JPH0699742B2 - Method of manufacturing low yield ratio hot dip galvanized steel sheet for building with excellent fire resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is a low yield ratio melting with excellent fire resistance for use in various structures in the fields of lightweight steel for construction, U column, other civil engineering and marine structures, etc. The present invention relates to a method for manufacturing a galvanized steel sheet.

(Prior Art) Hot-rolled steel sheets for buildings include general structural rolled steel sheets (JIS G 310
1), Welded structure rolled steel plate (JIS G 3106), Welded structure weather resistant hot rolled steel plate (JIS G 3114), High weather resistant rolled steel plate (JIS G 3125), (hereinafter known as steel plate), etc. are widely used. Has been done.

Fire resistance of a building is important, and various measures have been taken for large buildings and general housing. However, in general, as in the technique described in Japanese Patent Application Laid-Open No. 63-47451, a fireproof coating is generally used to prevent fire. Therefore, the construction cost rises and the space used for the building is narrowed.

In recent years, fire-resistant design has been reviewed, and as the new fire-resistant design method for buildings was legalized in 1987, the conventional allowable steel material temperature (350 ° C) during fire was removed and It became possible to determine the ability of the fireproof coating depending on the load actually applied to the building, and it became possible to use the steel sheet without coating when the high temperature strength of the raw steel sheet was secured.

However, there is currently no hot-rolled steel sheet that guarantees high-temperature strength for refractory, except for the invention of Japanese Patent Application No. 63-143740.

The invention of Japanese Patent Application No. 63-143740 has the same object as the present invention, but the present invention mainly relates to thick plates. However, among the buildings, lightweight steel frames and U-shaped columns often use hot-rolled steel strips or steel plates as raw materials. Hot-rolled steel strips or steel sheets are produced by a hot strip mill, but in this process, the finishing temperature cannot be unnecessarily lowered or the strip-passing speed cannot be extremely lowered due to continuous hot rolling. In order to produce a larger amount, there is a runout table quenching process and a winding process. For these reasons, imparting room temperature tensile properties and high temperature strength properties is significantly different from the thick plate manufacturing process.

In addition, there is an invention (number not arrived) filed on January 12, 1989 as a technology for applying this thick plate technology to a hot strip mill, but this invention is also based on the addition of Mo and is a high alloy steel. To a lesser extent, economics have not completely solved the problem.

Further, 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) In conventional steels, it is difficult to secure high-temperature strength by grain growth, coarsening of precipitates, dissolution of carbides, and the like. Further, although high alloy heat-resistant metals exist including iron-based metals, they are economically disadvantageous when consumed in large quantities for construction.

An object of the present invention is to provide a non-Mo ordinary steel that has excellent high-temperature characteristics, can reduce or omit the fireproof coating, can be easily processed at the time of forming a U column, etc., and has excellent base material corrosion resistance, and is also economical. The present invention provides a method for producing a galvanized steel sheet or strip having a low yield ratio and having similar steel components.

(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 10% is excellent in earthquake resistance, and it was achieved at the same time.

The gist of the present invention is as follows.

(1) In weight ratio, C ≦ 100ppm, Mn: 0.1 to 0.5%, P: 0.01 to
0.1%, Al ≦ 0.05%, Cu: 0.6 to 2.0%, Ti: 0.008 to
Steel containing 0.2% or / and Nb: 0.008 to 0.10% and the balance Fe and unavoidable impurities is made into a slab, and immediately or after heating to 1150 ° C or less, hot rolling and rolling at a temperature of 800 ° C or more. After completion, after cooling at an average cooling rate of 3 ° C / s or more, winding at 600 ° C or less, followed by pickling and electrolytic cleaning, and then heating to 500 to 600 ° C in a reducing atmosphere , A hot dip galvanizing method for buildings, which has a yield point strength at 600 ° C of 0.6 or more than the yield point strength at room temperature, characterized by being immersed in a hot dip galvanizing bath for hot dip galvanizing Steel plate manufacturing method.

