JPH07100816B2 - Manufacturing method of low yield ratio cold rolled steel sheet for building with excellent fire resistance - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a low yield ratio steel sheet having excellent fire resistance used for various constructions in the field of prefabricated building materials and other civil engineering and marine structures. .

(Prior Art) Cold-rolled steel for construction includes cold-rolled steel and steel strip (JIS G 31
41), high weather-resistant rolled steel plate (JIS G 3125), steel plate folded plate roof constituents (hereinafter known as steel plate), etc. are widely used.

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 Laid-Open No. 63-47451, fire protection is performed with a fireproof coating at present. For this reason,
Construction costs are rising, and the space available for buildings is being narrowed.

Recently, the fire resistance design was reviewed, and as the new fire resistance design method for buildings was legalized in 1987, the conventional allowable steel temperature (350 ° C) at the time of fire was removed, and the high temperature strength of steel plates Therefore, it becomes possible to determine the ability of the fireproof coating by the load actually applied to the building, and it is possible to use the uncoated steel sheet when the high temperature strength of the raw steel sheet is secured.

The applicant of the present invention has previously proposed Japanese Patent Application No. 63-143470, which is based on the addition of Mo and mainly proposes thick plates.

There is a technique described in Japanese Patent Application No. 63-143740, which is a technique for applying the thick plate technique to a hot strip mill, but this technique is also based on the addition of Mo, which is not so high as a high alloy steel, but is economical. In sex, the problem is not completely solved.

Recently, the present inventors have paid attention to the excellence of Cu-added steel for securing the high temperature strength, and invented the technique described in Japanese Patent Application No. 1-262525 as a technique of hot rolled steel sheet for fireproof construction. Furthermore, as a hot dip galvanizing technology for fireproof construction, Japanese Patent Application No. 1-1644
The invention described in Japanese Patent Application No. 6-73548 is invented.

However, among building materials, roofing materials, prefabricated building materials, and the like often use cold-rolled steel plates or steel strips as raw materials.

Cold-rolled steel strip or steel sheet is almost continuously annealed after cold rolling. In this process, it is impossible to extremely reduce the strip running speed due to mass production. Further, since the recrystallization is performed, the annealing temperature cannot be lowered unnecessarily. In addition, there are a quenching step and an overaging step after annealing.

For these reasons, imparting room temperature tensile properties and high temperature strength properties is significantly different from the manufacturing process of thick plates, hot-rolled steel sheets and hot-dip galvanized hot-rolled steel sheets. Currently, there is no technology for cold-rolled steel sheets for fireproof construction.

(Problems to be Solved by the Invention) In conventional steels, it is difficult to secure high temperature strength due to crystal 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 as they are consumed in large quantities for construction.

The present invention is a further development of the technology of Cu-added steel as a fire-resistant building steel sheet. The purpose of the present invention is to provide excellent high-temperature characteristics, reduce or omit the fire-resistant coating, and easily process it when forming roofing materials. Further, there is a method for producing a cold-rolled steel sheet or a cold-rolled steel strip having a low yield ratio which is also excellent in corrosion resistance of the base material.

(Means for Solving the Problems) As a result of research on the strength of steel sheet during a fire, the present inventors have found that it is an economical component system and the yield point strength at 600 ° C. of steel sheet is 0.6 times or more the normal temperature strength. Invented a manufacturing method.

Furthermore, as a result of the study on the steel plate strength during an earthquake,
It has also been clarified that a low yield ratio steel sheet having a yield ratio (yield point strength / tensile strength) at room temperature of 80% or less is excellent in seismic resistance, and it has also been achieved.

The gist of the present invention is that the weight ratio is C ≦ 0.018.
%, Mn: 0.1 to 0.5%, P: 0.01 to 0.1%, Al ≦ 0.1%, Cu: 0.6 to
In addition to 2.0%, Ti: 0.008-0.2% or / and Nb: 0.008
~ 0.10%, and if necessary B: 0.0001-0.003% or / and Ni-0.2 / 1.0 in Ni / Cu, with the balance Fe and unavoidable impurities made into steel slab, immediately or below 1150 ° C After heating, hot rolling is performed to finish rolling at a temperature of 800 ° C or higher, then cooling is performed at an average cooling rate of 3 ° C / s or higher, and then wound at 600 ° C or lower, and then cold reduction is 5 to 90%.
After continuous cold rolling at a temperature of 3 ℃ / s or more and an annealing temperature of 750 ℃ or higher in a continuous annealing facility, and then cooling at an average cooling rate of 3 ℃ / s or more, at an overaging temperature of 600 ℃ or less. It is a method for producing a cold-rolled steel sheet having a low yield ratio for construction, which is characterized by performing continuous annealing and which has a yield strength at 600 ° C of 0.6 times or more the yield strength at room temperature and is excellent in fire resistance.

