JPH03162519A - Manufacture of hot dip galvanized cold rolled steel sheet having low yield ratio for building having superior fire resistance - Google Patents

Abstract

PURPOSE:To obtain a hot dip galvanized steel sheet having low yield ratio and suitable for a building having superior fire resistance by hot rolling and cold rolling a low carbon steel slab contg. a specified amt. of Cu under specified conditions and subjecting the cold rolled sheet to heat treatment and hot dip galvanization in a continuous hot dip galvanizing line. CONSTITUTION:A steel contg., by weight, <=0.01% C, 0.1-0.5% Mn, <=0.1% P, <=0.06% Al and 0.6-2.0% Cu or further contg. 0.008-0.2% Ti or 0.008-0.10% Nb is prepd. and 0.0001-0.003% B is added to the steel or Ni is added in 0.2-1.0 ratio of Ni to Cu. The resulting steel is formed into a slab and this slab is hot rolled at the Ar3 transformation point or above optionally after reheating to <=1,150 deg.C. The hot rolled sheet is coiled and cold rolled at 5-90% draft. The cold rolled sheet is sent to a continuous hot dip galvanizing line, heated to 750-900 deg.C at >=1 deg.C/sec heating rate in a reducing atmosphere, cooled at >=3 deg.C/sec average cooling rate and hot dip galvanized by passing through a molten Zn bath. A hot dip galvanized steel sheet for a fire resistant building is obtd. The yield point strength of this steel sheet at 600 deg.C is >=60% as high as that at ordinary temp.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a low yield ratio hot-dip galvanized cold-rolled tunnel with excellent fire resistance used for prefabricated building materials and various other buildings in the fields of civil engineering and marine structures. This relates to the method of manufacturing the board.

(Prior art) Cold-rolled steel sheets for construction use include cold-rolling cracks and stripes (JISG
3141), highly weather resistant rolled steel plate (JIS G 312)
5) Folded steel plate roof structures or materials (hereinafter referred to as well-known steel plates) are widely used.

Fire resistance of buildings is important, and a variety of measures are being taken for everything from large buildings to general residential buildings. However, the current situation is that fire prevention measures are generally taken using fireproof coatings, such as the technique described in Japanese Patent Application 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 in 1986, the new fire-resistant design law for buildings was enacted, and the previous regulations regarding the allowable temperature of steel materials (350'C or less) in the event of a fire were removed.
The ability of fire-resistant coating can now be determined based on the high-temperature strength of the steel plate and the load actually applied to the building, and if the high-temperature strength of the material steel plate is ensured, it is also possible to use steel plate without coating. became.

The invention disclosed in Japanese Patent Application No. 63-143470 is based on the addition of Mo and is mainly applied to thick plates.

In addition, there is a technology described in Japanese Patent Application No. 1-3834 that was invented by applying this thick plate technology to hot strip mills, but this technology is also based on forced addition, and although it is not as effective as high alloy steel. In terms of economics, the problem cannot be said to have been completely resolved.

Recently, the present inventors have focused on the superiority of Cu-added steel in order to ensure high-temperature strength, and have applied patent application No. 1-1999 to the technology for hot-rolled steel sheets and hot-dip galvanized steel sheets for fire-resistant construction.
No. 26225 and patent application No. 1-16446.

However, in many cases, cold-rolled steel plates or steel strips are used as materials for roofing materials, building materials for breakouts, etc. of buildings.

Therefore, the present inventors invented the technology of Japanese Patent Application No. 1-27297 as a technology for cold-rolled steel sheets for fire-resistant construction.

As a building steel, it is also necessary to have corrosion resistance. Therefore, there is a need for a technology to galvanize cold-rolled steel sheets using a continuous hot-dip galvanizing line that is highly productive and economical.

When cold-rolled steel strips or steel plates are galvanized on a continuous hot-dip galvanizing line, the threading speed cannot be reduced to an extremely low level due to mass production in this process. In order to further recrystallize and have good plating adhesion,
The reduction temperature cannot be lowered unnecessarily. In addition, there is a rapid cooling step after annealing.

For these reasons, the process for imparting room-temperature tensile properties and high-temperature strength properties is significantly different from the manufacturing process for thick plates, hot-rolled steel sheets, and hot-dip galvanized hot-rolled steel sheets.

The present invention further develops the technology of Cu-added mesh as a fire-resistant architectural steel plate.

(Problems to be Solved by the Invention) In conventional steels, it is difficult to ensure high-temperature strength due to elongation of crystal grains, 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 purpose of the present invention is to provide a hot-dip galvanized cold coating with a low yield ratio that has excellent high-temperature properties, can reduce or omit the need for fire-resistant coating, can be easily processed when forming into roofing materials, prefabricated building materials, etc., and has excellent base material corrosion resistance. The object of the present invention is to provide a method for manufacturing a rolled steel plate or a copper strip.