(2) By weight ratio, C ≦ 100ppm, Mn: 0.1 to 0.5%, P: 0.01 to
0.1%, Al ≦ 0.05%, Cu: 0.6 to 2.0%, Ti: 0.008 to
0.2% or / and Nb: 0.008 to 0.10%, and further B: 1
~ 30ppm or / and Ni: 0.2-0.7%, the balance Fe and the unavoidable impurities made into steel slab, immediately,
Alternatively, after heating to 1150 ° C or lower, hot rolling is performed, rolling is completed at a temperature of 800 ° C or higher, and then cooling is performed at an average cooling rate of 3 ° C / s or higher, followed by winding at 600 ° C or lower, followed by pickling After electrolytic cleaning, after heating to 500 ~ 600 ℃ in a reducing atmosphere, it is immersed in a hot dip galvanizing bath to perform hot dip galvanizing, and the yield point strength at 600 ℃ is the yield point at room temperature. A method for producing a low yield ratio hot-dip galvanized steel sheet for buildings having a strength of 0.6 or more and excellent in fire resistance.

That is, the essence of the present invention is that a steel of a component system obtained by adding Ti or / and Nb to a very low carbon steel and adding a large amount of Cu is Cu.
Hot-rolling is performed under specific hot-rolling conditions so that predetermined characteristics are sufficiently imparted, and the strength is adjusted by P in some cases, and Ni is used for hot-cracking. There is a point to reinforce by addition.

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

C should be 100 ppm or less. In the present invention, C is a so-called IF steel (Interstitial Free Stee) from the viewpoints of formability at room temperature, especially stretch flangeability, and plating adhesion.
l) and add Ti and / or Nb. If the amount of C is large, the amount of Ti and / or Nb required for IF becomes large, which not only impairs the economical efficiency but also deteriorates the workability and toughness due to these carbides. In this sense, C is preferably 50 ppm or less.

Ti: 0.008 to 0.2% and / or Nb: 0.008 to 0.10% is necessary to form these carbides and fix C. Below the lower limit, the IF will not be sufficient and moldability and corrosion resistance will not be associated.
On the other hand, if the content exceeds the upper limit, the atomic equivalent of C is exceeded, which not only impairs the economical efficiency but also deteriorates the formability due to solid solution Ti and Nb. Preferably, Ti and / or Nb is added within the range of 0.05 ≧ 12/48 [Ti (%)] + 12/93 [Nb (%)] ≧ [C (%)].

Next, Mn is added in the 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, plating adhesion is deteriorated. In addition, in this component system, solid solution strengthening of Mn cannot be expected to be large, so unnecessarily increasing it impairs economic efficiency.

P is an element having a large solid solution strengthening, and interacts with Cu to bring about corrosion resistance. Therefore, 0.01% must be added. On the other hand, addition of more than 0.1% increases embrittlement, so the upper limit is made 0.1%.

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

Next, Cu is an extremely important element in the present invention.
That is, the high temperature strength, which is the main object of the present invention, is secured, the room temperature strength and the room temperature yield ratio are also taken into consideration, and further, the interaction with P also has excellent corrosion resistance. Although the mechanism of strengthening is not clear, it is considered that normal temperature strength is due to solid solution strengthening or slight cluster strengthening of Cu, and high temperature strength is due to cluster strengthening or precipitation strengthening of Cu. If Cu is added in an amount of less than 0.6%, the supersaturation degree of Cu will be insufficient and strength will not be imparted. Especially at high temperatures. Moreover, while addition of more than 2.0% tends to saturate these effects, it becomes difficult to avoid hot cracking, so the upper limit of the addition value is made 2.0%.

In the present invention, B or / and Ni are further added depending on the case. B is a grain boundary strengthening element, and in the IF steel as in the present invention, there is little solid solution carbon which is also a grain boundary strengthening element, and B is added in the sense of supplementing this. If it is less than 1 ppm, it has no effect, and if it exceeds 30 ppm, the effect is saturated. In addition, Ni is added 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 amount exceeds 0.7%, since Ni is an expensive metal, the economical efficiency, which is one of the major objects of the present invention, is impaired and the plating adhesion is deteriorated.