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

C is 0.018% or less. That is, from the viewpoint of formability at room temperature, so-called IF steel (Interstitial Free Stee
l) and / or Ti or / and Nb are added.
If the amount of C is large, the amount of Ti and / or Nb required for the IF steel 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 0.005% or less.

Ti: 0.008 to 0.2% or / and Nb: 0.008 to 0.10% is
It is necessary to form these carbides and fix C. If it is less than the lower limit, sufficient IF steel is not formed, and formability and corrosion resistance are not accompanied. On the other hand, if the amount 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, 0.05 ≧ 12/48 [Ti (%)] + 12/93 [Nb (%)] ≧ [C
(%)] Ti or / and Nb are added within the range shown by.

Next, Mn is added in the range of 0.1 to 0.5%. If it is less than the lower limit, FeS brittleness is likely to occur, and solid solution strengthening of Mn cannot be expected to be large in this component system, so unnecessarily increasing it impairs economic efficiency.

P is an element having a large solid solution strengthening, and provides corrosion resistance by interacting with Cu. 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 0.1%, inclusions increase and the ductility and toughness of the steel deteriorate. The lower limit is 0.
About 01% is usually taken, but Ti included in the present invention is
If it can be further reduced by deoxidation, etc., it may be about 0.003%.

Next, Cu is an extremely important element in the present invention. That is, to secure the high temperature strength is the main purpose of the present invention,
It also bears room temperature strength and room temperature yield ratio, and also has excellent corrosion resistance due to the interaction with P.

Although the mechanical mechanism for strengthening is not clear, room temperature strength is solid solution strengthening of Cu or slight cluster strengthening, and high temperature strength is C
It is considered to be responsible for the cluster strengthening or precipitation strengthening of u.

When Cu is added in an amount of less than 0.6%, the supersaturation degree of Cu is insufficient and strength is not imparted. Especially at high temperatures. Also, 2.
While addition of more than 0% tends to saturate these effects, it becomes difficult to avoid hot cracking, so the addition value is made 0.6 to 2.0%.

In the present invention, B or / and Ni are optionally added. 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 0.0001%, it has no effect, and if it exceeds 0.003%, the effect is saturated.

Further, Ni is added to completely eliminate hot cracking. The amount of Ni added depends on the amount of Cu added which causes hot cracking. When Ni / Cu is less than 0.2, the effect of reducing hot cracking due to Ni is not recognized, and when Ni / Cu exceeds 1.0, Ni is an expensive metal. Spoil.

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 preferable heating temperature is 1000 ° C which can be obtained by the current continuous hot rolling equipment. Under these conditions, solution treatment of Cu is sufficient.

The hot rolling finish temperature shall be 800 ° C or higher. When rolling is performed at a temperature lower than this temperature, Cu is strain-induced precipitation due to rolling, which makes it impossible to secure high-temperature strength thereafter. The upper limit of the hot rolling finish temperature is 900 ° C in relation to the lower limit of the heating temperature.
The following is preferable.

One of the hot rolling conditions of the present invention is to keep Cu in supersaturated solution in iron. 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 because 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 coiling temperature shall be 600 ° C or lower. If this temperature is exceeded, Cu will be overaged during gradual cooling after winding, and the required 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.

Further, 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 set to 450 ° C. or lower. The lower limit of the winding temperature is not particularly specified, but it may be room temperature,
If the coiling temperature is too low, it may not be possible to obtain sufficient IF steel, and excess 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.

When the above steel plate or strip is cold rolled, the cold reduction is 5 to 90%. If it is less than the lower limit, there is no effect of cold rolling, and if it exceeds the upper limit, it becomes difficult to maintain the flatness of the product.