(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 found that, in an economical system, the yield point strength at 600°C is 0.00% of the room temperature strength. He has invented a method for manufacturing steel sheets that is more than six times as large. Furthermore, as a result of examining the strength of steel plates during earthquakes, it was revealed that low yield ratio steel plates with a yield ratio (yield point strength/tensile strength) of 80% or less at room temperature have excellent earthquake resistance. I ended up doing it.

The gist of the present invention is as follows.

(1) Weight ratio: C≦0.0%, Mn: 0. 1
~0. 5%, P≦0.1%, Al≦0.06%, C
u: 0. 6-2. 0%, the balance being Fe and unavoidable impurities is made into a slab, immediately or after heating to 1150°C or less, hot rolled at Ar=transformation point or higher to form a coil, and then cold rolled. 5-9
After cold rolling at 0%, the temperature was raised on a continuous hot-dip galvanizing line at an average temperature increase rate of 1°C/s or more, heated to 750-900'C in a reducing atmosphere, and then 60 characterized by cooling at an average cooling rate of 3°C/s or more, followed by immersion 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 cold-rolled steel sheet for construction, which has an excellent fire resistance of 6 times or more.

Law.

(2) In terms of weight ratio, C≦0. Ol%, Mn: 0. 1
~0. 5%, P≦0.1%, Al≦0.06%, Cu
: 0. 6-2. 0% plus Ti; 0.0
08-0. 2% or/and Nb: o. o 0 8
~0. Immediately or after making a slab of steel containing 10% Fe and the remainder Fe and unavoidable impurities.
After heating to 150°C or lower, hot rolling is performed above the Ar3 transformation point to form a coil. After that, cold rolling is performed at a cold reduction rate of 5 to 90%, and then the average temperature is increased in a continuous hot-dip galvanizing line. The temperature was raised at a rate of 1°C/s or more, and the temperature was increased to 750°C in a reducing atmosphere.
Heating to ~900℃, then average cooling rate 3℃/s
For architectural use with excellent fire resistance, the yield point strength at 600°C is 0.6 times or more than the yield point strength at room temperature, characterized by cooling at the above temperature, followed by immersion in a hot-dip galvanizing bath and hot-dip galvanizing. A method for manufacturing low yield ratio hot-dip galvanized cold-rolled steel sheets.

(3) In the method described in 1 or 2 above, B: 0.0001 to 0.003% or/and N is further added to the wire.
The yield point strength at 600°C is 0.2 to 1.0 i in Ni/Cu. A method for manufacturing a cold-rolled steel sheet with a low yield ratio hot-dip galvanized for construction, which has an excellent fire resistance of more than 6 times. The reasons for limiting the numerical values of the structure and requirements of the present invention will be described below.

C shall be 0.01% or less. That is, it is a so-called ultra-low C type from the viewpoint of formability at room temperature.

Furthermore, in some cases, IP steel (Interst
Ti and/or Nb are added in order to make it tia IFree Steel). If the amount of C is large, I FM (Interstitial FreeSte
A large amount of Ti and/or Nb is required for el), 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 0.005% or less.

Next, Mn is added in a range of 0.1 to 0.5%. If it is less than the lower limit, FeS brittleness tends to occur, and if it exceeds the upper limit, the plating adhesion deteriorates.

P is an element with large solid solution strengthening, and may be added depending on the required strength level. Addition of more than 0.1% increases embrittlement, so the upper limit is 0.1%. 1%. Also,
P improves corrosion resistance through interaction with Cu, so
A preferable lower limit is 0.01%.

M is necessary as a deoxidizing agent, but if it exceeds 0.06%, plating adhesion decreases.

Next, Cu is an extremely important element in the present invention. That is, it ensures 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 exhibits excellent corrosion resistance through interaction with P. Although the strengthening mechanism is not clear, it is thought that the room temperature strength is due to solid solution strengthening or some cluster strengthening of Cu, and the high temperature strength is due to cluster strengthening or precipitation strengthening of Cu. 0. 6
If Cu is added in an amount less than %, the degree of supersaturation of Cu will be insufficient and strength will not be imparted. This is especially noticeable at high temperatures. Also, 2.
If the addition exceeds 0%, these effects tend to be saturated, but hot cracking becomes unavoidable, so the condition for the addition value is 0.
．． 6-2. Set to 0%.