The effects of the present invention are of course not brought about by specifying only the above component systems. That is, hot rolling conditions are also extremely important requirements. In particular, in the steel containing a large amount of Cu as in the present invention, hot brittleness called so-called Cu embrittlement occurs, and it is the current situation that 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 (so-called CC-direct rolling), or when heated, it is 1150 ° C or lower. If this condition is removed, hot cracking cannot be avoided. 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 is about 1000 ° C, which is used in the current continuous hot rolling equipment. Under these conditions, Cu solutionization is sufficient. The hot rolling finish temperature is 800 ° C or higher.
When rolling is performed at a temperature lower than this temperature, Cu is strain-induced precipitation due to rolling, and it becomes useless to secure high-temperature strength later. That is, one of the hot rolling conditions of the present invention is to keep Cu dissolved in iron in a supersaturated state. Cooling and winding conditions on the run-out table are also defined from this viewpoint. The former has an average cooling rate of 3 ° C / s or more.
When gradually cooled at a value lower than this cooling rate, Cu precipitates during cooling, and normal temperature strength, normal temperature yield ratio, and high temperature strength cannot be secured. It is preferable to set the temperature to 10 ° C./s or more, since Cu can be maintained in a more solid solution state. Although the upper limit of the cooling rate depends on the plate thickness, the effect of the present invention can be maintained even if the cooling rate is increased to 50 ° C./s, which is the maximum strength of the current equipment. The winding temperature is 60
Set to 0 ° C or less. If this temperature is exceeded, C will be generated during slow cooling after winding.
u is overaged and the necessary tensile properties cannot be obtained. The winding temperature is preferably 520 ° C. or lower in order to obtain the characteristics more stably in consideration of variations in winding. Furthermore, in order to obtain sufficient supersaturated Cu over the entire length of the hot-rolled coil to obtain sufficient room temperature strength / yield ratio, high temperature strength, etc., the coiling temperature is more preferably 450 ° C or lower. The lower limit of the coiling temperature is not particularly specified, and may be room temperature, but if the coiling temperature is too low, sometimes the IF steel may not be sufficient, and excessive solid solution carbon remains, impairing the ductility of the steel. From this point of view, the lower limit of the coiling temperature is preferably 300 ° C.
The wide range of cooling conditions and winding temperature conditions on the run-out table is one of the characteristics of the Mo-based refractory steel of the present invention.

When the above steel sheet or steel strip is subjected to pickling and then electrolytic cleaning and hot dip galvanizing, it is heated to 500 to 600 ° C. in a reducing atmosphere. If the upper limit is exceeded, Cu will be overaged and the required tensile properties cannot be obtained. If it is less than the lower limit value, the plating adhesion is deteriorated.

The steel of the present invention is usually made into steel in a converter, and is usually decarburized by a vacuum degassing method because of its extremely low C. And usually it is continuously cast into slabs. A hot-rolled coil is formed by a hot strip mill, and is pickled, then electrolytically cleaned, and then hot-dip galvanized. After that, in some cases, the plating layer may be alloyed by post-heating.

0.01 to 20% of Al may be added to the Zn plating bath. This makes it possible to form an Fe-Zn-Al ternary alloy layer at the interface between the plating and the base metal and improve the plating adhesion.

Further, addition of a low melting point alloy such as Pb, Cd, Sn, Sb or Mg to the Zn plating bath in an amount of 1% or less does not impair the effects of the present invention.

〔Example〕

 Next, examples of the present invention will be described.

After the steel having the components shown in Table 1 was tapped in a converter, it was continuously cast into a slab and immediately or after heating, hot rolling was performed. Table 2 shows hot rolling conditions and hot dip galvanizing conditions.
A JIS Z2201 No. 5 test piece was used for the tensile test at room temperature.
It went according to SZ2241. In the high temperature tensile test, a high temperature extensometer was attached to a test piece, the temperature was raised to 600 ° C. at a rate of 150 ° C./hour, the sample was pulled at this temperature, and the yield point was measured.

In addition, after the picked hot-rolled coil was pickled, a sample for rewinding test was taken on a skin pass line. So-called Cu of board
When rewinding the surface condition caused by the baldness on the skin pass line, the entire length of the coil was observed and rated as follows. ◎: Good, (same as general material), ○: Minor (passed product), △: Somewhat recognized (shipping impossible depending on the destination), ×: Large generation (defective product).