When continuous annealing is subsequently performed, the average heating rate is 1 ° C / s or more. If it is less than this value, Cu precipitates when passing through the Cu precipitation nose, and the required tensile properties cannot be obtained. The upper limit value is preferably 20 ° C./s which can be obtained with the current equipment.

The annealing temperature is 750 ° C or higher. 700 ° C. or higher is required for solid solution of Cu, but 750 ° C. or higher is required for recrystallization. The temperature is preferably 800 ° C. or higher so that recrystallization is sufficiently performed over the entire length of the coil and Cu is more solidly dissolved. The higher the annealing temperature, the more costly, so the upper limit is preferably 900 ° C.

The average cooling temperature is 3 ° C / s or higher. 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 it to 5 ° 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 will continue even at 100 ° C / s, which can be obtained with the strongest equipment at present.

The overaging temperature shall be 600 ° C or less. If this value is exceeded, Cu will be overaged during the subsequent cooling and the required tensile properties cannot be obtained. To obtain more stable characteristics, the overaging temperature is preferably 520 ° C or lower.

Furthermore, in order to obtain sufficient supersaturated Cu over the entire length of the coil and to obtain sufficient room temperature strength / yield ratio, high temperature strength, etc., it is more preferable to set the overaging temperature to 450 ° C. or lower. The lower limit is
The temperature is preferably 50 ° C. to maintain the flatness of the product.

(Examples) 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, cold rolling was performed, and then continuous annealing was performed. .

Table 2 shows hot rolling conditions, cold rolling conditions, and continuous annealing conditions.

A JIS Z 2201 No. 5 test piece was used for the tensile test at room temperature.
It was performed according to JIS Z 2241. 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 C of board
When rewinding the surface condition due to u-health on the skin pass line, the entire length of the coil was observed and the following scores were given. ◎: Good (same as general material), ○: Minor (shipped product), △: Slightly recognized (shipping impossible depending on the destination), ×: Large generation (defective product).

The workability of the material was evaluated by the bendability. The test piece is
Using JIS Z 2204 No. 3 test piece, the test method is JIS Z 22
Followed 48. The bending angle is 180 °.

Table 2 shows the characteristic values of the present invention steel and the comparative steel.

According to the present invention, the steel has no problem of Cu heggling at a practical level, and the tensile strength at room temperature is 400 MPa class or 490 MPa class tensile strength, and the yield strength is 245 MPa which is the standard value.
As described above, it is excellent in that it sufficiently satisfies 285 MPa or more and the yield ratio (yield point strength / tensile strength) is 80% or less. Also, the bendability is good.

Further, the yield strength at high temperature at 600 ° C. is sufficiently high, and the value of the yield strength at room temperature is 0.6 times or more, which is a high value of 0.7 times 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 circumstances, the present invention makes it possible to manufacture a cold-rolled steel sheet having excellent iron-based high temperature characteristics by continuous annealing capable of supplying a large amount.

Claims (2)

[Claims] 1. By weight ratio, in addition to C ≦ 0.018%, Mn: 0.1 to 0.5%, P: 0.01 to 0.1%, Al ≦ 0.1%, Cu: 0.6 to 2.0%, Ti: 0.008 to 0.2% , Or / and Nb: 0.008 to 0.10%, steel made of balance Fe and unavoidable impurities is made into a slab, and immediately or after heating to 1150 ° C or less, hot rolling is performed,
Rolling was completed at a temperature of 800 ° C or higher, then cooled at an average cooling rate of 3 ° C / s or higher, then wound at 600 ° C or lower, and then cold rolled at a cold reduction rate of 5 to 90%. After that, in the continuous annealing equipment, the average heating rate is 1 ° C / s or more, the annealing temperature is about 750 ° C or more, and then the average cooling rate is 3 ° C / s or more, and the continuous annealing is performed at the overaging temperature of 600 ° C or less. Characteristic 600 ℃
Of the cold-rolled steel sheet for low yield ratio for construction having excellent fire resistance, wherein the yield strength at room temperature is not less than 0.6 times the yield strength at room temperature. 2. The yield point strength at 600 ° C. according to claim 1, wherein B: 0.0001 to 0.003% by weight, and / or Ni / Ni / Cu in an amount of 0.2 to 1.0 is contained. A method for producing a cold-rolled steel sheet having a low yield ratio for construction, which is 0.6 times or more the point strength and is excellent in fire resistance.