general housing, and fire prevention is an important item.The present invention was developed under these circumstances. , which is manufactured using a hot strip mill that can supply iron-based materials with excellent high-temperature resistance characteristics in large quantities with a composition similar to that of ordinary steel, and which also makes it possible to hot-dip galvanize with excellent corrosion resistance. This will greatly contribute to solving social issues.

Claims (2)

[Claims] (1) Weight ratio: C≦100ppm, Mn: 0.1-0
．． 5%, P: 0.01-0.1%, Al≦0.05%,
In addition to Cu: 0.6~2.0%, Ti: 0.008~
After forming a slab of steel containing 0.2% or/and Nb: 0.008 to 0.10% and the remainder Fe and unavoidable impurities, hot rolling is performed immediately or after heating to 1150°C or less to 800°C. After finishing the rolling at the above temperature, and then cooling at an average cooling rate of 3℃/s or more, 600℃
600, which is characterized by being wound up, followed by pickling and electrolytic cleaning, heated to 500 to 600°C in a reducing atmosphere, and then immersed in a hot-dip galvanizing bath to perform hot-dip galvanizing. A method for producing a low-yield-ratio hot-dip galvanized steel sheet for construction, which has excellent fire resistance and whose yield point strength at °C is 0.6 or more of the yield point strength at room temperature. (2) Weight ratio: C≦100ppm, Mn: 0.1-0
．． 5%, P: 0.01-0.1%, M≦0.05%, C
In addition to u: 0.6-2.0%, Ti: 0.008-0
．． 2% or/and Nb: 0.008 to 0.10%, further B: 1 to 30 ppm or/and Ni: 0.
Immediately or at 1150°C after making a slab of steel containing 2 to 0.7% and the balance Fe and unavoidable impurities.
After heating, hot rolling is carried out and the rolling is completed at a temperature of 800°C or higher, after which cooling is performed at an average cooling rate of 3°C/s or higher, winding is performed at a temperature of 600°C or lower, followed by pickling and electrolytic cleaning. After heating to 500-600°C in a reducing atmosphere,
A low-yield-ratio molten zinc for architectural use with excellent fire resistance and whose yield point strength at 600°C is 0.6 or more of the yield point strength at room temperature, characterized by applying hot-dip galvanizing by immersing it in a hot-dip galvanizing bath. Method of manufacturing plated steel sheets.