Further, the workability of the material was evaluated by a hole expanding test showing stretch flangeability which is most needed for such hot-dip galvanized hot rolled steel sheet. The method is to open a 20 mmφ hole punched with a punch with a conical punch having an apex angle of 30 ° until the clutch is generated, and divide the hole diameter at that time by the original hole diameter (hole expansion ratio and Call) to evaluate.

The plating adhesion of the material was evaluated by a taping test. In the method, a tape was attached to a steel plate, the tape side was placed inside, the tape was peeled off from the steel plate after the contact bending process, and the amount of plating adhered to the tape was visually determined.
The evaluation is as follows. ×: Peeling off the entire surface (defective product),
△: Partial peeling (cannot be shipped depending on the destination), ○: Slightly spotted peeling (good product shipped), ◎: Several spotted peels (good) Table 2 shows hot rolling conditions and plating conditions The characteristic value of steel is shown. The steel according to the present invention has no problem of Cu heggling at a practical level, and has room temperature tensile properties of 40 kgf / mm ² grade, or 50 k
Compared to tensile properties of gf / mm ² class, the yield strength is sufficient to meet the standard values of 25 kgf / mm ² and 33 kgf / mm ² or more, and the yield ratio (yield strength / tensile strength) is 80% or less. It is a thing. Also, the hole expansion ratio is a good value of about 2.5 or more. Further, the high temperature yield point strength at 600 ° C. is also sufficiently high, and the value of 0.6 or more in the ratio with the normal temperature yield point strength is sufficiently satisfied, which is a high value of approximately 0.7 or more.

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

(Effects of the Invention) Countermeasures against building fires are a social issue, and high-performance homes are also required for general housing, and fire countermeasures are an important item among them. Under the above circumstances, the present invention is a hot strip mill capable of supplying a large amount of iron-based material having excellent high-temperature resistance characteristics with a composition system close to that of ordinary steel, and further provides hot-dip galvanizing with excellent corrosion resistance. It can be carried out and will greatly contribute to the solution of the social problems.

Claims (2)

[Claims] 1. A weight ratio of C ≦ 100 ppm, Mn: 0.1 to 0.5%, P:
0.01 to 0.1%, Al ≤ 0.05%, Cu: 0.6 to 2.0%, Ti:
A steel containing 0.008 to 0.2% and / or Nb: 0.008 to 0.10% and the balance Fe and unavoidable impurities is formed into a slab, and immediately or after heating to 1150 ° C or less, hot rolling is performed.
Rolling is completed at a temperature of 0 ° C or higher, and then the average cooling rate is 3
After cooling at ℃ / s or more, winding at 600 ℃ or less, followed by pickling and electrolytic cleaning, then 500 ~ 6 in a reducing atmosphere
After heating to 00 ℃, it is immersed in a hot dip galvanizing bath to apply hot dip galvanizing. The yield point strength at 600 ℃ is 0.6 or more of the yield point strength at room temperature. Yield ratio Hot-dip galvanized steel sheet manufacturing method. 2. A weight ratio of C ≦ 100 ppm, Mn: 0.1 to 0.5%, P:
0.01 to 0.1%, Al ≤ 0.05%, Cu: 0.6 to 2.0%, Ti:
0.008 to 0.2% or / and Nb: 0.008 to 0.10%, B: 1 to 30 ppm or / and Ni: 0.2 to 0.7%, balance F
After slabing steel consisting of e and unavoidable impurities,
Immediately or after heating to below 1150 ℃, hot rolling at 800 ℃
Rolling is completed at the above temperature, and then the average cooling rate is 3 ℃ / s.
After cooling as above, it is wound up at 600 ° C or lower, then pickled and then electrolytically washed, and then 500 to 600 ° C in a reducing atmosphere.
After being heated to room temperature, it is immersed in a hot dip galvanizing bath for hot dip galvanizing, and the yield point strength at 600 ℃ is 0.6 or more of the yield point strength at room temperature. Low yield ratio for construction with excellent fire resistance. Manufacturing method of hot-dip galvanized steel sheet.