Ti: 0.008-0. 2% or/and Nb:
0.008 to 0.10% is necessary to shape carbides and fix C. Below the lower limit, sufficient IF steel (In
terstitial free steel), and if there is no overaging treatment after immersion in a hot-dip galvanizing bath, problems such as stretcher strain will occur after bonding.

Moreover, when the upper limit is exceeded, the atomic equivalent of C is exceeded, which not only impairs economic efficiency but also deteriorates the appearance due to the solid solution of Ti and Nb. Preferably, 0.05≧12/48 (Ti(X)) +12/93
(Nb(X)) :li! (↑i or/and Nb is added within the range shown by C(χ).

In the present invention, B and/or Ni are further added depending on the case. B is a grain boundary strengthening element, and I
F @ (Interstitial Free St
el), there is little solid solution carbon, which is also a grain boundary strengthening element, and B is added to compensate for this. 0.0001%
If it is less than O. When O O exceeds 3%, the effect is saturated. Further, Ni is added in order to completely eliminate hot cracking. Ni is added depending on the amount of Cu added, which causes hot cracking. Ni/Cu is 0. If it is less than 2, no effect of reducing hot cracking due to Ni is observed;
/Cu is 1. If it exceeds 0, Ni is an expensive metal, which impairs economic efficiency, which is one of the main objectives of the present invention.

Hot rolling conditions are specified as follows.

Hot rolling is done immediately after slab casting (so-called CC-direct rolling)
If it is carried out or heated, the temperature should be 1 liter or less at 50°C. If this condition is not met, hot cracking is unavoidable. When CC-direct rolling is performed, there is no problem in keeping it warm or heating the edges to some extent. The lower limit of the preferred heating temperature is 1000°C, which can be achieved with current continuous hot rolling equipment. Under these conditions, solutionization of Cu is sufficient. The finishing temperature is equal to or higher than the Ar+transformation point.

If it is less than this, a processed structure will be formed and it will become hard. The upper limit is 95
The temperature is preferably 0°C. In addition, in the present invention, as described later, Cu is re-dissolved in the molten galvanizing process, so
The precipitation state of Cu during hot rolling is not particularly limited.

When cold rolling the above-mentioned steel plate or copper strip, the cold rolling reduction rate is 5 to 90%. If it is less than the lower limit, cold rolling will not be effective, and if it exceeds the upper limit, it will be difficult to maintain the flatness of the product.

When hot-dip galvanizing is subsequently performed on a continuous hot-dip galvanizing line, the average temperature increase rate shall be 1°C/s or more. If it is less than this value, Cu will precipitate when passing through the Cu precipitation region, making it impossible to obtain the necessary tensile properties.
Even if the upper limit is 70° C./s or more, which can be obtained in a direct-fire non-oxidation-reduction furnace, the effect will continue.

The reduction temperature is 750°C or higher. The present inventors investigated the precipitation behavior of Cu and found that in the steel of the present invention, Cu re-dissolves in solid solution at temperatures above 700"C. Therefore, heating to a temperature above 700"C is sufficient to dissolve Cu. However, in order to further recrystallize, a temperature of 750°C or higher is required.In order to sufficiently perform recrystallization over the entire length of the coil and to dissolve Cu more fully, it is recommended to set the temperature to 800°C or higher. Preferable.900
If it exceeds ℃, the crystal grains will become coarser and the strength at room temperature will decrease, and the strength at high temperature will also decrease, so the upper limit is 900'.
Let it be C.

Ru.

The average cooling rate shall be 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 heating rate is 5° C./s or more because Cu can be maintained in a solid solution state. The upper limit of the cooling rate is 100℃/s, which can be achieved with the current most powerful equipment, although it depends on the plate thickness.
Even if this is done, the effect will continue. After cooling, it is immersed in a hot-dip galvanizing bath to perform hot-dip galvanizing, but depending on the case, the plated layer may be alloyed by post-heating.

In the Zn plating bath, u@o. It may be added in an amount of ot to 20%.

In addition, in the Zn plating bath, Pb, Cd, Sn, S
Even if a low melting point alloy such as b or Mg is added in an amount of 1% or less, the effects of the present invention will not be impaired in any way.

Next, embodiments of the present invention will be described.

(Example) After steel having the precepts shown in Table 1 is taken out in a converter and made into a slab by continuous casting, it is hot-rolled immediately or after heating, cold-rolled, and then continuously melted. Melt galvanizing was performed on a galvanizing line.

Table 2 shows hot rolling conditions, cold rolling conditions, and continuous hot-dip galvanizing conditions. Tensile test at room temperature is JIS Z2201
It was conducted in accordance with JIS Z 2241 using a No. S test piece. In the high-temperature tensile test, a high-temperature extensometer was attached to the test piece, the temperature was raised to 600°C at a rate of 10°C/min, and after holding at that temperature for 15 minutes, a tensile test was conducted to measure the yield point.

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

When unwinding the surface condition caused by so-called Cu sagging, the entire length of the coil was observed and rated as follows. ◎: Good, (same as general materials), ○: Slight (product passed for shipment), △: Slightly observed (
(Cannot be shipped due to destination) ×: Large occurrence (defective product).

The workability of the material was evaluated by bendability. The test piece is JIS
Using Z 2204 No. 3 test piece, the test method was JI
In accordance with S Z 2248. The bending angle is 180°.

As for the rating, those that do not cause cracks in the bending test are ○.
, Those with cracks were rated as ×.

The plating adhesion of the material was evaluated using an impact test.

The method is to drop a hemispherical punch (diameter 12.7 φ) onto a steel plate, apply tape to the circular depression formed, peel the tape from the steel plate, and measure the amount of plating that has adhered to the tape. Judgment was made visually. The evaluation is as follows. ◎：
Several point-like peelings (good), O: Slightly many point-like peelings (product passed for shipment), Δ: Some peeling is observed in some areas (care required),
×: Large occurrence (defective product) Table 2 shows the characteristic values of the invention steel and comparative steel. The steel according to the present invention has no problem with the degree of Cu sagging at a practical level, and in terms of room temperature tensile strength, the yield point strength fully satisfies the standard value of 25 kgf/1 or more with respect to the tensile strength of 40 f/1 class. , and has an excellent yield ratio (yield point strength/tensile strength) of 80% or less. Moreover, the bendability is also good. Furthermore, the high temperature yield point strength at 600°C is sufficiently high, and the ratio to the yield point strength at room temperature is 0. It fully satisfies the value of 6 or more, and is generally a high value of 0.7 or more. Further, the steel according to the present invention also has good plating adhesion.

On the other hand, steel not according to the present invention has at least one of these characteristic values inferior to the steel according to the present invention.

(Effects of the invention) Fire countermeasures have become a major social issue as buildings become taller and housing becomes denser. Under these circumstances, the present invention has made it possible to manufacture hot-dip galvanized cold-rolled steel sheets, which are iron-based and have excellent high-temperature properties and corrosion resistance, on a continuous hot-dip galvanizing line capable of mass production. .

The present invention greatly contributes to solving the above social problems.

18 Q-

Claims (3)

[Claims] (1) Weight ratio: C≦0.01%, Mn: 0.1-0.
5%, P≦0.1%, Al≦0.06%, Cu:0.6
~2.0%, with the remainder being Fe and unavoidable impurities, immediately or at 1150°C.
After heating as follows, hot rolling is performed above Ar_3 transformation point to form a coil. After that, cold rolling is performed at a cold reduction rate of 5 to 90%, and then on a continuous hot-dip galvanizing line at an average heating rate of 1. The temperature is raised at a rate of ℃/s or more, and the temperature rises to 750 to 90℃ in a reducing atmosphere.
The yield point strength at 600°C is the yield point strength at room temperature, which is characterized by heating to 0°C, then cooling at an average cooling rate of 3°C/s or more, and then immersing in a hot-dip galvanizing bath to apply hot-dip galvanizing. A method for producing a cold-rolled cold-rolled steel sheet with a low yield ratio hot-dip galvanized steel sheet for construction, which has excellent fire resistance of 0.6 times or more. (2) Weight ratio: C≦0.01%, Mn: 0.1-0.
5%, P≦0.1%, Al≦0.06%, Cu:0.6
~2.0%, Ti: 0.008~0.2% or/and Nb: 0.008~0.10%, the balance F
After making the steel consisting of e and unavoidable impurities into a slab,
Immediately or after heating to 1150°C or lower, hot-rolled above the Ar_3 transformation point to form a coil, then cold-rolled at a cold reduction rate of 5 to 90%, and then on a continuous hot-dip galvanizing line. The temperature is raised at an average temperature increase rate of 1°C/s or more, heated to 750 to 900°C in a reducing atmosphere, then cooled at an average cooling rate of 3°C/s or more, and then immersed in a hot-dip galvanizing bath to remove molten zinc. 60 characterized by plating
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 cold-rolled steel sheet for construction, which has an excellent fire resistance of 6 times or more. (3) In the method according to claim 1 or 2, the steel further includes B: 0.0001 to 0.003% and/or Ni.
0.2 to 1.0 of Ni/Cu, the yield point strength at 600°C is 0.6 times or more the yield point strength at room temperature, and has excellent fire resistance for construction use. Method for manufacturing galvanized cold rolled steel sheet.